linux/arch/x86/entry/vsyscall/vsyscall_64.c
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   1// SPDX-License-Identifier: GPL-2.0
   2/*
   3 * Copyright (c) 2012-2014 Andy Lutomirski <luto@amacapital.net>
   4 *
   5 * Based on the original implementation which is:
   6 *  Copyright (C) 2001 Andrea Arcangeli <andrea@suse.de> SuSE
   7 *  Copyright 2003 Andi Kleen, SuSE Labs.
   8 *
   9 *  Parts of the original code have been moved to arch/x86/vdso/vma.c
  10 *
  11 * This file implements vsyscall emulation.  vsyscalls are a legacy ABI:
  12 * Userspace can request certain kernel services by calling fixed
  13 * addresses.  This concept is problematic:
  14 *
  15 * - It interferes with ASLR.
  16 * - It's awkward to write code that lives in kernel addresses but is
  17 *   callable by userspace at fixed addresses.
  18 * - The whole concept is impossible for 32-bit compat userspace.
  19 * - UML cannot easily virtualize a vsyscall.
  20 *
  21 * As of mid-2014, I believe that there is no new userspace code that
  22 * will use a vsyscall if the vDSO is present.  I hope that there will
  23 * soon be no new userspace code that will ever use a vsyscall.
  24 *
  25 * The code in this file emulates vsyscalls when notified of a page
  26 * fault to a vsyscall address.
  27 */
  28
  29#include <linux/kernel.h>
  30#include <linux/timer.h>
  31#include <linux/sched/signal.h>
  32#include <linux/mm_types.h>
  33#include <linux/syscalls.h>
  34#include <linux/ratelimit.h>
  35
  36#include <asm/vsyscall.h>
  37#include <asm/unistd.h>
  38#include <asm/fixmap.h>
  39#include <asm/traps.h>
  40#include <asm/paravirt.h>
  41
  42#define CREATE_TRACE_POINTS
  43#include "vsyscall_trace.h"
  44
  45static enum { EMULATE, XONLY, NONE } vsyscall_mode __ro_after_init =
  46#ifdef CONFIG_LEGACY_VSYSCALL_NONE
  47        NONE;
  48#elif defined(CONFIG_LEGACY_VSYSCALL_XONLY)
  49        XONLY;
  50#else
  51        EMULATE;
  52#endif
  53
  54static int __init vsyscall_setup(char *str)
  55{
  56        if (str) {
  57                if (!strcmp("emulate", str))
  58                        vsyscall_mode = EMULATE;
  59                else if (!strcmp("xonly", str))
  60                        vsyscall_mode = XONLY;
  61                else if (!strcmp("none", str))
  62                        vsyscall_mode = NONE;
  63                else
  64                        return -EINVAL;
  65
  66                return 0;
  67        }
  68
  69        return -EINVAL;
  70}
  71early_param("vsyscall", vsyscall_setup);
  72
  73static void warn_bad_vsyscall(const char *level, struct pt_regs *regs,
  74                              const char *message)
  75{
  76        if (!show_unhandled_signals)
  77                return;
  78
  79        printk_ratelimited("%s%s[%d] %s ip:%lx cs:%lx sp:%lx ax:%lx si:%lx di:%lx\n",
  80                           level, current->comm, task_pid_nr(current),
  81                           message, regs->ip, regs->cs,
  82                           regs->sp, regs->ax, regs->si, regs->di);
  83}
  84
  85static int addr_to_vsyscall_nr(unsigned long addr)
  86{
  87        int nr;
  88
  89        if ((addr & ~0xC00UL) != VSYSCALL_ADDR)
  90                return -EINVAL;
  91
  92        nr = (addr & 0xC00UL) >> 10;
  93        if (nr >= 3)
  94                return -EINVAL;
  95
  96        return nr;
  97}
  98
  99static bool write_ok_or_segv(unsigned long ptr, size_t size)
 100{
 101        /*
 102         * XXX: if access_ok, get_user, and put_user handled
 103         * sig_on_uaccess_err, this could go away.
