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SubjectRe: [PATCH RFC 01/10] mm: add Kernel Electric-Fence infrastructure
On Mon, Sep 7, 2020 at 3:41 PM Marco Elver <elver@google.com> wrote:

> + meta->addr = metadata_to_pageaddr(meta);
> + /* Unprotect if we're reusing this page. */
> + if (meta->state == KFENCE_OBJECT_FREED)
> + kfence_unprotect(meta->addr);
> +
> + /* Calculate address for this allocation. */
> + if (right)
> + meta->addr += PAGE_SIZE - size;
> + meta->addr = ALIGN_DOWN(meta->addr, cache->align);

I would move this ALIGN_DOWN under the (right) if.
Do I understand it correctly that it will work, but we expect it to do
nothing for !right? If cache align is >PAGE_SIZE, nothing good will
happen anyway, right?
The previous 2 lines look like part of the same calculation -- "figure
out the addr for the right case".


> + /* Update remaining metadata. */
> + metadata_update_state(meta, KFENCE_OBJECT_ALLOCATED);
> + /* Pairs with READ_ONCE() in kfence_shutdown_cache(). */
> + WRITE_ONCE(meta->cache, cache);
> + meta->size = right ? -size : size;
> + for_each_canary(meta, set_canary_byte);
> + virt_to_page(meta->addr)->slab_cache = cache;
> +
> + raw_spin_unlock_irqrestore(&meta->lock, flags);
> +
> + /* Memory initialization. */
> +
> + /*
> + * We check slab_want_init_on_alloc() ourselves, rather than letting
> + * SL*B do the initialization, as otherwise we might overwrite KFENCE's
> + * redzone.
> + */
> + addr = (void *)meta->addr;
> + if (unlikely(slab_want_init_on_alloc(gfp, cache)))
> + memzero_explicit(addr, size);
> + if (cache->ctor)
> + cache->ctor(addr);
> +
> + if (CONFIG_KFENCE_FAULT_INJECTION && !prandom_u32_max(CONFIG_KFENCE_FAULT_INJECTION))
> + kfence_protect(meta->addr); /* Random "faults" by protecting the object. */
> +
> + atomic_long_inc(&counters[KFENCE_COUNTER_ALLOCATED]);
> + atomic_long_inc(&counters-F[KFENCE_COUNTER_ALLOCS]);
> + return addr;
> +}
> +
> +static void kfence_guarded_free(void *addr, struct kfence_metadata *meta)
> +{
> + struct kcsan_scoped_access assert_page_exclusive;
> + unsigned long flags;
> +
> + raw_spin_lock_irqsave(&meta->lock, flags);
> +
> + if (meta->state != KFENCE_OBJECT_ALLOCATED || meta->addr != (unsigned long)addr) {
> + /* Invalid or double-free, bail out. */
> + atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
> + kfence_report_error((unsigned long)addr, meta, KFENCE_ERROR_INVALID_FREE);
> + raw_spin_unlock_irqrestore(&meta->lock, flags);
> + return;
> + }
> +
> + /* Detect racy use-after-free, or incorrect reallocation of this page by KFENCE. */
> + kcsan_begin_scoped_access((void *)ALIGN_DOWN((unsigned long)addr, PAGE_SIZE), PAGE_SIZE,
> + KCSAN_ACCESS_SCOPED | KCSAN_ACCESS_WRITE | KCSAN_ACCESS_ASSERT,
> + &assert_page_exclusive);
> +
> + if (CONFIG_KFENCE_FAULT_INJECTION)
> + kfence_unprotect((unsigned long)addr); /* To check canary bytes. */
> +
> + /* Restore page protection if there was an OOB access. */
> + if (meta->unprotected_page) {
> + kfence_protect(meta->unprotected_page);
> + meta->unprotected_page = 0;
> + }
> +
> + /* Check canary bytes for memory corruption. */
> + for_each_canary(meta, check_canary_byte);
> +
> + /*
> + * Clear memory if init-on-free is set. While we protect the page, the
> + * data is still there, and after a use-after-free is detected, we
> + * unprotect the page, so the data is still accessible.
