linux/fs/userfaultfd.c
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   1/*
   2 *  fs/userfaultfd.c
   3 *
   4 *  Copyright (C) 2007  Davide Libenzi <davidel@xmailserver.org>
   5 *  Copyright (C) 2008-2009 Red Hat, Inc.
   6 *  Copyright (C) 2015  Red Hat, Inc.
   7 *
   8 *  This work is licensed under the terms of the GNU GPL, version 2. See
   9 *  the COPYING file in the top-level directory.
  10 *
  11 *  Some part derived from fs/eventfd.c (anon inode setup) and
  12 *  mm/ksm.c (mm hashing).
  13 */
  14
  15#include <linux/list.h>
  16#include <linux/hashtable.h>
  17#include <linux/sched/signal.h>
  18#include <linux/sched/mm.h>
  19#include <linux/mm.h>
  20#include <linux/poll.h>
  21#include <linux/slab.h>
  22#include <linux/seq_file.h>
  23#include <linux/file.h>
  24#include <linux/bug.h>
  25#include <linux/anon_inodes.h>
  26#include <linux/syscalls.h>
  27#include <linux/userfaultfd_k.h>
  28#include <linux/mempolicy.h>
  29#include <linux/ioctl.h>
  30#include <linux/security.h>
  31#include <linux/hugetlb.h>
  32
  33static struct kmem_cache *userfaultfd_ctx_cachep __read_mostly;
  34
  35enum userfaultfd_state {
  36        UFFD_STATE_WAIT_API,
  37        UFFD_STATE_RUNNING,
  38};
  39
  40/*
  41 * Start with fault_pending_wqh and fault_wqh so they're more likely
  42 * to be in the same cacheline.
  43 */
  44struct userfaultfd_ctx {
  45        /* waitqueue head for the pending (i.e. not read) userfaults */
  46        wait_queue_head_t fault_pending_wqh;
  47        /* waitqueue head for the userfaults */
  48        wait_queue_head_t fault_wqh;
  49        /* waitqueue head for the pseudo fd to wakeup poll/read */
  50        wait_queue_head_t fd_wqh;
  51        /* waitqueue head for events */
  52        wait_queue_head_t event_wqh;
  53        /* a refile sequence protected by fault_pending_wqh lock */
  54        struct seqcount refile_seq;
  55        /* pseudo fd refcounting */
  56        atomic_t refcount;
  57        /* userfaultfd syscall flags */
  58        unsigned int flags;
  59        /* features requested from the userspace */
  60        unsigned int features;
  61        /* state machine */
  62        enum userfaultfd_state state;
  63        /* released */
  64        bool released;
  65        /* memory mappings are changing because of non-cooperative event */
  66        bool mmap_changing;
  67        /* mm with one ore more vmas attached to this userfaultfd_ctx */
  68        struct mm_struct *mm;
  69};
  70
  71struct userfaultfd_fork_ctx {
  72        struct userfaultfd_ctx *orig;
  73        struct userfaultfd_ctx *new;
  74        struct list_head list;
  75};
  76
  77struct userfaultfd_unmap_ctx {
  78        struct userfaultfd_ctx *ctx;
  79        unsigned long start;
  80        unsigned long end;
  81        struct list_head list;
  82};
  83
  84struct userfaultfd_wait_queue {
  85        struct uffd_msg msg;
  86        wait_queue_entry_t wq;
  87        struct userfaultfd_ctx *ctx;
  88        bool waken;
  89};
  90
  91struct userfaultfd_wake_range {
  92        unsigned long start;
  93        unsigned long len;
  94};
  95
  96static int userfaultfd_wake_function(wait_queue_entry_t *wq, unsigned mode,
  97                                     int wake_flags, void *key)
  98{
  99        struct userfaultfd_wake_range *range = key;
 100        int ret;
 101        struct userfaultfd_wait_queue *uwq;
 102        unsigned long start, len;
 103
 104        uwq = container_of(wq, struct userfaultfd_wait_queue, wq);
 105        ret = 0;
 106        /* len == 0 means wake all */
 107        start = range->start;
 108        len = range->len;
 109        if (len && (start > uwq->msg.arg.pagefault.address ||
 110                    start + len <= uwq->msg.arg.pagefault.address))
 111                goto out;
 112        WRITE_ONCE(uwq->waken, true);
 113        /*
 114         * The Program-Order guarantees provided by the scheduler
 115         * ensure uwq->waken is visible before the task is woken.
 116         */
 117        ret = wake_up_state(wq->private, mode);
 118        if (ret) {
 119                /*
 120                 * Wake only once, autoremove behavior.
 121                 *
 122                 * After the effect of list_del_init is visible to the other
 123                 * CPUs, the waitqueue may disappear from under us, see the
 124                 * !list_empty_careful() in handle_userfault().
 125                 *
 126                 * try_to_wake_up() has an implicit smp_mb(), and the
 127                 * wq->private is read before calling the extern function
 128                 * "wake_up_state" (which in turns calls try_to_wake_up).
 129                 */
 130                list_del_init(&wq->entry);
 131        }
 132out:
 133        return ret;
 134}
 135
 136/**
 137 * userfaultfd_ctx_get - Acquires a reference to the internal userfaultfd
 138 * context.
 139 * @ctx: [in] Pointer to the userfaultfd context.
 140 */
 141static void userfaultfd_ctx_get(struct userfaultfd_ctx *ctx)
 142{
 143        if (!atomic_inc_not_zero(&ctx->refcount))
 144                BUG();
 145}
 146
 147/**
 148 * userfaultfd_ctx_put - Releases a reference to the internal userfaultfd
 149 * context.
 150 * @ctx: [in] Pointer to userfaultfd context.
 151 *
 152 * The userfaultfd context reference must have been previously acquired either
 153 * with userfaultfd_ctx_get() or userfaultfd_ctx_fdget().
 154 */
 155static void userfaultfd_ctx_put(struct userfaultfd_ctx *ctx)
 156{
 157        if (atomic_dec_and_test(&ctx->refcount)) {
 158                VM_BUG_ON(spin_is_locked(&ctx->fault_pending_wqh.lock));
 159                VM_BUG_ON(waitqueue_active(&ctx->fault_pending_wqh));
 160                VM_BUG_ON(spin_is_locked(&ctx->fault_wqh.lock));
 161                VM_BUG_ON(waitqueue_active(&ctx->fault_wqh));
 162                VM_BUG_ON(spin_is_locked(&ctx->event_wqh.lock));
 163                VM_BUG_ON(waitqueue_active(&ctx->event_wqh));
 164                VM_BUG_ON(spin_is_locked(&ctx->fd_wqh.lock));
 165                VM_BUG_ON(waitqueue_active(&ctx->fd_wqh));
 166                mmdrop(ctx->mm);
 167                kmem_cache_free(userfaultfd_ctx_cachep, ctx);
 168        }
 169}
 170
 171static inline void msg_init(struct uffd_msg *msg)
 172{
 173        BUILD_BUG_ON(sizeof(struct uffd_msg) != 32);
 174        /*
 175         * Must use memset to zero out the paddings or kernel data is
 176         * leaked to userland.
 177         */
 178        memset(msg, 0, sizeof(struct uffd_msg));
 179}
 180
 181static inline struct uffd_msg userfault_msg(unsigned long address,
 182                                            unsigned int flags,
 183                                            unsigned long reason,
 184                                            unsigned int features)
 185{
 186        struct uffd_msg msg;
 187        msg_init(&msg);
 188        msg.event = UFFD_EVENT_PAGEFAULT;
 189        msg.arg.pagefault.address = address;
 190        if (flags & FAULT_FLAG_WRITE)
 191                /*
 192                 * If UFFD_FEATURE_PAGEFAULT_FLAG_WP was set in the
 193                 * uffdio_api.features and UFFD_PAGEFAULT_FLAG_WRITE
 194                 * was not set in a UFFD_EVENT_PAGEFAULT, it means it
 195                 * was a read fault, otherwise if set it means it's
 196                 * a write fault.
 197                 */
 198                msg.arg.pagefault.flags |= UFFD_PAGEFAULT_FLAG_WRITE;
 199        if (reason & VM_UFFD_WP)
 200                /*
 201                 * If UFFD_FEATURE_PAGEFAULT_FLAG_WP was set in the
 202                 * uffdio_api.features and UFFD_PAGEFAULT_FLAG_WP was
 203                 * not set in a UFFD_EVENT_PAGEFAULT, it means it was
 204                 * a missing fault, otherwise if set it means it's a
 205                 * write protect fault.
 206                 */
 207                msg.arg.pagefault.flags |= UFFD_PAGEFAULT_FLAG_WP;
 208        if (features & UFFD_FEATURE_THREAD_ID)
 209                msg.arg.pagefault.feat.ptid = task_pid_vnr(current);
 210        return msg;
 211}
 212
 213#ifdef CONFIG_HUGETLB_PAGE
 214/*
 215 * Same functionality as userfaultfd_must_wait below with modifications for
 216 * hugepmd ranges.
 217 */
 218static inline bool userfaultfd_huge_must_wait(struct userfaultfd_ctx *ctx,
 219                                         struct vm_area_struct *vma,
 220                                         unsigned long address,
 221                                         unsigned long flags,
 222                                         unsigned long reason)
 223{
 224        struct mm_struct *mm = ctx->mm;
 225        pte_t *ptep, pte;
 226        bool ret = true;
 227
 228        VM_BUG_ON(!rwsem_is_locked(&mm->mmap_sem));
 229
 230        ptep = huge_pte_offset(mm, address, vma_mmu_pagesize(vma));
 231
 232        if (!ptep)
 233                goto out;
 234
 235        ret = false;
 236        pte = huge_ptep_get(ptep);
 237
 238        /*
 239         * Lockless access: we're in a wait_event so it's ok if it
 240         * changes under us.
