// SPDX-License-Identifier: GPL-2.0
/* Support for MMIO probes.
 * Benefit many code from kprobes
 * (C) 2002 Louis Zhuang <louis.zhuang@intel.com>.
 *     2007 Alexander Eichner
 *     2008 Pekka Paalanen <pq@iki.fi>
 */

#define pr_fmt(fmt) KBUILD_MODNAME ": " fmt

#include <linux/list.h>
#include <linux/rculist.h>
#include <linux/spinlock.h>
#include <linux/hash.h>
#include <linux/export.h>
#include <linux/kernel.h>
#include <linux/uaccess.h>
#include <linux/ptrace.h>
#include <linux/preempt.h>
#include <linux/percpu.h>
#include <linux/kdebug.h>
#include <linux/mutex.h>
#include <linux/io.h>
#include <linux/slab.h>
#include <asm/cacheflush.h>
#include <asm/tlbflush.h>
#include <linux/errno.h>
#include <asm/debugreg.h>
#include <linux/mmiotrace.h>

#define KMMIO_PAGE_HASH_BITS 4
#define KMMIO_PAGE_TABLE_SIZE (1 << KMMIO_PAGE_HASH_BITS)

struct kmmio_fault_page {
        struct list_head list;
        struct kmmio_fault_page *release_next;
        unsigned long addr; /* the requested address */
        pteval_t old_presence; /* page presence prior to arming */
        bool armed;

        /*
         * Number of times this page has been registered as a part
         * of a probe. If zero, page is disarmed and this may be freed.
         * Used only by writers (RCU) and post_kmmio_handler().
         * Protected by kmmio_lock, when linked into kmmio_page_table.
         */
        int count;

        bool scheduled_for_release;
};

struct kmmio_delayed_release {
        struct rcu_head rcu;
        struct kmmio_fault_page *release_list;
};

struct kmmio_context {
        struct kmmio_fault_page *fpage;
        struct kmmio_probe *probe;
        unsigned long saved_flags;
        unsigned long addr;
        int active;
};

static DEFINE_SPINLOCK(kmmio_lock);

/* Protected by kmmio_lock */
unsigned int kmmio_count;

/* Read-protected by RCU, write-protected by kmmio_lock. */
static struct list_head kmmio_page_table[KMMIO_PAGE_TABLE_SIZE];
static LIST_HEAD(kmmio_probes);

static struct list_head *kmmio_page_list(unsigned long addr)
{
        unsigned int l;
        pte_t *pte = lookup_address(addr, &l);

        if (!pte)
                return NULL;
        addr &= page_level_mask(l);

        return &kmmio_page_table[hash_long(addr, KMMIO_PAGE_HASH_BITS)];
}

/* Accessed per-cpu */
static DEFINE_PER_CPU(struct kmmio_context, kmmio_ctx);

/*
 * this is basically a dynamic stabbing problem:
 * Could use the existing prio tree code or
 * Possible better implementations:
 * The Interval Skip List: A Data Structure for Finding All Intervals That
 * Overlap a Point (might be simple)
 * Space Efficient Dynamic Stabbing with Fast Queries - Mikkel Thorup
 */
/* Get the kmmio at this addr (if any). You must be holding RCU read lock. */
static struct kmmio_probe *get_kmmio_probe(unsigned long addr)
{
        struct kmmio_probe *p;
        list_for_each_entry_rcu(p, &kmmio_probes, list) {
                if (addr >= p->addr && addr < (p->addr + p->len))
                        return p;
        }
        return NULL;
}