> + */
> + if (unlikely(slab_want_init_on_free(meta->cache)))
> + memzero_explicit(addr, abs(meta->size));
> +
> + /* Mark the object as freed. */
> + metadata_update_state(meta, KFENCE_OBJECT_FREED);
> +
> + raw_spin_unlock_irqrestore(&meta->lock, flags);
> +
> + /* Protect to detect use-after-frees. */
> + kfence_protect((unsigned long)addr);
> +
> + /* Add it to the tail of the freelist for reuse. */
> + raw_spin_lock_irqsave(&kfence_freelist_lock, flags);
> + KFENCE_WARN_ON(!list_empty(&meta->list));
> + list_add_tail(&meta->list, &kfence_freelist);
> + kcsan_end_scoped_access(&assert_page_exclusive);
> + raw_spin_unlock_irqrestore(&kfence_freelist_lock, flags);
> +
> + atomic_long_dec(&counters[KFENCE_COUNTER_ALLOCATED]);
> + atomic_long_inc(&counters[KFENCE_COUNTER_FREES]);
> +}
> +
> +static void rcu_guarded_free(struct rcu_head *h)
> +{
> + struct kfence_metadata *meta = container_of(h, struct kfence_metadata, rcu_head);
> +
> + kfence_guarded_free((void *)meta->addr, meta);
> +}
> +
> +static bool __init kfence_initialize_pool(void)
> +{
> + unsigned long addr;
> + struct page *pages;
> + int i;
> +
> + if (!arch_kfence_initialize_pool())
> + return false;
> +
> + addr = (unsigned long)__kfence_pool;
> + pages = virt_to_page(addr);
> +
> + /*
> + * Set up non-redzone pages: they must have PG_slab set, to avoid
> + * freeing these as real pages.
> + *
> + * We also want to avoid inserting kfence_free() in the kfree()
> + * fast-path in SLUB, and therefore need to ensure kfree() correctly
> + * enters __slab_free() slow-path.
> + */
> + for (i = 0; i < KFENCE_POOL_SIZE / PAGE_SIZE; i++) {
> + if (!i || (i % 2))
> + continue;
> +
> + __SetPageSlab(&pages[i]);
> + }
> +
> + /*
> + * Protect the first 2 pages. The first page is mostly unnecessary, and
> + * merely serves as an extended guard page. However, adding one
> + * additional page in the beginning gives us an even number of pages,
> + * which simplifies the mapping of address to metadata index.
> + */
> + for (i = 0; i < 2; i++) {
> + if (unlikely(!kfence_protect(addr)))
> + return false;
> +
> + addr += PAGE_SIZE;
> + }
> +
> + for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
> + struct kfence_metadata *meta = &kfence_metadata[i];
> +
> + /* Initialize metadata. */
> + INIT_LIST_HEAD(&meta->list);
> + raw_spin_lock_init(&meta->lock);
> + meta->state = KFENCE_OBJECT_UNUSED;
> + meta->addr = addr; /* Initialize for validation in metadata_to_pageaddr(). */
> + list_add_tail(&meta->list, &kfence_freelist);
> +
> + /* Protect the right redzone. */
> + if (unlikely(!kfence_protect(addr + PAGE_SIZE)))
> + return false;
> +
> + addr += 2 * PAGE_SIZE;
> + }
> +
> + return true;
> +}
> +
> +/* === DebugFS Interface ==================================================== */
> +
> +static int stats_show(struct seq_file *seq, void *v)
> +{
> + int i;
> +
> + seq_printf(seq, "enabled: %i\n", READ_ONCE(kfence_enabled));
> + for (i = 0; i < KFENCE_COUNTER_COUNT; i++)
> + seq_printf(seq, "%s: %ld\n", counter_names[i], atomic_long_read(&counters[i]));
> +
> + return 0;
> +}
> +DEFINE_SHOW_ATTRIBUTE(stats);
> +
> +/*
> + * debugfs seq_file operations for /sys/kernel/debug/kfence/objects.