 241         */
 242        if (huge_pte_none(pte))
 243                ret = true;
 244        if (!huge_pte_write(pte) && (reason & VM_UFFD_WP))
 245                ret = true;
 246out:
 247        return ret;
 248}
 249#else
 250static inline bool userfaultfd_huge_must_wait(struct userfaultfd_ctx *ctx,
 251                                         struct vm_area_struct *vma,
 252                                         unsigned long address,
 253                                         unsigned long flags,
 254                                         unsigned long reason)
 255{
 256        return false;   /* should never get here */
 257}
 258#endif /* CONFIG_HUGETLB_PAGE */
 259
 260/*
 261 * Verify the pagetables are still not ok after having reigstered into
 262 * the fault_pending_wqh to avoid userland having to UFFDIO_WAKE any
 263 * userfault that has already been resolved, if userfaultfd_read and
 264 * UFFDIO_COPY|ZEROPAGE are being run simultaneously on two different
 265 * threads.
 266 */
 267static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx,
 268                                         unsigned long address,
 269                                         unsigned long flags,
 270                                         unsigned long reason)
 271{
 272        struct mm_struct *mm = ctx->mm;
 273        pgd_t *pgd;
 274        p4d_t *p4d;
 275        pud_t *pud;
 276        pmd_t *pmd, _pmd;
 277        pte_t *pte;
 278        bool ret = true;
 279
 280        VM_BUG_ON(!rwsem_is_locked(&mm->mmap_sem));
 281
 282        pgd = pgd_offset(mm, address);
 283        if (!pgd_present(*pgd))
 284                goto out;
 285        p4d = p4d_offset(pgd, address);
 286        if (!p4d_present(*p4d))
 287                goto out;
 288        pud = pud_offset(p4d, address);
 289        if (!pud_present(*pud))
 290                goto out;
 291        pmd = pmd_offset(pud, address);
 292        /*
 293         * READ_ONCE must function as a barrier with narrower scope
 294         * and it must be equivalent to:
 295         *      _pmd = *pmd; barrier();
 296         *
 297         * This is to deal with the instability (as in
 298         * pmd_trans_unstable) of the pmd.
 299         */
 300        _pmd = READ_ONCE(*pmd);
 301        if (pmd_none(_pmd))
 302                goto out;
 303
 304        ret = false;
 305        if (!pmd_present(_pmd))
 306                goto out;
 307
 308        if (pmd_trans_huge(_pmd))
 309                goto out;
 310
 311        /*
 312         * the pmd is stable (as in !pmd_trans_unstable) so we can re-read it
 313         * and use the standard pte_offset_map() instead of parsing _pmd.
 314         */
 315        pte = pte_offset_map(pmd, address);
 316        /*
 317         * Lockless access: we're in a wait_event so it's ok if it
 318         * changes under us.
 319         */
 320        if (pte_none(*pte))
 321                ret = true;
 322        pte_unmap(pte);
 323
 324out:
 325        return ret;
 326}
 327
 328/*
 329 * The locking rules involved in returning VM_FAULT_RETRY depending on
 330 * FAULT_FLAG_ALLOW_RETRY, FAULT_FLAG_RETRY_NOWAIT and
 331 * FAULT_FLAG_KILLABLE are not straightforward. The "Caution"
 332 * recommendation in __lock_page_or_retry is not an understatement.
 333 *
 334 * If FAULT_FLAG_ALLOW_RETRY is set, the mmap_sem must be released
 335 * before returning VM_FAULT_RETRY only if FAULT_FLAG_RETRY_NOWAIT is
 336 * not set.
 337 *
 338 * If FAULT_FLAG_ALLOW_RETRY is set but FAULT_FLAG_KILLABLE is not
 339 * set, VM_FAULT_RETRY can still be returned if and only if there are
 340 * fatal_signal_pending()s, and the mmap_sem must be released before
 341 * returning it.
 342 */
 343vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
 344{
 345        struct mm_struct *mm = vmf->vma->vm_mm;
 346        struct userfaultfd_ctx *ctx;
 347        struct userfaultfd_wait_queue uwq;
 348        vm_fault_t ret = VM_FAULT_SIGBUS;
 349        bool must_wait, return_to_userland;
 350        long blocking_state;
 351
 352        /*
 353         * We don't do userfault handling for the final child pid update.
 354         *
 355         * We also don't do userfault handling during
 356         * coredumping. hugetlbfs has the special
 357         * follow_hugetlb_page() to skip missing pages in the
 358         * FOLL_DUMP case, anon memory also checks for FOLL_DUMP with
 359         * the no_page_table() helper in follow_page_mask(), but the
 360         * shmem_vm_ops->fault method is invoked even during
 361         * coredumping without mmap_sem and it ends up here.
 362         */
 363        if (current->flags & (PF_EXITING|PF_DUMPCORE))
 364                goto out;
 365
 366        /*
 367         * Coredumping runs without mmap_sem so we can only check that
 368         * the mmap_sem is held, if PF_DUMPCORE was not set.
 369         */
 370        WARN_ON_ONCE(!rwsem_is_locked(&mm->mmap_sem));
 371
 372        ctx = vmf->vma->vm_userfaultfd_ctx.ctx;
 373        if (!ctx)
 374                goto out;
 375
 376        BUG_ON(ctx->mm != mm);
 377
 378        VM_BUG_ON(reason & ~(VM_UFFD_MISSING|VM_UFFD_WP));
 379        VM_BUG_ON(!(reason & VM_UFFD_MISSING) ^ !!(reason & VM_UFFD_WP));
 380
 381        if (ctx->features & UFFD_FEATURE_SIGBUS)
 382                goto out;
 383
 384        /*
 385         * If it's already released don't get it. This avoids to loop
 386         * in __get_user_pages if userfaultfd_release waits on the
 387         * caller of handle_userfault to release the mmap_sem.
 388         */
 389        if (unlikely(READ_ONCE(ctx->released))) {
 390                /*
 391                 * Don't return VM_FAULT_SIGBUS in this case, so a non
 392                 * cooperative manager can close the uffd after the
 393                 * last UFFDIO_COPY, without risking to trigger an
 394                 * involuntary SIGBUS if the process was starting the
 395                 * userfaultfd while the userfaultfd was still armed
 396                 * (but after the last UFFDIO_COPY). If the uffd
 397                 * wasn't already closed when the userfault reached
 398                 * this point, that would normally be solved by
 399                 * userfaultfd_must_wait returning 'false'.
 400                 *
 401                 * If we were to return VM_FAULT_SIGBUS here, the non
 402                 * cooperative manager would be instead forced to
 403                 * always call UFFDIO_UNREGISTER before it can safely
 404                 * close the uffd.
 405                 */
 406                ret = VM_FAULT_NOPAGE;
 407                goto out;
 408        }
 409
 410        /*
 411         * Check that we can return VM_FAULT_RETRY.
 412         *
 413         * NOTE: it should become possible to return VM_FAULT_RETRY
 414         * even if FAULT_FLAG_TRIED is set without leading to gup()
 415         * -EBUSY failures, if the userfaultfd is to be extended for
 416         * VM_UFFD_WP tracking and we intend to arm the userfault
 417         * without first stopping userland access to the memory. For
 418         * VM_UFFD_MISSING userfaults this is enough for now.
 419         */
 420        if (unlikely(!(vmf->flags & FAULT_FLAG_ALLOW_RETRY))) {
 421                /*
 422                 * Validate the invariant that nowait must allow retry
 423                 * to be sure not to return SIGBUS erroneously on
 424                 * nowait invocations.
 425                 */
 426                BUG_ON(vmf->flags & FAULT_FLAG_RETRY_NOWAIT);
 427#ifdef CONFIG_DEBUG_VM
 428                if (printk_ratelimit()) {
 429                        printk(KERN_WARNING
 430                               "FAULT_FLAG_ALLOW_RETRY missing %x\n",
 431                               vmf->flags);
 432                        dump_stack();
 433                }
 434#endif
 435                goto out;
 436        }
 437
 438        /*
 439         * Handle nowait, not much to do other than tell it to retry
 440         * and wait.
 441         */
 442        ret = VM_FAULT_RETRY;
 443        if (vmf->flags & FAULT_FLAG_RETRY_NOWAIT)
 444                goto out;
 445
 446        /* take the reference before dropping the mmap_sem */
 447        userfaultfd_ctx_get(ctx);
 448
 449        init_waitqueue_func_entry(&uwq.wq, userfaultfd_wake_function);
 450        uwq.wq.private = current;
 451        uwq.msg = userfault_msg(vmf->address, vmf->flags, reason,
 452                        ctx->features);
 453        uwq.ctx = ctx;
 454        uwq.waken = false;
 455
 456        return_to_userland =
 457                (vmf->flags & (FAULT_FLAG_USER|FAULT_FLAG_KILLABLE)) ==
 458                (FAULT_FLAG_USER|FAULT_FLAG_KILLABLE);
 459        blocking_state = return_to_userland ? TASK_INTERRUPTIBLE :
 460                         TASK_KILLABLE;
 461
 462        spin_lock(&ctx->fault_pending_wqh.lock);
 463        /*
 464         * After the __add_wait_queue the uwq is visible to userland
 465         * through poll/read().
 466         */
 467        __add_wait_queue(&ctx->fault_pending_wqh, &uwq.wq);
 468        /*
 469         * The smp_mb() after __set_current_state prevents the reads
 470         * following the spin_unlock to happen before the list_add in
 471         * __add_wait_queue.