/* You must be holding RCU read lock. */
static struct kmmio_fault_page *get_kmmio_fault_page(unsigned long addr)
{
        struct list_head *head;
        struct kmmio_fault_page *f;
        unsigned int l;
        pte_t *pte = lookup_address(addr, &l);

        if (!pte)
                return NULL;
        addr &= page_level_mask(l);
        head = kmmio_page_list(addr);
        list_for_each_entry_rcu(f, head, list) {
                if (f->addr == addr)
                        return f;
        }
        return NULL;
}

static void clear_pmd_presence(pmd_t *pmd, bool clear, pmdval_t *old)
{
        pmd_t new_pmd;
        pmdval_t v = pmd_val(*pmd);
        if (clear) {
                *old = v;
                new_pmd = pmd_mkinvalid(*pmd);
        } else {
                /* Presume this has been called with clear==true previously */
                new_pmd = __pmd(*old);
        }
        set_pmd(pmd, new_pmd);
}

static void clear_pte_presence(pte_t *pte, bool clear, pteval_t *old)
{
        pteval_t v = pte_val(*pte);
        if (clear) {
                *old = v;
                /* Nothing should care about address */
                pte_clear(&init_mm, 0, pte);
        } else {
                /* Presume this has been called with clear==true previously */
                set_pte_atomic(pte, __pte(*old));
        }
}

static int clear_page_presence(struct kmmio_fault_page *f, bool clear)
{
        unsigned int level;
        pte_t *pte = lookup_address(f->addr, &level);

        if (!pte) {
                pr_err("no pte for addr 0x%08lx\n", f->addr);
                return -1;
        }

        switch (level) {
        case PG_LEVEL_2M:
                clear_pmd_presence((pmd_t *)pte, clear, &f->old_presence);
                break;
        case PG_LEVEL_4K:
                clear_pte_presence(pte, clear, &f->old_presence);
                break;
        default:
                pr_err("unexpected page level 0x%x.\n", level);
                return -1;
        }

        flush_tlb_one_kernel(f->addr);
        return 0;
}

/*
 * Mark the given page as not present. Access to it will trigger a fault.
 *
 * Struct kmmio_fault_page is protected by RCU and kmmio_lock, but the
 * protection is ignored here. RCU read lock is assumed held, so the struct
 * will not disappear unexpectedly. Furthermore, the caller must guarantee,
 * that double arming the same virtual address (page) cannot occur.
 *
 * Double disarming on the other hand is allowed, and may occur when a fault
 * and mmiotrace shutdown happen simultaneously.
 */
static int arm_kmmio_fault_page(struct kmmio_fault_page *f)
{
        int ret;
        WARN_ONCE(f->armed, KERN_ERR pr_fmt("kmmio page already armed.\n"));
        if (f->armed) {
                pr_warn("double-arm: addr 0x%08lx, ref %d, old %d\n",
                        f->addr, f->count, !!f->old_presence);
        }
        ret = clear_page_presence(f, true);
        WARN_ONCE(ret < 0, KERN_ERR pr_fmt("arming at 0x%08lx failed.\n"),
                  f->addr);
        f->armed = true;
        return ret;
}

/** Restore the given page to saved presence state. */
static void disarm_kmmio_fault_page(struct kmmio_fault_page *f)
{
        int ret = clear_page_presence(f, false);
        WARN_ONCE(ret < 0,
                        KERN_ERR "kmmio disarming at 0x%08lx failed.\n", f->addr);
        f->armed = false;
}

/*
 * This is being called from do_page_fault().
 *
 * We may be in an interrupt or a critical section. Also prefecthing may
 * trigger a page fault. We may be in the middle of process switch.
 * We cannot take any locks, because we could be executing especially
 * within a kmmio critical section.
 *
 * Local interrupts are disabled, so preemption cannot happen.
 * Do not enable interrupts, do not sleep, and watch out for other CPUs.
 */
/*
 * Interrupts are disabled on entry as trap3 is an interrupt gate
 * and they remain disabled throughout this function.
 */
int kmmio_handler(struct pt_regs *regs, unsigned long addr)
{
        struct kmmio_context *ctx;
        struct kmmio_fault_page *faultpage;
        int ret = 0; /* default to fault not handled */
        unsigned long page_base = addr;
        unsigned int l;
        pte_t *pte = lookup_address(addr, &l);
        if (!pte)
                return -EINVAL;
        page_base &= page_level_mask(l);