> + * start_object() and next_object() return the object index + 1, because NULL is used
> + * to stop iteration.
> + */
> +static void *start_object(struct seq_file *seq, loff_t *pos)
> +{
> + if (*pos < CONFIG_KFENCE_NUM_OBJECTS)
> + return (void *)((long)*pos + 1);
> + return NULL;
> +}
> +
> +static void stop_object(struct seq_file *seq, void *v)
> +{
> +}
> +
> +static void *next_object(struct seq_file *seq, void *v, loff_t *pos)
> +{
> + ++*pos;
> + if (*pos < CONFIG_KFENCE_NUM_OBJECTS)
> + return (void *)((long)*pos + 1);
> + return NULL;
> +}
> +
> +static int show_object(struct seq_file *seq, void *v)
> +{
> + struct kfence_metadata *meta = &kfence_metadata[(long)v - 1];
> + unsigned long flags;
> +
> + raw_spin_lock_irqsave(&meta->lock, flags);
> + kfence_print_object(seq, meta);
> + raw_spin_unlock_irqrestore(&meta->lock, flags);
> + seq_puts(seq, "---------------------------------\n");
> +
> + return 0;
> +}
> +
> +static const struct seq_operations object_seqops = {
> + .start = start_object,
> + .next = next_object,
> + .stop = stop_object,
> + .show = show_object,
> +};
> +
> +static int open_objects(struct inode *inode, struct file *file)
> +{
> + return seq_open(file, &object_seqops);
> +}
> +
> +static const struct file_operations objects_fops = {
> + .open = open_objects,
> + .read = seq_read,
> + .llseek = seq_lseek,
> +};
> +
> +static int __init kfence_debugfs_init(void)
> +{
> + struct dentry *kfence_dir = debugfs_create_dir("kfence", NULL);
> +
> + debugfs_create_file("stats", 0400, kfence_dir, NULL, &stats_fops);
> + debugfs_create_file("objects", 0400, kfence_dir, NULL, &objects_fops);
> + return 0;
> +}
> +
> +late_initcall(kfence_debugfs_init);
> +
> +/* === Allocation Gate Timer ================================================ */
> +
> +/*
> + * Set up delayed work, which will enable and disable the static key. We need to
> + * use a work queue (rather than a simple timer), since enabling and disabling a
> + * static key cannot be done from an interrupt.
> + */
> +static struct delayed_work kfence_timer;
> +static void toggle_allocation_gate(struct work_struct *work)
> +{
> + if (!READ_ONCE(kfence_enabled))
> + return;
> +
> + /* Enable static key, and await allocation to happen. */
> + atomic_set(&allocation_gate, 0);
> + static_branch_enable(&kfence_allocation_key);
> + wait_event(allocation_wait, atomic_read(&allocation_gate) != 0);
> +
> + /* Disable static key and reset timer. */
> + static_branch_disable(&kfence_allocation_key);
> + schedule_delayed_work(&kfence_timer, msecs_to_jiffies(kfence_sample_interval));
> +}
> +static DECLARE_DELAYED_WORK(kfence_timer, toggle_allocation_gate);
> +
> +/* === Public interface ===================================================== */
> +
> +void __init kfence_init(void)
> +{
> + /* Setting kfence_sample_interval to 0 on boot disables KFENCE. */
> + if (!kfence_sample_interval)
> + return;
> +
> + if (!kfence_initialize_pool()) {
> + pr_err("%s failed\n", __func__);
> + return;
> + }
> +
> + schedule_delayed_work(&kfence_timer, 0);
> + WRITE_ONCE(kfence_enabled, true);

Can toggle_allocation_gate run before we set kfence_enabled? If yes,
it can break. If not, it's still somewhat confusing.