 472         */
 473        set_current_state(blocking_state);
 474        spin_unlock(&ctx->fault_pending_wqh.lock);
 475
 476        if (!is_vm_hugetlb_page(vmf->vma))
 477                must_wait = userfaultfd_must_wait(ctx, vmf->address, vmf->flags,
 478                                                  reason);
 479        else
 480                must_wait = userfaultfd_huge_must_wait(ctx, vmf->vma,
 481                                                       vmf->address,
 482                                                       vmf->flags, reason);
 483        up_read(&mm->mmap_sem);
 484
 485        if (likely(must_wait && !READ_ONCE(ctx->released) &&
 486                   (return_to_userland ? !signal_pending(current) :
 487                    !fatal_signal_pending(current)))) {
 488                wake_up_poll(&ctx->fd_wqh, EPOLLIN);
 489                schedule();
 490                ret |= VM_FAULT_MAJOR;
 491
 492                /*
 493                 * False wakeups can orginate even from rwsem before
 494                 * up_read() however userfaults will wait either for a
 495                 * targeted wakeup on the specific uwq waitqueue from
 496                 * wake_userfault() or for signals or for uffd
 497                 * release.
 498                 */
 499                while (!READ_ONCE(uwq.waken)) {
 500                        /*
 501                         * This needs the full smp_store_mb()
 502                         * guarantee as the state write must be
 503                         * visible to other CPUs before reading
 504                         * uwq.waken from other CPUs.
 505                         */
 506                        set_current_state(blocking_state);
 507                        if (READ_ONCE(uwq.waken) ||
 508                            READ_ONCE(ctx->released) ||
 509                            (return_to_userland ? signal_pending(current) :
 510                             fatal_signal_pending(current)))
 511                                break;
 512                        schedule();
 513                }
 514        }
 515
 516        __set_current_state(TASK_RUNNING);
 517
 518        if (return_to_userland) {
 519                if (signal_pending(current) &&
 520                    !fatal_signal_pending(current)) {
 521                        /*
 522                         * If we got a SIGSTOP or SIGCONT and this is
 523                         * a normal userland page fault, just let
 524                         * userland return so the signal will be
 525                         * handled and gdb debugging works.  The page
 526                         * fault code immediately after we return from
 527                         * this function is going to release the
 528                         * mmap_sem and it's not depending on it
 529                         * (unlike gup would if we were not to return
 530                         * VM_FAULT_RETRY).
 531                         *
 532                         * If a fatal signal is pending we still take
 533                         * the streamlined VM_FAULT_RETRY failure path
 534                         * and there's no need to retake the mmap_sem
 535                         * in such case.
 536                         */
 537                        down_read(&mm->mmap_sem);
 538                        ret = VM_FAULT_NOPAGE;
 539                }
 540        }
 541
 542        /*
 543         * Here we race with the list_del; list_add in
 544         * userfaultfd_ctx_read(), however because we don't ever run
 545         * list_del_init() to refile across the two lists, the prev
 546         * and next pointers will never point to self. list_add also
 547         * would never let any of the two pointers to point to
 548         * self. So list_empty_careful won't risk to see both pointers
 549         * pointing to self at any time during the list refile. The
 550         * only case where list_del_init() is called is the full
 551         * removal in the wake function and there we don't re-list_add
 552         * and it's fine not to block on the spinlock. The uwq on this
 553         * kernel stack can be released after the list_del_init.
 554         */
 555        if (!list_empty_careful(&uwq.wq.entry)) {
 556                spin_lock(&ctx->fault_pending_wqh.lock);
 557                /*
 558                 * No need of list_del_init(), the uwq on the stack
 559                 * will be freed shortly anyway.
 560                 */
 561                list_del(&uwq.wq.entry);
 562                spin_unlock(&ctx->fault_pending_wqh.lock);
 563        }
 564
 565        /*
 566         * ctx may go away after this if the userfault pseudo fd is
 567         * already released.
 568         */
 569        userfaultfd_ctx_put(ctx);
 570
 571out:
 572        return ret;
 573}
 574
 575static void userfaultfd_event_wait_completion(struct userfaultfd_ctx *ctx,
 576                                              struct userfaultfd_wait_queue *ewq)
 577{
 578        struct userfaultfd_ctx *release_new_ctx;
 579
 580        if (WARN_ON_ONCE(current->flags & PF_EXITING))
 581                goto out;
 582
 583        ewq->ctx = ctx;
 584        init_waitqueue_entry(&ewq->wq, current);
 585        release_new_ctx = NULL;
 586
 587        spin_lock(&ctx->event_wqh.lock);
 588        /*
 589         * After the __add_wait_queue the uwq is visible to userland
 590         * through poll/read().
 591         */
 592        __add_wait_queue(&ctx->event_wqh, &ewq->wq);
 593        for (;;) {
 594                set_current_state(TASK_KILLABLE);
 595                if (ewq->msg.event == 0)
 596                        break;
 597                if (READ_ONCE(ctx->released) ||
 598                    fatal_signal_pending(current)) {
 599                        /*
 600                         * &ewq->wq may be queued in fork_event, but
 601                         * __remove_wait_queue ignores the head
 602                         * parameter. It would be a problem if it
 603                         * didn't.
 604                         */
 605                        __remove_wait_queue(&ctx->event_wqh, &ewq->wq);
 606                        if (ewq->msg.event == UFFD_EVENT_FORK) {
 607                                struct userfaultfd_ctx *new;
 608
 609                                new = (struct userfaultfd_ctx *)
 610                                        (unsigned long)
 611                                        ewq->msg.arg.reserved.reserved1;
 612                                release_new_ctx = new;
 613                        }
 614                        break;
 615                }
 616
 617                spin_unlock(&ctx->event_wqh.lock);
 618
 619                wake_up_poll(&ctx->fd_wqh, EPOLLIN);
 620                schedule();
 621
 622                spin_lock(&ctx->event_wqh.lock);
 623        }
 624        __set_current_state(TASK_RUNNING);
 625        spin_unlock(&ctx->event_wqh.lock);
 626
 627        if (release_new_ctx) {
 628                struct vm_area_struct *vma;
 629                struct mm_struct *mm = release_new_ctx->mm;
 630
 631                /* the various vma->vm_userfaultfd_ctx still points to it */
 632                down_write(&mm->mmap_sem);
 633                for (vma = mm->mmap; vma; vma = vma->vm_next)
 634                        if (vma->vm_userfaultfd_ctx.ctx == release_new_ctx) {
 635                                vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
 636                                vma->vm_flags &= ~(VM_UFFD_WP | VM_UFFD_MISSING);
 637                        }
 638                up_write(&mm->mmap_sem);
 639
 640                userfaultfd_ctx_put(release_new_ctx);
 641        }
 642
 643        /*
 644         * ctx may go away after this if the userfault pseudo fd is
 645         * already released.