 104         */
 105
 106        if (!access_ok((void __user *)ptr, size)) {
 107                struct thread_struct *thread = &current->thread;
 108
 109                thread->error_code      = X86_PF_USER | X86_PF_WRITE;
 110                thread->cr2             = ptr;
 111                thread->trap_nr         = X86_TRAP_PF;
 112
 113                force_sig_fault(SIGSEGV, SEGV_MAPERR, (void __user *)ptr);
 114                return false;
 115        } else {
 116                return true;
 117        }
 118}
 119
 120bool emulate_vsyscall(unsigned long error_code,
 121                      struct pt_regs *regs, unsigned long address)
 122{
 123        struct task_struct *tsk;
 124        unsigned long caller;
 125        int vsyscall_nr, syscall_nr, tmp;
 126        int prev_sig_on_uaccess_err;
 127        long ret;
 128        unsigned long orig_dx;
 129
 130        /* Write faults or kernel-privilege faults never get fixed up. */
 131        if ((error_code & (X86_PF_WRITE | X86_PF_USER)) != X86_PF_USER)
 132                return false;
 133
 134        if (!(error_code & X86_PF_INSTR)) {
 135                /* Failed vsyscall read */
 136                if (vsyscall_mode == EMULATE)
 137                        return false;
 138
 139                /*
 140                 * User code tried and failed to read the vsyscall page.
 141                 */
 142                warn_bad_vsyscall(KERN_INFO, regs, "vsyscall read attempt denied -- look up the vsyscall kernel parameter if you need a workaround");
 143                return false;
 144        }
 145
 146        /*
 147         * No point in checking CS -- the only way to get here is a user mode
 148         * trap to a high address, which means that we're in 64-bit user code.
 149         */
 150
 151        WARN_ON_ONCE(address != regs->ip);
 152
 153        if (vsyscall_mode == NONE) {
 154                warn_bad_vsyscall(KERN_INFO, regs,
 155                                  "vsyscall attempted with vsyscall=none");
 156                return false;
 157        }
 158
 159        vsyscall_nr = addr_to_vsyscall_nr(address);
 160
 161        trace_emulate_vsyscall(vsyscall_nr);
 162
 163        if (vsyscall_nr < 0) {
 164                warn_bad_vsyscall(KERN_WARNING, regs,
 165                                  "misaligned vsyscall (exploit attempt or buggy program) -- look up the vsyscall kernel parameter if you need a workaround");
 166                goto sigsegv;
 167        }
 168
 169        if (get_user(caller, (unsigned long __user *)regs->sp) != 0) {
 170                warn_bad_vsyscall(KERN_WARNING, regs,
 171                                  "vsyscall with bad stack (exploit attempt?)");
 172                goto sigsegv;
 173        }
 174
 175        tsk = current;
 176
 177        /*
 178         * Check for access_ok violations and find the syscall nr.
 179         *
 180         * NULL is a valid user pointer (in the access_ok sense) on 32-bit and
 181         * 64-bit, so we don't need to special-case it here.  For all the
 182         * vsyscalls, NULL means "don't write anything" not "write it at
 183         * address 0".
 184         */
 185        switch (vsyscall_nr) {
 186        case 0:
 187                if (!write_ok_or_segv(regs->di, sizeof(struct __kernel_old_timeval)) ||
 188                    !write_ok_or_segv(regs->si, sizeof(struct timezone))) {
 189                        ret = -EFAULT;
 190                        goto check_fault;
 191                }
 192
 193                syscall_nr = __NR_gettimeofday;
 194                break;
 195
 196        case 1:
 197                if (!write_ok_or_segv(regs->di, sizeof(__kernel_old_time_t))) {
 198                        ret = -EFAULT;
 199                        goto check_fault;
 200                }
 201
 202                syscall_nr = __NR_time;
 203                break;
 204
 205        case 2:
 206                if (!write_ok_or_segv(regs->di, sizeof(unsigned)) ||
 207                    !write_ok_or_segv(regs->si, sizeof(unsigned))) {
 208                        ret = -EFAULT;
 209                        goto check_fault;
 210                }
 211
 212                syscall_nr = __NR_getcpu;
 213                break;
 214        }
 215
 216        /*
 217         * Handle seccomp.  regs->ip must be the original value.