> + pr_info("initialized - using %zu bytes for %d objects", KFENCE_POOL_SIZE,
> + CONFIG_KFENCE_NUM_OBJECTS);
> + if (IS_ENABLED(CONFIG_DEBUG_KERNEL))
> + pr_cont(" at 0x%px-0x%px\n", (void *)__kfence_pool,
> + (void *)(__kfence_pool + KFENCE_POOL_SIZE));
> + else
> + pr_cont("\n");
> +}
> +
> +bool kfence_shutdown_cache(struct kmem_cache *s)
> +{
> + unsigned long flags;
> + struct kfence_metadata *meta;
> + int i;
> +
> + for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
> + bool in_use;
> +
> + meta = &kfence_metadata[i];
> +
> + /*
> + * If we observe some inconsistent cache and state pair where we
> + * should have returned false here, cache destruction is racing
> + * with either kmem_cache_alloc() or kmem_cache_free(). Taking
> + * the lock will not help, as different critical section
> + * serialization will have the same outcome.
> + */
> + if (READ_ONCE(meta->cache) != s ||
> + READ_ONCE(meta->state) != KFENCE_OBJECT_ALLOCATED)
> + continue;
> +
> + raw_spin_lock_irqsave(&meta->lock, flags);
> + in_use = meta->cache == s && meta->state == KFENCE_OBJECT_ALLOCATED;
> + raw_spin_unlock_irqrestore(&meta->lock, flags);
> +
> + if (in_use)
> + return false;
> + }
> +
> + for (i = 0; i < CONFIG_KFENCE_NUM_OBJECTS; i++) {
> + meta = &kfence_metadata[i];
> +
> + /* See above. */
> + if (READ_ONCE(meta->cache) != s || READ_ONCE(meta->state) != KFENCE_OBJECT_FREED)
> + continue;
> +
> + raw_spin_lock_irqsave(&meta->lock, flags);
> + if (meta->cache == s && meta->state == KFENCE_OBJECT_FREED)
> + meta->cache = NULL;
> + raw_spin_unlock_irqrestore(&meta->lock, flags);
> + }
> +
> + return true;
> +}
> +
> +void *__kfence_alloc(struct kmem_cache *s, size_t size, gfp_t flags)
> +{
> + /*
> + * allocation_gate only needs to become non-zero, so it doesn't make
> + * sense to continue writing to it and pay the associated contention
> + * cost, in case we have a large number of concurrent allocations.
> + */
> + if (atomic_read(&allocation_gate) || atomic_inc_return(&allocation_gate) > 1)
> + return NULL;
> + wake_up(&allocation_wait);
> +
> + if (!READ_ONCE(kfence_enabled))
> + return NULL;
> +
> + if (size > PAGE_SIZE)
> + return NULL;
> +
> + return kfence_guarded_alloc(s, size, flags);
> +}
> +
> +size_t kfence_ksize(const void *addr)
> +{
> + const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
> +
> + /*
> + * Read locklessly -- if there is a race with __kfence_alloc(), this
> + * most certainly is either a use-after-free, or invalid access.
> + */
> + return meta ? abs(meta->size) : 0;
> +}
> +
> +void *kfence_object_start(const void *addr)
> +{
> + const struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
> +
> + /*
> + * Read locklessly -- if there is a race with __kfence_alloc(), this
> + * most certainly is either a use-after-free, or invalid access.
> + */
> + return meta ? (void *)meta->addr : NULL;
> +}
> +
> +void __kfence_free(void *addr)
> +{
> + struct kfence_metadata *meta = addr_to_metadata((unsigned long)addr);
> +
> + if (unlikely(meta->cache->flags & SLAB_TYPESAFE_BY_RCU))

This may deserve a comment as to why we apply rcu on object level
whereas SLAB_TYPESAFE_BY_RCU means slab level only.