 646         */
 647out:
 648        WRITE_ONCE(ctx->mmap_changing, false);
 649        userfaultfd_ctx_put(ctx);
 650}
 651
 652static void userfaultfd_event_complete(struct userfaultfd_ctx *ctx,
 653                                       struct userfaultfd_wait_queue *ewq)
 654{
 655        ewq->msg.event = 0;
 656        wake_up_locked(&ctx->event_wqh);
 657        __remove_wait_queue(&ctx->event_wqh, &ewq->wq);
 658}
 659
 660int dup_userfaultfd(struct vm_area_struct *vma, struct list_head *fcs)
 661{
 662        struct userfaultfd_ctx *ctx = NULL, *octx;
 663        struct userfaultfd_fork_ctx *fctx;
 664
 665        octx = vma->vm_userfaultfd_ctx.ctx;
 666        if (!octx || !(octx->features & UFFD_FEATURE_EVENT_FORK)) {
 667                vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
 668                vma->vm_flags &= ~(VM_UFFD_WP | VM_UFFD_MISSING);
 669                return 0;
 670        }
 671
 672        list_for_each_entry(fctx, fcs, list)
 673                if (fctx->orig == octx) {
 674                        ctx = fctx->new;
 675                        break;
 676                }
 677
 678        if (!ctx) {
 679                fctx = kmalloc(sizeof(*fctx), GFP_KERNEL);
 680                if (!fctx)
 681                        return -ENOMEM;
 682
 683                ctx = kmem_cache_alloc(userfaultfd_ctx_cachep, GFP_KERNEL);
 684                if (!ctx) {
 685                        kfree(fctx);
 686                        return -ENOMEM;
 687                }
 688
 689                atomic_set(&ctx->refcount, 1);
 690                ctx->flags = octx->flags;
 691                ctx->state = UFFD_STATE_RUNNING;
 692                ctx->features = octx->features;
 693                ctx->released = false;
 694                ctx->mmap_changing = false;
 695                ctx->mm = vma->vm_mm;
 696                mmgrab(ctx->mm);
 697
 698                userfaultfd_ctx_get(octx);
 699                WRITE_ONCE(octx->mmap_changing, true);
 700                fctx->orig = octx;
 701                fctx->new = ctx;
 702                list_add_tail(&fctx->list, fcs);
 703        }
 704
 705        vma->vm_userfaultfd_ctx.ctx = ctx;
 706        return 0;
 707}
 708
 709static void dup_fctx(struct userfaultfd_fork_ctx *fctx)
 710{
 711        struct userfaultfd_ctx *ctx = fctx->orig;
 712        struct userfaultfd_wait_queue ewq;
 713
 714        msg_init(&ewq.msg);
 715
 716        ewq.msg.event = UFFD_EVENT_FORK;
 717        ewq.msg.arg.reserved.reserved1 = (unsigned long)fctx->new;
 718
 719        userfaultfd_event_wait_completion(ctx, &ewq);
 720}
 721
 722void dup_userfaultfd_complete(struct list_head *fcs)
 723{
 724        struct userfaultfd_fork_ctx *fctx, *n;
 725
 726        list_for_each_entry_safe(fctx, n, fcs, list) {
 727                dup_fctx(fctx);
 728                list_del(&fctx->list);
 729                kfree(fctx);
 730        }
 731}
 732
 733void mremap_userfaultfd_prep(struct vm_area_struct *vma,
 734                             struct vm_userfaultfd_ctx *vm_ctx)
 735{
 736        struct userfaultfd_ctx *ctx;
 737
 738        ctx = vma->vm_userfaultfd_ctx.ctx;
 739        if (ctx && (ctx->features & UFFD_FEATURE_EVENT_REMAP)) {
 740                vm_ctx->ctx = ctx;
 741                userfaultfd_ctx_get(ctx);
 742                WRITE_ONCE(ctx->mmap_changing, true);
 743        }
 744}
 745
 746void mremap_userfaultfd_complete(struct vm_userfaultfd_ctx *vm_ctx,
 747                                 unsigned long from, unsigned long to,
 748                                 unsigned long len)
 749{
 750        struct userfaultfd_ctx *ctx = vm_ctx->ctx;
 751        struct userfaultfd_wait_queue ewq;
 752
 753        if (!ctx)
 754                return;
 755
 756        if (to & ~PAGE_MASK) {
 757                userfaultfd_ctx_put(ctx);
 758                return;
 759        }
 760
 761        msg_init(&ewq.msg);
 762
 763        ewq.msg.event = UFFD_EVENT_REMAP;
 764        ewq.msg.arg.remap.from = from;
 765        ewq.msg.arg.remap.to = to;
 766        ewq.msg.arg.remap.len = len;
 767
 768        userfaultfd_event_wait_completion(ctx, &ewq);
 769}
 770
 771bool userfaultfd_remove(struct vm_area_struct *vma,
 772                        unsigned long start, unsigned long end)
 773{
 774        struct mm_struct *mm = vma->vm_mm;
 775        struct userfaultfd_ctx *ctx;
 776        struct userfaultfd_wait_queue ewq;
 777
 778        ctx = vma->vm_userfaultfd_ctx.ctx;
 779        if (!ctx || !(ctx->features & UFFD_FEATURE_EVENT_REMOVE))
 780                return true;
 781
 782        userfaultfd_ctx_get(ctx);
 783        WRITE_ONCE(ctx->mmap_changing, true);
 784        up_read(&mm->mmap_sem);
 785
 786        msg_init(&ewq.msg);
 787
 788        ewq.msg.event = UFFD_EVENT_REMOVE;
 789        ewq.msg.arg.remove.start = start;
 790        ewq.msg.arg.remove.end = end;
 791
 792        userfaultfd_event_wait_completion(ctx, &ewq);
 793
 794        return false;
 795}
 796
 797static bool has_unmap_ctx(struct userfaultfd_ctx *ctx, struct list_head *unmaps,
 798                          unsigned long start, unsigned long end)
 799{
 800        struct userfaultfd_unmap_ctx *unmap_ctx;
 801
 802        list_for_each_entry(unmap_ctx, unmaps, list)
 803                if (unmap_ctx->ctx == ctx && unmap_ctx->start == start &&
 804                    unmap_ctx->end == end)
 805                        return true;
 806
 807        return false;
 808}
 809
 810int userfaultfd_unmap_prep(struct vm_area_struct *vma,
 811                           unsigned long start, unsigned long end,
 812                           struct list_head *unmaps)
 813{
 814        for ( ; vma && vma->vm_start < end; vma = vma->vm_next) {
 815                struct userfaultfd_unmap_ctx *unmap_ctx;
 816                struct userfaultfd_ctx *ctx = vma->vm_userfaultfd_ctx.ctx;
 817
 818                if (!ctx || !(ctx->features & UFFD_FEATURE_EVENT_UNMAP) ||
 819                    has_unmap_ctx(ctx, unmaps, start, end))
 820                        continue;
 821
 822                unmap_ctx = kzalloc(sizeof(*unmap_ctx), GFP_KERNEL);
 823                if (!unmap_ctx)
 824                        return -ENOMEM;
 825
 826                userfaultfd_ctx_get(ctx);
 827                WRITE_ONCE(ctx->mmap_changing, true);
 828                unmap_ctx->ctx = ctx;
 829                unmap_ctx->start = start;
 830                unmap_ctx->end = end;
 831                list_add_tail(&unmap_ctx->list, unmaps);
 832        }
 833
 834        return 0;
 835}
 836
 837void userfaultfd_unmap_complete(struct mm_struct *mm, struct list_head *uf)
 838{
 839        struct userfaultfd_unmap_ctx *ctx, *n;
 840        struct userfaultfd_wait_queue ewq;
 841
 842        list_for_each_entry_safe(ctx, n, uf, list) {
 843                msg_init(&ewq.msg);
 844
 845                ewq.msg.event = UFFD_EVENT_UNMAP;
 846                ewq.msg.arg.remove.start = ctx->start;
 847                ewq.msg.arg.remove.end = ctx->end;
 848
 849                userfaultfd_event_wait_completion(ctx->ctx, &ewq);
 850
 851                list_del(&ctx->list);
 852                kfree(ctx);
 853        }
 854}
 855
 856static int userfaultfd_release(struct inode *inode, struct file *file)
 857{
 858        struct userfaultfd_ctx *ctx = file->private_data;
 859        struct mm_struct *mm = ctx->mm;
 860        struct vm_area_struct *vma, *prev;
 861        /* len == 0 means wake all */
 862        struct userfaultfd_wake_range range = { .len = 0, };
 863        unsigned long new_flags;
 864
 865        WRITE_ONCE(ctx->released, true);
 866
 867        if (!mmget_not_zero(mm))
 868                goto wakeup;
 869
 870        /*
 871         * Flush page faults out of all CPUs. NOTE: all page faults
 872         * must be retried without returning VM_FAULT_SIGBUS if
 873         * userfaultfd_ctx_get() succeeds but vma->vma_userfault_ctx
 874         * changes while handle_userfault released the mmap_sem. So
 875         * it's critical that released is set to true (above), before
 876         * taking the mmap_sem for writing.
 877         */
 878        down_write(&mm->mmap_sem);
 879        prev = NULL;
 880        for (vma = mm->mmap; vma; vma = vma->vm_next) {
 881                cond_resched();
 882                BUG_ON(!!vma->vm_userfaultfd_ctx.ctx ^
 883                       !!(vma->vm_flags & (VM_UFFD_MISSING | VM_UFFD_WP)));
 884                if (vma->vm_userfaultfd_ctx.ctx != ctx) {
 885                        prev = vma;
 886                        continue;
 887                }
 888                new_flags = vma->vm_flags & ~(VM_UFFD_MISSING | VM_UFFD_WP);
 889                prev = vma_merge(mm, prev, vma->vm_start, vma->vm_end,
 890                                 new_flags, vma->anon_vma,
 891                                 vma->vm_file, vma->vm_pgoff,
 892                                 vma_policy(vma),
 893                                 NULL_VM_UFFD_CTX);
 894                if (prev)
 895                        vma = prev;
 896                else
 897                        prev = vma;
 898                vma->vm_flags = new_flags;
 899                vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
 900        }
 901        up_write(&mm->mmap_sem);
 902        mmput(mm);
 903wakeup:
 904        /*
 905         * After no new page faults can wait on this fault_*wqh, flush
 906         * the last page faults that may have been already waiting on
 907         * the fault_*wqh.
 908         */
 909        spin_lock(&ctx->fault_pending_wqh.lock);
 910        __wake_up_locked_key(&ctx->fault_pending_wqh, TASK_NORMAL, &range);
 911        __wake_up(&ctx->fault_wqh, TASK_NORMAL, 1, &range);
 912        spin_unlock(&ctx->fault_pending_wqh.lock);
 913
 914        /* Flush pending events that may still wait on event_wqh */
 915        wake_up_all(&ctx->event_wqh);
 916
 917        wake_up_poll(&ctx->fd_wqh, EPOLLHUP);
 918        userfaultfd_ctx_put(ctx);
 919        return 0;
 920}
 921
 922/* fault_pending_wqh.lock must be hold by the caller */
 923static inline struct userfaultfd_wait_queue *find_userfault_in(
 924                wait_queue_head_t *wqh)
 925{
 926        wait_queue_entry_t *wq;
 927        struct userfaultfd_wait_queue *uwq;
 928
 929        VM_BUG_ON(!spin_is_locked(&wqh->lock));
 930
 931        uwq = NULL;
 932        if (!waitqueue_active(wqh))
 933                goto out;
 934        /* walk in reverse to provide FIFO behavior to read userfaults */
 935        wq = list_last_entry(&wqh->head, typeof(*wq), entry);
 936        uwq = container_of(wq, struct userfaultfd_wait_queue, wq);
 937out:
 938        return uwq;
 939}
 940
 941static inline struct userfaultfd_wait_queue *find_userfault(
 942                struct userfaultfd_ctx *ctx)
 943{
 944        return find_userfault_in(&ctx->fault_pending_wqh);
 945}
 946
 947static inline struct userfaultfd_wait_queue *find_userfault_evt(
 948                struct userfaultfd_ctx *ctx)
 949{
 950        return find_userfault_in(&ctx->event_wqh);
 951}
 952
 953static __poll_t userfaultfd_poll(struct file *file, poll_table *wait)
 954{
 955        struct userfaultfd_ctx *ctx = file->private_data;
 956        __poll_t ret;
 957
 958        poll_wait(file, &ctx->fd_wqh, wait);
 959
 960        switch (ctx->state) {
 961        case UFFD_STATE_WAIT_API:
 962                return EPOLLERR;
 963        case UFFD_STATE_RUNNING:
 964                /*
 965                 * poll() never guarantees that read won't block.