 218         * See seccomp_send_sigsys and Documentation/userspace-api/seccomp_filter.rst.
 219         *
 220         * We could optimize the seccomp disabled case, but performance
 221         * here doesn't matter.
 222         */
 223        regs->orig_ax = syscall_nr;
 224        regs->ax = -ENOSYS;
 225        tmp = secure_computing();
 226        if ((!tmp && regs->orig_ax != syscall_nr) || regs->ip != address) {
 227                warn_bad_vsyscall(KERN_DEBUG, regs,
 228                                  "seccomp tried to change syscall nr or ip");
 229                force_exit_sig(SIGSYS);
 230                return true;
 231        }
 232        regs->orig_ax = -1;
 233        if (tmp)
 234                goto do_ret;  /* skip requested */
 235
 236        /*
 237         * With a real vsyscall, page faults cause SIGSEGV.  We want to
 238         * preserve that behavior to make writing exploits harder.
 239         */
 240        prev_sig_on_uaccess_err = current->thread.sig_on_uaccess_err;
 241        current->thread.sig_on_uaccess_err = 1;
 242
 243        ret = -EFAULT;
 244        switch (vsyscall_nr) {
 245        case 0:
 246                /* this decodes regs->di and regs->si on its own */
 247                ret = __x64_sys_gettimeofday(regs);
 248                break;
 249
 250        case 1:
 251                /* this decodes regs->di on its own */
 252                ret = __x64_sys_time(regs);
 253                break;
 254
 255        case 2:
 256                /* while we could clobber regs->dx, we didn't in the past... */
 257                orig_dx = regs->dx;
 258                regs->dx = 0;
 259                /* this decodes regs->di, regs->si and regs->dx on its own */
 260                ret = __x64_sys_getcpu(regs);
 261                regs->dx = orig_dx;
 262                break;
 263        }
 264
 265        current->thread.sig_on_uaccess_err = prev_sig_on_uaccess_err;
 266
 267check_fault:
 268        if (ret == -EFAULT) {
 269                /* Bad news -- userspace fed a bad pointer to a vsyscall. */
 270                warn_bad_vsyscall(KERN_INFO, regs,
 271                                  "vsyscall fault (exploit attempt?)");
 272
 273                /*
 274                 * If we failed to generate a signal for any reason,
 275                 * generate one here.  (This should be impossible.)