> + call_rcu(&meta->rcu_head, rcu_guarded_free);
> + else
> + kfence_guarded_free(addr, meta);
> +}
> +
> +bool kfence_handle_page_fault(unsigned long addr)
> +{
> + const int page_index = (addr - (unsigned long)__kfence_pool) / PAGE_SIZE;
> + struct kfence_metadata *to_report = NULL;
> + enum kfence_error_type error_type;
> + unsigned long flags;
> +
> + if (!is_kfence_address((void *)addr))
> + return false;
> +
> + if (!READ_ONCE(kfence_enabled)) /* If disabled at runtime ... */
> + return kfence_unprotect(addr); /* ... unprotect and proceed. */
> +
> + atomic_long_inc(&counters[KFENCE_COUNTER_BUGS]);
> +
> + if (page_index % 2) {
> + /* This is a redzone, report a buffer overflow. */
> + struct kfence_metadata *meta = NULL;
> + int distance = 0;
> +
> + meta = addr_to_metadata(addr - PAGE_SIZE);
> + if (meta && READ_ONCE(meta->state) == KFENCE_OBJECT_ALLOCATED) {
> + to_report = meta;
> + /* Data race ok; distance calculation approximate. */
> + distance = addr - data_race(meta->addr + abs(meta->size));
> + }
> +
> + meta = addr_to_metadata(addr + PAGE_SIZE);
> + if (meta && READ_ONCE(meta->state) == KFENCE_OBJECT_ALLOCATED) {
> + /* Data race ok; distance calculation approximate. */
> + if (!to_report || distance > data_race(meta->addr) - addr)
> + to_report = meta;
> + }
> +
> + if (!to_report)
> + goto out;
> +
> + raw_spin_lock_irqsave(&to_report->lock, flags);
> + to_report->unprotected_page = addr;
> + error_type = KFENCE_ERROR_OOB;
> +
> + /*
> + * If the object was freed before we took the look we can still
> + * report this as an OOB -- the report will simply show the
> + * stacktrace of the free as well.
> + */
> + } else {
> + to_report = addr_to_metadata(addr);
> + if (!to_report)
> + goto out;
> +
> + raw_spin_lock_irqsave(&to_report->lock, flags);
> + error_type = KFENCE_ERROR_UAF;
> + /*
> + * We may race with __kfence_alloc(), and it is possible that a
> + * freed object may be reallocated. We simply report this as a
> + * use-after-free, with the stack trace showing the place where
> + * the object was re-allocated.
> + */
> + }
> +
> +out:
> + if (to_report) {
> + kfence_report_error(addr, to_report, error_type);
> + raw_spin_unlock_irqrestore(&to_report->lock, flags);
> + } else {
> + /* This may be a UAF or OOB access, but we can't be sure. */
> + kfence_report_error(addr, NULL, KFENCE_ERROR_INVALID);
> + }
> +
> + return kfence_unprotect(addr); /* Unprotect and let access proceed. */
> +}
> diff --git a/mm/kfence/kfence.h b/mm/kfence/kfence.h
> new file mode 100644
> index 000000000000..25ce2c0dc092
> --- /dev/null
> +++ b/mm/kfence/kfence.h
> @@ -0,0 +1,104 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +
> +#ifndef MM_KFENCE_KFENCE_H
> +#define MM_KFENCE_KFENCE_H
> +
> +#include <linux/mm.h>
> +#include <linux/slab.h>
> +#include <linux/spinlock.h>
> +#include <linux/types.h>
> +
> +#include "../slab.h" /* for struct kmem_cache */
> +
> +/* For non-debug builds, avoid leaking kernel pointers into dmesg. */
> +#ifdef CONFIG_DEBUG_KERNEL
> +#define PTR_FMT "%px"
> +#else
> +#define PTR_FMT "%p"
> +#endif
> +
> +/*
> + * Get the canary byte pattern for @addr. Use a pattern that varies based on the
> + * lower 3 bits of the address, to detect memory corruptions with higher
> + * probability, where similar constants are used.