 966                 * userfaults can be waken before they're read().
 967                 */
 968                if (unlikely(!(file->f_flags & O_NONBLOCK)))
 969                        return EPOLLERR;
 970                /*
 971                 * lockless access to see if there are pending faults
 972                 * __pollwait last action is the add_wait_queue but
 973                 * the spin_unlock would allow the waitqueue_active to
 974                 * pass above the actual list_add inside
 975                 * add_wait_queue critical section. So use a full
 976                 * memory barrier to serialize the list_add write of
 977                 * add_wait_queue() with the waitqueue_active read
 978                 * below.
 979                 */
 980                ret = 0;
 981                smp_mb();
 982                if (waitqueue_active(&ctx->fault_pending_wqh))
 983                        ret = EPOLLIN;
 984                else if (waitqueue_active(&ctx->event_wqh))
 985                        ret = EPOLLIN;
 986
 987                return ret;
 988        default:
 989                WARN_ON_ONCE(1);
 990                return EPOLLERR;
 991        }
 992}
 993
 994static const struct file_operations userfaultfd_fops;
 995
 996static int resolve_userfault_fork(struct userfaultfd_ctx *ctx,
 997                                  struct userfaultfd_ctx *new,
 998                                  struct uffd_msg *msg)
 999{
1000        int fd;
1001
1002        fd = anon_inode_getfd("[userfaultfd]", &userfaultfd_fops, new,
1003                              O_RDWR | (new->flags & UFFD_SHARED_FCNTL_FLAGS));
1004        if (fd < 0)
1005                return fd;
1006
1007        msg->arg.reserved.reserved1 = 0;
1008        msg->arg.fork.ufd = fd;
1009        return 0;
1010}
1011
1012static ssize_t userfaultfd_ctx_read(struct userfaultfd_ctx *ctx, int no_wait,
1013                                    struct uffd_msg *msg)
1014{
1015        ssize_t ret;
1016        DECLARE_WAITQUEUE(wait, current);
1017        struct userfaultfd_wait_queue *uwq;
1018        /*
1019         * Handling fork event requires sleeping operations, so
1020         * we drop the event_wqh lock, then do these ops, then
1021         * lock it back and wake up the waiter. While the lock is
1022         * dropped the ewq may go away so we keep track of it
1023         * carefully.
1024         */
1025        LIST_HEAD(fork_event);
1026        struct userfaultfd_ctx *fork_nctx = NULL;
1027
1028        /* always take the fd_wqh lock before the fault_pending_wqh lock */
1029        spin_lock_irq(&ctx->fd_wqh.lock);
1030        __add_wait_queue(&ctx->fd_wqh, &wait);
1031        for (;;) {
1032                set_current_state(TASK_INTERRUPTIBLE);
1033                spin_lock(&ctx->fault_pending_wqh.lock);
1034                uwq = find_userfault(ctx);
1035                if (uwq) {
1036                        /*
1037                         * Use a seqcount to repeat the lockless check
1038                         * in wake_userfault() to avoid missing
1039                         * wakeups because during the refile both
1040                         * waitqueue could become empty if this is the
1041                         * only userfault.
1042                         */
1043                        write_seqcount_begin(&ctx->refile_seq);
1044
1045                        /*
1046                         * The fault_pending_wqh.lock prevents the uwq
1047                         * to disappear from under us.
1048                         *
1049                         * Refile this userfault from
1050                         * fault_pending_wqh to fault_wqh, it's not
1051                         * pending anymore after we read it.
1052                         *
1053                         * Use list_del() by hand (as
1054                         * userfaultfd_wake_function also uses
1055                         * list_del_init() by hand) to be sure nobody
1056                         * changes __remove_wait_queue() to use
1057                         * list_del_init() in turn breaking the
1058                         * !list_empty_careful() check in
1059                         * handle_userfault(). The uwq->wq.head list
1060                         * must never be empty at any time during the
1061                         * refile, or the waitqueue could disappear
1062                         * from under us. The "wait_queue_head_t"
1063                         * parameter of __remove_wait_queue() is unused
1064                         * anyway.
1065                         */
1066                        list_del(&uwq->wq.entry);
1067                        add_wait_queue(&ctx->fault_wqh, &uwq->wq);
1068
1069                        write_seqcount_end(&ctx->refile_seq);
1070
1071                        /* careful to always initialize msg if ret == 0 */
1072                        *msg = uwq->msg;
1073                        spin_unlock(&ctx->fault_pending_wqh.lock);
1074                        ret = 0;
1075                        break;
1076                }
1077                spin_unlock(&ctx->fault_pending_wqh.lock);
1078
1079                spin_lock(&ctx->event_wqh.lock);
1080                uwq = find_userfault_evt(ctx);
1081                if (uwq) {
1082                        *msg = uwq->msg;
1083
1084                        if (uwq->msg.event == UFFD_EVENT_FORK) {
1085                                fork_nctx = (struct userfaultfd_ctx *)
1086                                        (unsigned long)
1087                                        uwq->msg.arg.reserved.reserved1;
1088                                list_move(&uwq->wq.entry, &fork_event);
1089                                /*
1090                                 * fork_nctx can be freed as soon as
1091                                 * we drop the lock, unless we take a
1092                                 * reference on it.
1093                                 */
1094                                userfaultfd_ctx_get(fork_nctx);
1095                                spin_unlock(&ctx->event_wqh.lock);
1096                                ret = 0;
1097                                break;
1098                        }
1099
1100                        userfaultfd_event_complete(ctx, uwq);
1101                        spin_unlock(&ctx->event_wqh.lock);
1102                        ret = 0;
1103                        break;
1104                }
1105                spin_unlock(&ctx->event_wqh.lock);
1106
1107                if (signal_pending(current)) {
1108                        ret = -ERESTARTSYS;
1109                        break;
1110                }
1111                if (no_wait) {
1112                        ret = -EAGAIN;
1113                        break;
1114                }
1115                spin_unlock_irq(&ctx->fd_wqh.lock);
1116                schedule();
1117                spin_lock_irq(&ctx->fd_wqh.lock);
1118        }
1119        __remove_wait_queue(&ctx->fd_wqh, &wait);
1120        __set_current_state(TASK_RUNNING);
1121        spin_unlock_irq(&ctx->fd_wqh.lock);
1122
1123        if (!ret && msg->event == UFFD_EVENT_FORK) {
1124                ret = resolve_userfault_fork(ctx, fork_nctx, msg);
1125                spin_lock(&ctx->event_wqh.lock);
1126                if (!list_empty(&fork_event)) {
1127                        /*
1128                         * The fork thread didn't abort, so we can
1129                         * drop the temporary refcount.
1130                         */
1131                        userfaultfd_ctx_put(fork_nctx);
1132
1133                        uwq = list_first_entry(&fork_event,
1134                                               typeof(*uwq),
1135                                               wq.entry);
1136                        /*
1137                         * If fork_event list wasn't empty and in turn
1138                         * the event wasn't already released by fork
1139                         * (the event is allocated on fork kernel
1140                         * stack), put the event back to its place in
1141                         * the event_wq. fork_event head will be freed
1142                         * as soon as we return so the event cannot
1143                         * stay queued there no matter the current
1144                         * "ret" value.
1145                         */
1146                        list_del(&uwq->wq.entry);
1147                        __add_wait_queue(&ctx->event_wqh, &uwq->wq);
1148
1149                        /*
1150                         * Leave the event in the waitqueue and report
1151                         * error to userland if we failed to resolve
1152                         * the userfault fork.
1153                         */
1154                        if (likely(!ret))
1155                                userfaultfd_event_complete(ctx, uwq);
1156                } else {
1157                        /*
1158                         * Here the fork thread aborted and the
1159                         * refcount from the fork thread on fork_nctx
1160                         * has already been released. We still hold
1161                         * the reference we took before releasing the
1162                         * lock above. If resolve_userfault_fork
1163                         * failed we've to drop it because the
1164                         * fork_nctx has to be freed in such case. If
1165                         * it succeeded we'll hold it because the new
1166                         * uffd references it.
1167                         */
1168                        if (ret)
1169                                userfaultfd_ctx_put(fork_nctx);
1170                }
1171                spin_unlock(&ctx->event_wqh.lock);
1172        }
1173
1174        return ret;
1175}
1176
1177static ssize_t userfaultfd_read(struct file *file, char __user *buf,
1178                                size_t count, loff_t *ppos)
1179{
1180        struct userfaultfd_ctx *ctx = file->private_data;
1181        ssize_t _ret, ret = 0;
1182        struct uffd_msg msg;
1183        int no_wait = file->f_flags & O_NONBLOCK;
1184
1185        if (ctx->state == UFFD_STATE_WAIT_API)
1186                return -EINVAL;
1187
1188        for (;;) {
1189                if (count < sizeof(msg))
1190                        return ret ? ret : -EINVAL;
1191                _ret = userfaultfd_ctx_read(ctx, no_wait, &msg);
1192                if (_ret < 0)
1193                        return ret ? ret : _ret;
1194                if (copy_to_user((__u64 __user *) buf, &msg, sizeof(msg)))
1195                        return ret ? ret : -EFAULT;
1196                ret += sizeof(msg);
1197                buf += sizeof(msg);
1198                count -= sizeof(msg);
1199                /*
1200                 * Allow to read more than one fault at time but only
1201                 * block if waiting for the very first one.