 276                 */
 277                if (WARN_ON_ONCE(!sigismember(&tsk->pending.signal, SIGBUS) &&
 278                                 !sigismember(&tsk->pending.signal, SIGSEGV)))
 279                        goto sigsegv;
 280
 281                return true;  /* Don't emulate the ret. */
 282        }
 283
 284        regs->ax = ret;
 285
 286do_ret:
 287        /* Emulate a ret instruction. */
 288        regs->ip = caller;
 289        regs->sp += 8;
 290        return true;
 291
 292sigsegv:
 293        force_sig(SIGSEGV);
 294        return true;
 295}
 296
 297/*
 298 * A pseudo VMA to allow ptrace access for the vsyscall page.  This only
 299 * covers the 64bit vsyscall page now. 32bit has a real VMA now and does
 300 * not need special handling anymore:
 301 */
 302static const char *gate_vma_name(struct vm_area_struct *vma)
 303{
 304        return "[vsyscall]";
 305}
 306static const struct vm_operations_struct gate_vma_ops = {
 307        .name = gate_vma_name,
 308};
 309static struct vm_area_struct gate_vma __ro_after_init = {
 310        .vm_start       = VSYSCALL_ADDR,
 311        .vm_end         = VSYSCALL_ADDR + PAGE_SIZE,
 312        .vm_page_prot   = PAGE_READONLY_EXEC,
 313        .vm_flags       = VM_READ | VM_EXEC,
 314        .vm_ops         = &gate_vma_ops,
 315};
 316
 317struct vm_area_struct *get_gate_vma(struct mm_struct *mm)
 318{
 319#ifdef CONFIG_COMPAT
 320        if (!mm || !(mm->context.flags & MM_CONTEXT_HAS_VSYSCALL))
 321                return NULL;
 322#endif
 323        if (vsyscall_mode == NONE)
 324                return NULL;
 325        return &gate_vma;
 326}
 327
 328int in_gate_area(struct mm_struct *mm, unsigned long addr)
 329{
 330        struct vm_area_struct *vma = get_gate_vma(mm);
 331
 332        if (!vma)
 333                return 0;
 334
 335        return (addr >= vma->vm_start) && (addr < vma->vm_end);
 336}
 337
 338/*
 339 * Use this when you have no reliable mm, typically from interrupt
 340 * context. It is less reliable than using a task's mm and may give
 341 * false positives.
 342 */
 343int in_gate_area_no_mm(unsigned long addr)
 344{
 345        return vsyscall_mode != NONE && (addr & PAGE_MASK) == VSYSCALL_ADDR;
 346}
 347
 348/*
 349 * The VSYSCALL page is the only user-accessible page in the kernel address
 350 * range.  Normally, the kernel page tables can have _PAGE_USER clear, but
 351 * the tables covering VSYSCALL_ADDR need _PAGE_USER set if vsyscalls
 352 * are enabled.
 353 *
 354 * Some day we may create a "minimal" vsyscall mode in which we emulate
 355 * vsyscalls but leave the page not present.  If so, we skip calling
 356 * this.
 357 */
 358void __init set_vsyscall_pgtable_user_bits(pgd_t *root)
 359{
 360        pgd_t *pgd;
 361        p4d_t *p4d;
 362        pud_t *pud;
 363        pmd_t *pmd;
 364
 365        pgd = pgd_offset_pgd(root, VSYSCALL_ADDR);
 366        set_pgd(pgd, __pgd(pgd_val(*pgd) | _PAGE_USER));
 367        p4d = p4d_offset(pgd, VSYSCALL_ADDR);
 368#if CONFIG_PGTABLE_LEVELS >= 5
 369        set_p4d(p4d, __p4d(p4d_val(*p4d) | _PAGE_USER));
 370#endif
 371        pud = pud_offset(p4d, VSYSCALL_ADDR);
 372        set_pud(pud, __pud(pud_val(*pud) | _PAGE_USER));
 373        pmd = pmd_offset(pud, VSYSCALL_ADDR);
 374        set_pmd(pmd, __pmd(pmd_val(*pmd) | _PAGE_USER));
 375}
 376
 377void __init map_vsyscall(void)
 378{
 379        extern char __vsyscall_page;
 380        unsigned long physaddr_vsyscall = __pa_symbol(&__vsyscall_page);
 381
 382        /*
 383         * For full emulation, the page needs to exist for real.  In
 384         * execute-only mode, there is no PTE at all backing the vsyscall
 385         * page.
 386         */
 387        if (vsyscall_mode == EMULATE) {
 388                __set_fixmap(VSYSCALL_PAGE, physaddr_vsyscall,
 389                             PAGE_KERNEL_VVAR);
 390                set_vsyscall_pgtable_user_bits(swapper_pg_dir);
 391        }
 392
 393        if (vsyscall_mode == XONLY)
 394                gate_vma.vm_flags = VM_EXEC;
 395
 396        BUILD_BUG_ON((unsigned long)__fix_to_virt(VSYSCALL_PAGE) !=
 397                     (unsigned long)VSYSCALL_ADDR);
 398}
 399