> + */
> +#define KFENCE_CANARY_PATTERN(addr) ((u8)0xaa ^ (u8)((unsigned long)addr & 0x7))
> +
> +/* Maximum stack depth for reports. */
> +#define KFENCE_STACK_DEPTH 64
> +
> +/* KFENCE object states. */
> +enum kfence_object_state {
> + KFENCE_OBJECT_UNUSED, /* Object is unused. */
> + KFENCE_OBJECT_ALLOCATED, /* Object is currently allocated. */
> + KFENCE_OBJECT_FREED, /* Object was allocated, and then freed. */
> +};
> +
> +/* KFENCE metadata per guarded allocation. */
> +struct kfence_metadata {
> + struct list_head list; /* Freelist node; access under kfence_freelist_lock. */
> + struct rcu_head rcu_head; /* For delayed freeing. */
> +
> + /*
> + * Lock protecting below data; to ensure consistency of the below data,
> + * since the following may execute concurrently: __kfence_alloc(),
> + * __kfence_free(), kfence_handle_page_fault(). However, note that we
> + * cannot grab the same metadata off the freelist twice, and multiple
> + * __kfence_alloc() cannot run concurrently on the same metadata.
> + */
> + raw_spinlock_t lock;
> +
> + /* The current state of the object; see above. */
> + enum kfence_object_state state;
> +
> + /*
> + * Allocated object address; cannot be calculated from size, because of
> + * alignment requirements.
> + *
> + * Invariant: ALIGN_DOWN(addr, PAGE_SIZE) is constant.
> + */
> + unsigned long addr;
> +
> + /*
> + * The size of the original allocation:
> + * size > 0: left page alignment
> + * size < 0: right page alignment
> + */
> + int size;
> +
> + /*
> + * The kmem_cache cache of the last allocation; NULL if never allocated
> + * or the cache has already been destroyed.
> + */
> + struct kmem_cache *cache;
> +
> + /*
> + * In case of an invalid access, the page that was unprotected; we
> + * optimistically only store address.
> + */
> + unsigned long unprotected_page;
> +
> + /* Allocation and free stack information. */
> + int num_alloc_stack;
> + int num_free_stack;
> + unsigned long alloc_stack[KFENCE_STACK_DEPTH];
> + unsigned long free_stack[KFENCE_STACK_DEPTH];
> +};
> +
> +extern struct kfence_metadata kfence_metadata[CONFIG_KFENCE_NUM_OBJECTS];
> +
> +/* KFENCE error types for report generation. */
> +enum kfence_error_type {
> + KFENCE_ERROR_OOB, /* Detected a out-of-bounds access. */
> + KFENCE_ERROR_UAF, /* Detected a use-after-free access. */
> + KFENCE_ERROR_CORRUPTION, /* Detected a memory corruption on free. */
> + KFENCE_ERROR_INVALID, /* Invalid access of unknown type. */
> + KFENCE_ERROR_INVALID_FREE, /* Invalid free. */
> +};
> +
> +void kfence_report_error(unsigned long address, const struct kfence_metadata *meta,
> + enum kfence_error_type type);
> +
> +void kfence_print_object(struct seq_file *seq, const struct kfence_metadata *meta);
> +
> +#endif /* MM_KFENCE_KFENCE_H */
> diff --git a/mm/kfence/report.c b/mm/kfence/report.c
> new file mode 100644
> index 000000000000..8c28200e7433
> --- /dev/null
> +++ b/mm/kfence/report.c
> @@ -0,0 +1,201 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <stdarg.h>
> +
> +#include <linux/kernel.h>
> +#include <linux/lockdep.h>
> +#include <linux/printk.h>
> +#include <linux/seq_file.h>
> +#include <linux/stacktrace.h>
> +#include <linux/string.h>
> +
> +#include <asm/kfence.h>
> +
> +#include "kfence.h"
> +
> +/* Helper function to either print to a seq_file or to console. */
> +static void seq_con_printf(struct seq_file *seq, const char *fmt, ...)