        /*
         * Preemption is now disabled to prevent process switch during
         * single stepping. We can only handle one active kmmio trace
         * per cpu, so ensure that we finish it before something else
         * gets to run. We also hold the RCU read lock over single
         * stepping to avoid looking up the probe and kmmio_fault_page
         * again.
         */
        preempt_disable();
        rcu_read_lock();

        faultpage = get_kmmio_fault_page(page_base);
        if (!faultpage) {
                /*
                 * Either this page fault is not caused by kmmio, or
                 * another CPU just pulled the kmmio probe from under
                 * our feet. The latter case should not be possible.
                 */
                goto no_kmmio;
        }

        ctx = this_cpu_ptr(&kmmio_ctx);
        if (ctx->active) {
                if (page_base == ctx->addr) {
                        /*
                         * A second fault on the same page means some other
                         * condition needs handling by do_page_fault(), the
                         * page really not being present is the most common.
                         */
                        pr_debug("secondary hit for 0x%08lx CPU %d.\n",
                                 addr, smp_processor_id());

                        if (!faultpage->old_presence)
                                pr_info("unexpected secondary hit for address 0x%08lx on CPU %d.\n",
                                        addr, smp_processor_id());
                } else {
                        /*
                         * Prevent overwriting already in-flight context.
                         * This should not happen, let's hope disarming at
                         * least prevents a panic.
                         */
                        pr_emerg("recursive probe hit on CPU %d, for address 0x%08lx. Ignoring.\n",
                                 smp_processor_id(), addr);
                        pr_emerg("previous hit was at 0x%08lx.\n", ctx->addr);
                        disarm_kmmio_fault_page(faultpage);
                }
                goto no_kmmio;
        }
        ctx->active++;

        ctx->fpage = faultpage;
        ctx->probe = get_kmmio_probe(page_base);
        ctx->saved_flags = (regs->flags & (X86_EFLAGS_TF | X86_EFLAGS_IF));
        ctx->addr = page_base;

        if (ctx->probe && ctx->probe->pre_handler)
                ctx->probe->pre_handler(ctx->probe, regs, addr);

        /*
         * Enable single-stepping and disable interrupts for the faulting
         * context. Local interrupts must not get enabled during stepping.
         */
        regs->flags |= X86_EFLAGS_TF;
        regs->flags &= ~X86_EFLAGS_IF;

        /* Now we set present bit in PTE and single step. */
        disarm_kmmio_fault_page(ctx->fpage);

        /*
         * If another cpu accesses the same page while we are stepping,
         * the access will not be caught. It will simply succeed and the
         * only downside is we lose the event. If this becomes a problem,
         * the user should drop to single cpu before tracing.
         */

        return 1; /* fault handled */

no_kmmio:
        rcu_read_unlock();
        preempt_enable_no_resched();
        return ret;
}

/*
 * Interrupts are disabled on entry as trap1 is an interrupt gate
 * and they remain disabled throughout this function.
 * This must always get called as the pair to kmmio_handler().
 */
static int post_kmmio_handler(unsigned long condition, struct pt_regs *regs)
{
        int ret = 0;
        struct kmmio_context *ctx = this_cpu_ptr(&kmmio_ctx);

        if (!ctx->active) {
                /*
                 * debug traps without an active context are due to either
                 * something external causing them (f.e. using a debugger while
                 * mmio tracing enabled), or erroneous behaviour
                 */
                pr_warn("unexpected debug trap on CPU %d.\n", smp_processor_id());
                goto out;
        }

        if (ctx->probe && ctx->probe->post_handler)
                ctx->probe->post_handler(ctx->probe, condition, regs);

        /* Prevent racing against release_kmmio_fault_page(). */
        spin_lock(&kmmio_lock);
        if (ctx->fpage->count)
                arm_kmmio_fault_page(ctx->fpage);
        spin_unlock(&kmmio_lock);

        regs->flags &= ~X86_EFLAGS_TF;
        regs->flags |= ctx->saved_flags;

        /* These were acquired in kmmio_handler(). */
        ctx->active--;
        BUG_ON(ctx->active);
        rcu_read_unlock();
        preempt_enable_no_resched();