1202                 */
1203                no_wait = O_NONBLOCK;
1204        }
1205}
1206
1207static void __wake_userfault(struct userfaultfd_ctx *ctx,
1208                             struct userfaultfd_wake_range *range)
1209{
1210        spin_lock(&ctx->fault_pending_wqh.lock);
1211        /* wake all in the range and autoremove */
1212        if (waitqueue_active(&ctx->fault_pending_wqh))
1213                __wake_up_locked_key(&ctx->fault_pending_wqh, TASK_NORMAL,
1214                                     range);
1215        if (waitqueue_active(&ctx->fault_wqh))
1216                __wake_up(&ctx->fault_wqh, TASK_NORMAL, 1, range);
1217        spin_unlock(&ctx->fault_pending_wqh.lock);
1218}
1219
1220static __always_inline void wake_userfault(struct userfaultfd_ctx *ctx,
1221                                           struct userfaultfd_wake_range *range)
1222{
1223        unsigned seq;
1224        bool need_wakeup;
1225
1226        /*
1227         * To be sure waitqueue_active() is not reordered by the CPU
1228         * before the pagetable update, use an explicit SMP memory
1229         * barrier here. PT lock release or up_read(mmap_sem) still
1230         * have release semantics that can allow the
1231         * waitqueue_active() to be reordered before the pte update.
1232         */
1233        smp_mb();
1234
1235        /*
1236         * Use waitqueue_active because it's very frequent to
1237         * change the address space atomically even if there are no
1238         * userfaults yet. So we take the spinlock only when we're
1239         * sure we've userfaults to wake.
1240         */
1241        do {
1242                seq = read_seqcount_begin(&ctx->refile_seq);
1243                need_wakeup = waitqueue_active(&ctx->fault_pending_wqh) ||
1244                        waitqueue_active(&ctx->fault_wqh);
1245                cond_resched();
1246        } while (read_seqcount_retry(&ctx->refile_seq, seq));
1247        if (need_wakeup)
1248                __wake_userfault(ctx, range);
1249}
1250
1251static __always_inline int validate_range(struct mm_struct *mm,
1252                                          __u64 start, __u64 len)
1253{
1254        __u64 task_size = mm->task_size;
1255
1256        if (start & ~PAGE_MASK)
1257                return -EINVAL;
1258        if (len & ~PAGE_MASK)
1259                return -EINVAL;
1260        if (!len)
1261                return -EINVAL;
1262        if (start < mmap_min_addr)
1263                return -EINVAL;
1264        if (start >= task_size)
1265                return -EINVAL;
1266        if (len > task_size - start)
1267                return -EINVAL;
1268        return 0;
1269}
1270
1271static inline bool vma_can_userfault(struct vm_area_struct *vma)
1272{
1273        return vma_is_anonymous(vma) || is_vm_hugetlb_page(vma) ||
1274                vma_is_shmem(vma);
1275}
1276
1277static int userfaultfd_register(struct userfaultfd_ctx *ctx,
1278                                unsigned long arg)
1279{
1280        struct mm_struct *mm = ctx->mm;
1281        struct vm_area_struct *vma, *prev, *cur;
1282        int ret;
1283        struct uffdio_register uffdio_register;
1284        struct uffdio_register __user *user_uffdio_register;
1285        unsigned long vm_flags, new_flags;
1286        bool found;
1287        bool basic_ioctls;
1288        unsigned long start, end, vma_end;
1289
1290        user_uffdio_register = (struct uffdio_register __user *) arg;
1291
1292        ret = -EFAULT;
1293        if (copy_from_user(&uffdio_register, user_uffdio_register,
1294                           sizeof(uffdio_register)-sizeof(__u64)))
1295                goto out;
1296
1297        ret = -EINVAL;
1298        if (!uffdio_register.mode)
1299                goto out;
1300        if (uffdio_register.mode & ~(UFFDIO_REGISTER_MODE_MISSING|
1301                                     UFFDIO_REGISTER_MODE_WP))
1302                goto out;
1303        vm_flags = 0;
1304        if (uffdio_register.mode & UFFDIO_REGISTER_MODE_MISSING)
1305                vm_flags |= VM_UFFD_MISSING;
1306        if (uffdio_register.mode & UFFDIO_REGISTER_MODE_WP) {
1307                vm_flags |= VM_UFFD_WP;
1308                /*
1309                 * FIXME: remove the below error constraint by
1310                 * implementing the wprotect tracking mode.
1311                 */
1312                ret = -EINVAL;
1313                goto out;
1314        }
1315
1316        ret = validate_range(mm, uffdio_register.range.start,
1317                             uffdio_register.range.len);
1318        if (ret)
1319                goto out;
1320
1321        start = uffdio_register.range.start;
1322        end = start + uffdio_register.range.len;
1323
1324        ret = -ENOMEM;
1325        if (!mmget_not_zero(mm))
1326                goto out;
1327
1328        down_write(&mm->mmap_sem);
1329        vma = find_vma_prev(mm, start, &prev);
1330        if (!vma)
1331                goto out_unlock;
1332
1333        /* check that there's at least one vma in the range */
1334        ret = -EINVAL;
1335        if (vma->vm_start >= end)
1336                goto out_unlock;
1337
1338        /*
1339         * If the first vma contains huge pages, make sure start address
1340         * is aligned to huge page size.
1341         */
1342        if (is_vm_hugetlb_page(vma)) {
1343                unsigned long vma_hpagesize = vma_kernel_pagesize(vma);
1344
1345                if (start & (vma_hpagesize - 1))
1346                        goto out_unlock;
1347        }
1348
1349        /*
1350         * Search for not compatible vmas.
1351         */
1352        found = false;
1353        basic_ioctls = false;
1354        for (cur = vma; cur && cur->vm_start < end; cur = cur->vm_next) {
1355                cond_resched();
1356
1357                BUG_ON(!!cur->vm_userfaultfd_ctx.ctx ^
1358                       !!(cur->vm_flags & (VM_UFFD_MISSING | VM_UFFD_WP)));
1359
1360                /* check not compatible vmas */
1361                ret = -EINVAL;
1362                if (!vma_can_userfault(cur))
1363                        goto out_unlock;
1364
1365                /*
1366                 * UFFDIO_COPY will fill file holes even without
1367                 * PROT_WRITE. This check enforces that if this is a
1368                 * MAP_SHARED, the process has write permission to the backing
1369                 * file. If VM_MAYWRITE is set it also enforces that on a
1370                 * MAP_SHARED vma: there is no F_WRITE_SEAL and no further
1371                 * F_WRITE_SEAL can be taken until the vma is destroyed.
1372                 */
1373                ret = -EPERM;
1374                if (unlikely(!(cur->vm_flags & VM_MAYWRITE)))
1375                        goto out_unlock;
1376
1377                /*
1378                 * If this vma contains ending address, and huge pages
1379                 * check alignment.
1380                 */
1381                if (is_vm_hugetlb_page(cur) && end <= cur->vm_end &&
1382                    end > cur->vm_start) {
1383                        unsigned long vma_hpagesize = vma_kernel_pagesize(cur);
1384
1385                        ret = -EINVAL;
1386
1387                        if (end & (vma_hpagesize - 1))
1388                                goto out_unlock;
1389                }
1390
1391                /*
1392                 * Check that this vma isn't already owned by a
1393                 * different userfaultfd. We can't allow more than one
1394                 * userfaultfd to own a single vma simultaneously or we
1395                 * wouldn't know which one to deliver the userfaults to.
1396                 */
1397                ret = -EBUSY;
1398                if (cur->vm_userfaultfd_ctx.ctx &&
1399                    cur->vm_userfaultfd_ctx.ctx != ctx)
1400                        goto out_unlock;
1401
1402                /*
1403                 * Note vmas containing huge pages
1404                 */
1405                if (is_vm_hugetlb_page(cur))
1406                        basic_ioctls = true;
1407
1408                found = true;
1409        }
1410        BUG_ON(!found);
1411
1412        if (vma->vm_start < start)
1413                prev = vma;
1414
1415        ret = 0;
1416        do {
1417                cond_resched();
1418
1419                BUG_ON(!vma_can_userfault(vma));
1420                BUG_ON(vma->vm_userfaultfd_ctx.ctx &&
1421                       vma->vm_userfaultfd_ctx.ctx != ctx);
1422                WARN_ON(!(vma->vm_flags & VM_MAYWRITE));
1423
1424                /*
1425                 * Nothing to do: this vma is already registered into this
1426                 * userfaultfd and with the right tracking mode too.
1427                 */
1428                if (vma->vm_userfaultfd_ctx.ctx == ctx &&
1429                    (vma->vm_flags & vm_flags) == vm_flags)
1430                        goto skip;
1431
1432                if (vma->vm_start > start)
1433                        start = vma->vm_start;
1434                vma_end = min(end, vma->vm_end);
1435
1436                new_flags = (vma->vm_flags & ~vm_flags) | vm_flags;
1437                prev = vma_merge(mm, prev, start, vma_end, new_flags,
1438                                 vma->anon_vma, vma->vm_file, vma->vm_pgoff,
1439                                 vma_policy(vma),
1440                                 ((struct vm_userfaultfd_ctx){ ctx }));
1441                if (prev) {
1442                        vma = prev;
1443                        goto next;
1444                }
1445                if (vma->vm_start < start) {
1446                        ret = split_vma(mm, vma, start, 1);
1447                        if (ret)
1448                                break;
1449                }
1450                if (vma->vm_end > end) {
1451                        ret = split_vma(mm, vma, end, 0);
1452                        if (ret)
1453                                break;
1454                }
1455        next:
1456                /*
1457                 * In the vma_merge() successful mprotect-like case 8:
1458                 * the next vma was merged into the current one and
1459                 * the current one has not been updated yet.