> +{
> + va_list args;
> +
> + va_start(args, fmt);
> + if (seq)
> + seq_vprintf(seq, fmt, args);
> + else
> + vprintk(fmt, args);
> + va_end(args);
> +}
> +
> +/* Get the number of stack entries to skip get out of MM internals. */
> +static int get_stack_skipnr(const unsigned long stack_entries[], int num_entries,
> + enum kfence_error_type type)
> +{
> + char buf[64];
> + int skipnr, fallback = 0;
> +
> + for (skipnr = 0; skipnr < num_entries; skipnr++) {
> + int len = scnprintf(buf, sizeof(buf), "%ps", (void *)stack_entries[skipnr]);
> +
> + /* Depending on error type, find different stack entries. */
> + switch (type) {
> + case KFENCE_ERROR_UAF:
> + case KFENCE_ERROR_OOB:
> + case KFENCE_ERROR_INVALID:
> + if (!strncmp(buf, KFENCE_SKIP_ARCH_FAULT_HANDLER, len))
> + goto found;
> + break;
> + case KFENCE_ERROR_CORRUPTION:
> + case KFENCE_ERROR_INVALID_FREE:
> + if (str_has_prefix(buf, "kfence_") || str_has_prefix(buf, "__kfence_"))
> + fallback = skipnr + 1; /* In case kfree tail calls into kfence. */
> +
> + /* Also the *_bulk() variants by only checking prefixes. */
> + if (str_has_prefix(buf, "kfree") || str_has_prefix(buf, "kmem_cache_free"))
> + goto found;
> + break;
> + }
> + }
> + if (fallback < num_entries)
> + return fallback;
> +found:
> + skipnr++;
> + return skipnr < num_entries ? skipnr : 0;
> +}
> +
> +static void kfence_print_stack(struct seq_file *seq, const struct kfence_metadata *meta,
> + bool show_alloc)
> +{
> + const unsigned long *entries = show_alloc ? meta->alloc_stack : meta->free_stack;
> + const int nentries = show_alloc ? meta->num_alloc_stack : meta->num_free_stack;
> +
> + if (nentries) {
> + int i;
> +
> + /* stack_trace_seq_print() does not exist; open code our own. */
> + for (i = 0; i < nentries; i++)
> + seq_con_printf(seq, " %pS\n", entries[i]);
> + } else {
> + seq_con_printf(seq, " no %s stack\n", show_alloc ? "allocation" : "deallocation");
> + }
> +}
> +
> +void kfence_print_object(struct seq_file *seq, const struct kfence_metadata *meta)
> +{
> + const int size = abs(meta->size);

This negative encoding is somewhat confusing. We do lots of abs, but
do we even look at the sign anywhere? I can't find any use that is not
abs.

> + const unsigned long start = meta->addr;
> + const struct kmem_cache *const cache = meta->cache;
> +
> + lockdep_assert_held(&meta->lock);
> +
> + if (meta->state == KFENCE_OBJECT_UNUSED) {
> + seq_con_printf(seq, "kfence-#%zd unused\n", meta - kfence_metadata);
> + return;
> + }
> +
> + seq_con_printf(seq,
> + "kfence-#%zd [0x" PTR_FMT "-0x" PTR_FMT
> + ", size=%d, cache=%s] allocated in:\n",
> + meta - kfence_metadata, (void *)start, (void *)(start + size - 1), size,
> + (cache && cache->name) ? cache->name : "<destroyed>");
> + kfence_print_stack(seq, meta, true);
> +
> + if (meta->state == KFENCE_OBJECT_FREED) {
> + seq_con_printf(seq, "freed in:\n");
> + kfence_print_stack(seq, meta, false);
> + }
> +}
> +
> +/*
> + * Show bytes at @addr that are different from the expected canary values, up to
> + * @max_bytes.