        /*
         * if somebody else is singlestepping across a probe point, flags
         * will have TF set, in which case, continue the remaining processing
         * of do_debug, as if this is not a probe hit.
         */
        if (!(regs->flags & X86_EFLAGS_TF))
                ret = 1;
out:
        return ret;
}

/* You must be holding kmmio_lock. */
static int add_kmmio_fault_page(unsigned long addr)
{
        struct kmmio_fault_page *f;

        f = get_kmmio_fault_page(addr);
        if (f) {
                if (!f->count)
                        arm_kmmio_fault_page(f);
                f->count++;
                return 0;
        }

        f = kzalloc(sizeof(*f), GFP_ATOMIC);
        if (!f)
                return -1;

        f->count = 1;
        f->addr = addr;

        if (arm_kmmio_fault_page(f)) {
                kfree(f);
                return -1;
        }

        list_add_rcu(&f->list, kmmio_page_list(f->addr));

        return 0;
}

/* You must be holding kmmio_lock. */
static void release_kmmio_fault_page(unsigned long addr,
                                struct kmmio_fault_page **release_list)
{
        struct kmmio_fault_page *f;

        f = get_kmmio_fault_page(addr);
        if (!f)
                return;

        f->count--;
        BUG_ON(f->count < 0);
        if (!f->count) {
                disarm_kmmio_fault_page(f);
                if (!f->scheduled_for_release) {
                        f->release_next = *release_list;
                        *release_list = f;
                        f->scheduled_for_release = true;
                }
        }
}

/*
 * With page-unaligned ioremaps, one or two armed pages may contain
 * addresses from outside the intended mapping. Events for these addresses
 * are currently silently dropped. The events may result only from programming
 * mistakes by accessing addresses before the beginning or past the end of a
 * mapping.
 */
int register_kmmio_probe(struct kmmio_probe *p)
{
        unsigned long flags;
        int ret = 0;
        unsigned long size = 0;
        unsigned long addr = p->addr & PAGE_MASK;
        const unsigned long size_lim = p->len + (p->addr & ~PAGE_MASK);
        unsigned int l;
        pte_t *pte;

        spin_lock_irqsave(&kmmio_lock, flags);
        if (get_kmmio_probe(addr)) {
                ret = -EEXIST;
                goto out;
        }

        pte = lookup_address(addr, &l);
        if (!pte) {
                ret = -EINVAL;
                goto out;
        }

        kmmio_count++;
        list_add_rcu(&p->list, &kmmio_probes);
        while (size < size_lim) {
                if (add_kmmio_fault_page(addr + size))
                        pr_err("Unable to set page fault.\n");
                size += page_level_size(l);
        }
out:
        spin_unlock_irqrestore(&kmmio_lock, flags);
        /*
         * XXX: What should I do here?
         * Here was a call to global_flush_tlb(), but it does not exist
         * anymore. It seems it's not needed after all.
         */
        return ret;
}
EXPORT_SYMBOL(register_kmmio_probe);

static void rcu_free_kmmio_fault_pages(struct rcu_head *head)
{
        struct kmmio_delayed_release *dr = container_of(
                                                head,
                                                struct kmmio_delayed_release,
                                                rcu);
        struct kmmio_fault_page *f = dr->release_list;
        while (f) {
                struct kmmio_fault_page *next = f->release_next;
                BUG_ON(f->count);
                kfree(f);
                f = next;
        }
        kfree(dr);
}

static void remove_kmmio_fault_pages(struct rcu_head *head)
{
        struct kmmio_delayed_release *dr =
                container_of(head, struct kmmio_delayed_release, rcu);
        struct kmmio_fault_page *f = dr->release_list;
        struct kmmio_fault_page **prevp = &dr->release_list;
        unsigned long flags;

        spin_lock_irqsave(&kmmio_lock, flags);
        while (f) {
                if (!f->count) {
                        list_del_rcu(&f->list);
                        prevp = &f->release_next;
                } else {
                        *prevp = f->release_next;
                        f->release_next = NULL;
                        f->scheduled_for_release = false;
                }
                f = *prevp;
        }
        spin_unlock_irqrestore(&kmmio_lock, flags);