1460                 */
1461                vma->vm_flags = new_flags;
1462                vma->vm_userfaultfd_ctx.ctx = ctx;
1463
1464        skip:
1465                prev = vma;
1466                start = vma->vm_end;
1467                vma = vma->vm_next;
1468        } while (vma && vma->vm_start < end);
1469out_unlock:
1470        up_write(&mm->mmap_sem);
1471        mmput(mm);
1472        if (!ret) {
1473                /*
1474                 * Now that we scanned all vmas we can already tell
1475                 * userland which ioctls methods are guaranteed to
1476                 * succeed on this range.
1477                 */
1478                if (put_user(basic_ioctls ? UFFD_API_RANGE_IOCTLS_BASIC :
1479                             UFFD_API_RANGE_IOCTLS,
1480                             &user_uffdio_register->ioctls))
1481                        ret = -EFAULT;
1482        }
1483out:
1484        return ret;
1485}
1486
1487static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
1488                                  unsigned long arg)
1489{
1490        struct mm_struct *mm = ctx->mm;
1491        struct vm_area_struct *vma, *prev, *cur;
1492        int ret;
1493        struct uffdio_range uffdio_unregister;
1494        unsigned long new_flags;
1495        bool found;
1496        unsigned long start, end, vma_end;
1497        const void __user *buf = (void __user *)arg;
1498
1499        ret = -EFAULT;
1500        if (copy_from_user(&uffdio_unregister, buf, sizeof(uffdio_unregister)))
1501                goto out;
1502
1503        ret = validate_range(mm, uffdio_unregister.start,
1504                             uffdio_unregister.len);
1505        if (ret)
1506                goto out;
1507
1508        start = uffdio_unregister.start;
1509        end = start + uffdio_unregister.len;
1510
1511        ret = -ENOMEM;
1512        if (!mmget_not_zero(mm))
1513                goto out;
1514
1515        down_write(&mm->mmap_sem);
1516        vma = find_vma_prev(mm, start, &prev);
1517        if (!vma)
1518                goto out_unlock;
1519
1520        /* check that there's at least one vma in the range */
1521        ret = -EINVAL;
1522        if (vma->vm_start >= end)
1523                goto out_unlock;
1524
1525        /*
1526         * If the first vma contains huge pages, make sure start address
1527         * is aligned to huge page size.
1528         */
1529        if (is_vm_hugetlb_page(vma)) {
1530                unsigned long vma_hpagesize = vma_kernel_pagesize(vma);
1531
1532                if (start & (vma_hpagesize - 1))
1533                        goto out_unlock;
1534        }
1535
1536        /*
1537         * Search for not compatible vmas.
1538         */
1539        found = false;
1540        ret = -EINVAL;
1541        for (cur = vma; cur && cur->vm_start < end; cur = cur->vm_next) {
1542                cond_resched();
1543
1544                BUG_ON(!!cur->vm_userfaultfd_ctx.ctx ^
1545                       !!(cur->vm_flags & (VM_UFFD_MISSING | VM_UFFD_WP)));
1546
1547                /*
1548                 * Check not compatible vmas, not strictly required
1549                 * here as not compatible vmas cannot have an
1550                 * userfaultfd_ctx registered on them, but this
1551                 * provides for more strict behavior to notice
1552                 * unregistration errors.
1553                 */
1554                if (!vma_can_userfault(cur))
1555                        goto out_unlock;
1556
1557                found = true;
1558        }
1559        BUG_ON(!found);
1560
1561        if (vma->vm_start < start)
1562                prev = vma;
1563
1564        ret = 0;
1565        do {
1566                cond_resched();
1567
1568                BUG_ON(!vma_can_userfault(vma));
1569
1570                /*
1571                 * Nothing to do: this vma is already registered into this
1572                 * userfaultfd and with the right tracking mode too.
1573                 */
1574                if (!vma->vm_userfaultfd_ctx.ctx)
1575                        goto skip;
1576
1577                WARN_ON(!(vma->vm_flags & VM_MAYWRITE));
1578
1579                if (vma->vm_start > start)
1580                        start = vma->vm_start;
1581                vma_end = min(end, vma->vm_end);
1582
1583                if (userfaultfd_missing(vma)) {
1584                        /*
1585                         * Wake any concurrent pending userfault while
1586                         * we unregister, so they will not hang
1587                         * permanently and it avoids userland to call
1588                         * UFFDIO_WAKE explicitly.
1589                         */
1590                        struct userfaultfd_wake_range range;
1591                        range.start = start;
1592                        range.len = vma_end - start;
1593                        wake_userfault(vma->vm_userfaultfd_ctx.ctx, &range);
1594                }
1595
1596                new_flags = vma->vm_flags & ~(VM_UFFD_MISSING | VM_UFFD_WP);
1597                prev = vma_merge(mm, prev, start, vma_end, new_flags,
1598                                 vma->anon_vma, vma->vm_file, vma->vm_pgoff,
1599                                 vma_policy(vma),
1600                                 NULL_VM_UFFD_CTX);
1601                if (prev) {
1602                        vma = prev;
1603                        goto next;
1604                }
1605                if (vma->vm_start < start) {
1606                        ret = split_vma(mm, vma, start, 1);
1607                        if (ret)
1608                                break;
1609                }
1610                if (vma->vm_end > end) {
1611                        ret = split_vma(mm, vma, end, 0);
1612                        if (ret)
1613                                break;
1614                }
1615        next:
1616                /*
1617                 * In the vma_merge() successful mprotect-like case 8:
1618                 * the next vma was merged into the current one and
1619                 * the current one has not been updated yet.
1620                 */
1621                vma->vm_flags = new_flags;
1622                vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
1623
1624        skip:
1625                prev = vma;
1626                start = vma->vm_end;
1627                vma = vma->vm_next;
1628        } while (vma && vma->vm_start < end);
1629out_unlock:
1630        up_write(&mm->mmap_sem);
1631        mmput(mm);
1632out:
1633        return ret;
1634}
1635
1636/*
1637 * userfaultfd_wake may be used in combination with the
1638 * UFFDIO_*_MODE_DONTWAKE to wakeup userfaults in batches.
1639 */
1640static int userfaultfd_wake(struct userfaultfd_ctx *ctx,
1641                            unsigned long arg)
1642{
1643        int ret;
1644        struct uffdio_range uffdio_wake;
1645        struct userfaultfd_wake_range range;
1646        const void __user *buf = (void __user *)arg;
1647
1648        ret = -EFAULT;
1649        if (copy_from_user(&uffdio_wake, buf, sizeof(uffdio_wake)))
1650                goto out;
1651
1652        ret = validate_range(ctx->mm, uffdio_wake.start, uffdio_wake.len);
1653        if (ret)
1654                goto out;
1655
1656        range.start = uffdio_wake.start;
1657        range.len = uffdio_wake.len;
1658
1659        /*
1660         * len == 0 means wake all and we don't want to wake all here,
1661         * so check it again to be sure.
1662         */
1663        VM_BUG_ON(!range.len);
1664
1665        wake_userfault(ctx, &range);
1666        ret = 0;
1667
1668out:
1669        return ret;
1670}
1671
1672static int userfaultfd_copy(struct userfaultfd_ctx *ctx,
1673                            unsigned long arg)
1674{
1675        __s64 ret;
1676        struct uffdio_copy uffdio_copy;
1677        struct uffdio_copy __user *user_uffdio_copy;
1678        struct userfaultfd_wake_range range;
1679
1680        user_uffdio_copy = (struct uffdio_copy __user *) arg;
1681
1682        ret = -EAGAIN;
1683        if (READ_ONCE(ctx->mmap_changing))
1684                goto out;
1685
1686        ret = -EFAULT;
1687        if (copy_from_user(&uffdio_copy, user_uffdio_copy,
1688                           /* don't copy "copy" last field */
1689                           sizeof(uffdio_copy)-sizeof(__s64)))
1690                goto out;
1691
1692        ret = validate_range(ctx->mm, uffdio_copy.dst, uffdio_copy.len);
1693        if (ret)
1694                goto out;
1695        /*
1696         * double check for wraparound just in case. copy_from_user()
1697         * will later check uffdio_copy.src + uffdio_copy.len to fit
1698         * in the userland range.