> + */
> +static void print_diff_canary(const u8 *addr, size_t max_bytes)
> +{
> + const u8 *max_addr = min((const u8 *)PAGE_ALIGN((unsigned long)addr), addr + max_bytes);
> +
> + pr_cont("[");
> + for (; addr < max_addr; addr++) {
> + if (*addr == KFENCE_CANARY_PATTERN(addr))
> + pr_cont(" .");
> + else if (IS_ENABLED(CONFIG_DEBUG_KERNEL))
> + pr_cont(" 0x%02x", *addr);
> + else /* Do not leak kernel memory in non-debug builds. */
> + pr_cont(" !");
> + }
> + pr_cont(" ]");
> +}
> +
> +void kfence_report_error(unsigned long address, const struct kfence_metadata *meta,
> + enum kfence_error_type type)
> +{
> + unsigned long stack_entries[KFENCE_STACK_DEPTH] = { 0 };
> + int num_stack_entries = stack_trace_save(stack_entries, KFENCE_STACK_DEPTH, 1);
> + int skipnr = get_stack_skipnr(stack_entries, num_stack_entries, type);
> +
> + /* KFENCE_ERROR_OOB requires non-NULL meta; for the rest it's optional. */
> + if (WARN_ON(type == KFENCE_ERROR_OOB && !meta))
> + return;
> +
> + if (meta)
> + lockdep_assert_held(&meta->lock);
> + /*
> + * Because we may generate reports in printk-unfriendly parts of the
> + * kernel, such as scheduler code, the use of printk() could deadlock.
> + * Until such time that all printing code here is safe in all parts of
> + * the kernel, accept the risk, and just get our message out (given the
> + * system might already behave unpredictably due to the memory error).
> + * As such, also disable lockdep to hide warnings, and avoid disabling
> + * lockdep for the rest of the kernel.
> + */
> + lockdep_off();
> +
> + pr_err("==================================================================\n");
> + /* Print report header. */
> + switch (type) {
> + case KFENCE_ERROR_OOB:
> + pr_err("BUG: KFENCE: out-of-bounds in %pS\n\n", (void *)stack_entries[skipnr]);
> + pr_err("Out-of-bounds access at 0x" PTR_FMT " (%s of kfence-#%zd):\n",
> + (void *)address, address < meta->addr ? "left" : "right",
> + meta - kfence_metadata);
> + break;
> + case KFENCE_ERROR_UAF:
> + pr_err("BUG: KFENCE: use-after-free in %pS\n\n", (void *)stack_entries[skipnr]);
> + pr_err("Use-after-free access at 0x" PTR_FMT ":\n", (void *)address);
> + break;
> + case KFENCE_ERROR_CORRUPTION:
> + pr_err("BUG: KFENCE: memory corruption in %pS\n\n", (void *)stack_entries[skipnr]);
> + pr_err("Detected corrupted memory at 0x" PTR_FMT " ", (void *)address);
> + print_diff_canary((u8 *)address, 16);
> + pr_cont(":\n");
> + break;
> + case KFENCE_ERROR_INVALID:
> + pr_err("BUG: KFENCE: invalid access in %pS\n\n", (void *)stack_entries[skipnr]);
> + pr_err("Invalid access at 0x" PTR_FMT ":\n", (void *)address);
> + break;
> + case KFENCE_ERROR_INVALID_FREE:
> + pr_err("BUG: KFENCE: invalid free in %pS\n\n", (void *)stack_entries[skipnr]);
> + pr_err("Invalid free of 0x" PTR_FMT ":\n", (void *)address);
> + break;
> + }
> +
> + /* Print stack trace and object info. */
> + stack_trace_print(stack_entries + skipnr, num_stack_entries - skipnr, 0);
> +
> + if (meta) {
> + pr_err("\n");
> + kfence_print_object(NULL, meta);
> + }
> +
> + /* Print report footer. */
> + pr_err("\n");
> + dump_stack_print_info(KERN_DEFAULT);
> + pr_err("==================================================================\n");
> +
> + lockdep_on();
> +
> + if (panic_on_warn)
> + panic("panic_on_warn set ...\n");
> +
> + /* We encountered a memory unsafety error, taint the kernel! */
> + add_taint(TAINT_WARN, LOCKDEP_STILL_OK);
> +}
> --
> 2.28.0.526.ge36021eeef-goog
>

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