        /* This is the real RCU destroy call. */
        call_rcu(&dr->rcu, rcu_free_kmmio_fault_pages);
}

/*
 * Remove a kmmio probe. You have to synchronize_rcu() before you can be
 * sure that the callbacks will not be called anymore. Only after that
 * you may actually release your struct kmmio_probe.
 *
 * Unregistering a kmmio fault page has three steps:
 * 1. release_kmmio_fault_page()
 *    Disarm the page, wait a grace period to let all faults finish.
 * 2. remove_kmmio_fault_pages()
 *    Remove the pages from kmmio_page_table.
 * 3. rcu_free_kmmio_fault_pages()
 *    Actually free the kmmio_fault_page structs as with RCU.
 */
void unregister_kmmio_probe(struct kmmio_probe *p)
{
        unsigned long flags;
        unsigned long size = 0;
        unsigned long addr = p->addr & PAGE_MASK;
        const unsigned long size_lim = p->len + (p->addr & ~PAGE_MASK);
        struct kmmio_fault_page *release_list = NULL;
        struct kmmio_delayed_release *drelease;
        unsigned int l;
        pte_t *pte;

        pte = lookup_address(addr, &l);
        if (!pte)
                return;

        spin_lock_irqsave(&kmmio_lock, flags);
        while (size < size_lim) {
                release_kmmio_fault_page(addr + size, &release_list);
                size += page_level_size(l);
        }
        list_del_rcu(&p->list);
        kmmio_count--;
        spin_unlock_irqrestore(&kmmio_lock, flags);

        if (!release_list)
                return;

        drelease = kmalloc(sizeof(*drelease), GFP_ATOMIC);
        if (!drelease) {
                pr_crit("leaking kmmio_fault_page objects.\n");
                return;
        }
        drelease->release_list = release_list;

        /*
         * This is not really RCU here. We have just disarmed a set of
         * pages so that they cannot trigger page faults anymore. However,
         * we cannot remove the pages from kmmio_page_table,
         * because a probe hit might be in flight on another CPU. The
         * pages are collected into a list, and they will be removed from
         * kmmio_page_table when it is certain that no probe hit related to
         * these pages can be in flight. RCU grace period sounds like a
         * good choice.
         *
         * If we removed the pages too early, kmmio page fault handler might
         * not find the respective kmmio_fault_page and determine it's not
         * a kmmio fault, when it actually is. This would lead to madness.
         */
        call_rcu(&drelease->rcu, remove_kmmio_fault_pages);
}
EXPORT_SYMBOL(unregister_kmmio_probe);

static int
kmmio_die_notifier(struct notifier_block *nb, unsigned long val, void *args)
{
        struct die_args *arg = args;
        unsigned long* dr6_p = (unsigned long *)ERR_PTR(arg->err);

        if (val == DIE_DEBUG && (*dr6_p & DR_STEP))
                if (post_kmmio_handler(*dr6_p, arg->regs) == 1) {
                        /*
                         * Reset the BS bit in dr6 (pointed by args->err) to
                         * denote completion of processing
                         */
                        *dr6_p &= ~DR_STEP;
                        return NOTIFY_STOP;
                }

        return NOTIFY_DONE;
}

static struct notifier_block nb_die = {
        .notifier_call = kmmio_die_notifier
};

int kmmio_init(void)
{
        int i;

        for (i = 0; i < KMMIO_PAGE_TABLE_SIZE; i++)
                INIT_LIST_HEAD(&kmmio_page_table[i]);

        return register_die_notifier(&nb_die);
}

void kmmio_cleanup(void)
{
        int i;

        unregister_die_notifier(&nb_die);
        for (i = 0; i < KMMIO_PAGE_TABLE_SIZE; i++) {
                WARN_ONCE(!list_empty(&kmmio_page_table[i]),
                        KERN_ERR "kmmio_page_table not empty at cleanup, any further tracing will leak memory.\n");
        }
}