1699         */
1700        ret = -EINVAL;
1701        if (uffdio_copy.src + uffdio_copy.len <= uffdio_copy.src)
1702                goto out;
1703        if (uffdio_copy.mode & ~UFFDIO_COPY_MODE_DONTWAKE)
1704                goto out;
1705        if (mmget_not_zero(ctx->mm)) {
1706                ret = mcopy_atomic(ctx->mm, uffdio_copy.dst, uffdio_copy.src,
1707                                   uffdio_copy.len, &ctx->mmap_changing);
1708                mmput(ctx->mm);
1709        } else {
1710                return -ESRCH;
1711        }
1712        if (unlikely(put_user(ret, &user_uffdio_copy->copy)))
1713                return -EFAULT;
1714        if (ret < 0)
1715                goto out;
1716        BUG_ON(!ret);
1717        /* len == 0 would wake all */
1718        range.len = ret;
1719        if (!(uffdio_copy.mode & UFFDIO_COPY_MODE_DONTWAKE)) {
1720                range.start = uffdio_copy.dst;
1721                wake_userfault(ctx, &range);
1722        }
1723        ret = range.len == uffdio_copy.len ? 0 : -EAGAIN;
1724out:
1725        return ret;
1726}
1727
1728static int userfaultfd_zeropage(struct userfaultfd_ctx *ctx,
1729                                unsigned long arg)
1730{
1731        __s64 ret;
1732        struct uffdio_zeropage uffdio_zeropage;
1733        struct uffdio_zeropage __user *user_uffdio_zeropage;
1734        struct userfaultfd_wake_range range;
1735
1736        user_uffdio_zeropage = (struct uffdio_zeropage __user *) arg;
1737
1738        ret = -EAGAIN;
1739        if (READ_ONCE(ctx->mmap_changing))
1740                goto out;
1741
1742        ret = -EFAULT;
1743        if (copy_from_user(&uffdio_zeropage, user_uffdio_zeropage,
1744                           /* don't copy "zeropage" last field */
1745                           sizeof(uffdio_zeropage)-sizeof(__s64)))
1746                goto out;
1747
1748        ret = validate_range(ctx->mm, uffdio_zeropage.range.start,
1749                             uffdio_zeropage.range.len);
1750        if (ret)
1751                goto out;
1752        ret = -EINVAL;
1753        if (uffdio_zeropage.mode & ~UFFDIO_ZEROPAGE_MODE_DONTWAKE)
1754                goto out;
1755
1756        if (mmget_not_zero(ctx->mm)) {
1757                ret = mfill_zeropage(ctx->mm, uffdio_zeropage.range.start,
1758                                     uffdio_zeropage.range.len,
1759                                     &ctx->mmap_changing);
1760                mmput(ctx->mm);
1761        } else {
1762                return -ESRCH;
1763        }
1764        if (unlikely(put_user(ret, &user_uffdio_zeropage->zeropage)))
1765                return -EFAULT;
1766        if (ret < 0)
1767                goto out;
1768        /* len == 0 would wake all */
1769        BUG_ON(!ret);
1770        range.len = ret;
1771        if (!(uffdio_zeropage.mode & UFFDIO_ZEROPAGE_MODE_DONTWAKE)) {
1772                range.start = uffdio_zeropage.range.start;
1773                wake_userfault(ctx, &range);
1774        }
1775        ret = range.len == uffdio_zeropage.range.len ? 0 : -EAGAIN;
1776out:
1777        return ret;
1778}
1779
1780static inline unsigned int uffd_ctx_features(__u64 user_features)
1781{
1782        /*
1783         * For the current set of features the bits just coincide
1784         */
1785        return (unsigned int)user_features;
1786}
1787
1788/*
1789 * userland asks for a certain API version and we return which bits
1790 * and ioctl commands are implemented in this kernel for such API
1791 * version or -EINVAL if unknown.
1792 */
1793static int userfaultfd_api(struct userfaultfd_ctx *ctx,
1794                           unsigned long arg)
1795{
1796        struct uffdio_api uffdio_api;
1797        void __user *buf = (void __user *)arg;
1798        int ret;
1799        __u64 features;
1800
1801        ret = -EINVAL;
1802        if (ctx->state != UFFD_STATE_WAIT_API)
1803                goto out;
1804        ret = -EFAULT;
1805        if (copy_from_user(&uffdio_api, buf, sizeof(uffdio_api)))
1806                goto out;
1807        features = uffdio_api.features;
1808        if (uffdio_api.api != UFFD_API || (features & ~UFFD_API_FEATURES)) {
1809                memset(&uffdio_api, 0, sizeof(uffdio_api));
1810                if (copy_to_user(buf, &uffdio_api, sizeof(uffdio_api)))
1811                        goto out;
1812                ret = -EINVAL;
1813                goto out;
1814        }
1815        /* report all available features and ioctls to userland */
1816        uffdio_api.features = UFFD_API_FEATURES;
1817        uffdio_api.ioctls = UFFD_API_IOCTLS;
1818        ret = -EFAULT;
1819        if (copy_to_user(buf, &uffdio_api, sizeof(uffdio_api)))
1820                goto out;
1821        ctx->state = UFFD_STATE_RUNNING;
1822        /* only enable the requested features for this uffd context */
1823        ctx->features = uffd_ctx_features(features);
1824        ret = 0;
1825out:
1826        return ret;
1827}
1828
1829static long userfaultfd_ioctl(struct file *file, unsigned cmd,
1830                              unsigned long arg)
1831{
1832        int ret = -EINVAL;
1833        struct userfaultfd_ctx *ctx = file->private_data;
1834
1835        if (cmd != UFFDIO_API && ctx->state == UFFD_STATE_WAIT_API)
1836                return -EINVAL;
1837
1838        switch(cmd) {
1839        case UFFDIO_API:
1840                ret = userfaultfd_api(ctx, arg);
1841                break;
1842        case UFFDIO_REGISTER:
1843                ret = userfaultfd_register(ctx, arg);
1844                break;
1845        case UFFDIO_UNREGISTER:
1846                ret = userfaultfd_unregister(ctx, arg);
1847                break;
1848        case UFFDIO_WAKE:
1849                ret = userfaultfd_wake(ctx, arg);
1850                break;
1851        case UFFDIO_COPY:
1852                ret = userfaultfd_copy(ctx, arg);
1853                break;
1854        case UFFDIO_ZEROPAGE:
1855                ret = userfaultfd_zeropage(ctx, arg);
1856                break;
1857        }
1858        return ret;
1859}
1860
1861#ifdef CONFIG_PROC_FS
1862static void userfaultfd_show_fdinfo(struct seq_file *m, struct file *f)
1863{
1864        struct userfaultfd_ctx *ctx = f->private_data;
1865        wait_queue_entry_t *wq;
1866        unsigned long pending = 0, total = 0;
1867
1868        spin_lock(&ctx->fault_pending_wqh.lock);
1869        list_for_each_entry(wq, &ctx->fault_pending_wqh.head, entry) {
1870                pending++;
1871                total++;
1872        }
1873        list_for_each_entry(wq, &ctx->fault_wqh.head, entry) {
1874                total++;
1875        }
1876        spin_unlock(&ctx->fault_pending_wqh.lock);
1877
1878        /*
1879         * If more protocols will be added, there will be all shown
1880         * separated by a space. Like this:
1881         *      protocols: aa:... bb:...
1882         */
1883        seq_printf(m, "pending:\t%lu\ntotal:\t%lu\nAPI:\t%Lx:%x:%Lx\n",
1884                   pending, total, UFFD_API, ctx->features,
1885                   UFFD_API_IOCTLS|UFFD_API_RANGE_IOCTLS);
1886}
1887#endif
1888
1889static const struct file_operations userfaultfd_fops = {
1890#ifdef CONFIG_PROC_FS
1891        .show_fdinfo    = userfaultfd_show_fdinfo,
1892#endif
1893        .release        = userfaultfd_release,
1894        .poll           = userfaultfd_poll,
1895        .read           = userfaultfd_read,
1896        .unlocked_ioctl = userfaultfd_ioctl,
1897        .compat_ioctl   = userfaultfd_ioctl,
1898        .llseek         = noop_llseek,
1899};
1900
1901static void init_once_userfaultfd_ctx(void *mem)
1902{
1903        struct userfaultfd_ctx *ctx = (struct userfaultfd_ctx *) mem;
1904
1905        init_waitqueue_head(&ctx->fault_pending_wqh);
1906        init_waitqueue_head(&ctx->fault_wqh);
1907        init_waitqueue_head(&ctx->event_wqh);
1908        init_waitqueue_head(&ctx->fd_wqh);
1909        seqcount_init(&ctx->refile_seq);
1910}
1911
1912SYSCALL_DEFINE1(userfaultfd, int, flags)
1913{
1914        struct userfaultfd_ctx *ctx;
1915        int fd;
1916
1917        BUG_ON(!current->mm);
1918
1919        /* Check the UFFD_* constants for consistency.  */
1920        BUILD_BUG_ON(UFFD_CLOEXEC != O_CLOEXEC);
1921        BUILD_BUG_ON(UFFD_NONBLOCK != O_NONBLOCK);
1922
1923        if (flags & ~UFFD_SHARED_FCNTL_FLAGS)
1924                return -EINVAL;
1925
1926        ctx = kmem_cache_alloc(userfaultfd_ctx_cachep, GFP_KERNEL);
1927        if (!ctx)
1928                return -ENOMEM;
1929
1930        atomic_set(&ctx->refcount, 1);
1931        ctx->flags = flags;
1932        ctx->features = 0;
1933        ctx->state = UFFD_STATE_WAIT_API;
1934        ctx->released = false;
1935        ctx->mmap_changing = false;
1936        ctx->mm = current->mm;
1937        /* prevent the mm struct to be freed */
1938        mmgrab(ctx->mm);
1939
1940        fd = anon_inode_getfd("[userfaultfd]", &userfaultfd_fops, ctx,
1941                              O_RDWR | (flags & UFFD_SHARED_FCNTL_FLAGS));
1942        if (fd < 0) {
1943                mmdrop(ctx->mm);
1944                kmem_cache_free(userfaultfd_ctx_cachep, ctx);
1945        }
1946        return fd;
1947}
1948
1949static int __init userfaultfd_init(void)
1950{
1951        userfaultfd_ctx_cachep = kmem_cache_create("userfaultfd_ctx_cache",
1952                                                sizeof(struct userfaultfd_ctx),
1953                                                0,
1954                                                SLAB_HWCACHE_ALIGN|SLAB_PANIC,
1955                                                init_once_userfaultfd_ctx);
1956        return 0;
1957}
1958__initcall(userfaultfd_init);
1959