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linux/mm/kasan/common.c
Andrey Konovalov ca77f290cf kasan: check KASAN_NO_FREE_META in __kasan_metadata_size
Patch series "kasan: switch tag-based modes to stack ring from per-object
metadata", v3.

This series makes the tag-based KASAN modes use a ring buffer for storing
stack depot handles for alloc/free stack traces for slab objects instead
of per-object metadata.  This ring buffer is referred to as the stack
ring.

On each alloc/free of a slab object, the tagged address of the object and
the current stack trace are recorded in the stack ring.

On each bug report, if the accessed address belongs to a slab object, the
stack ring is scanned for matching entries.  The newest entries are used
to print the alloc/free stack traces in the report: one entry for alloc
and one for free.

The advantages of this approach over storing stack trace handles in
per-object metadata with the tag-based KASAN modes:

- Allows to find relevant stack traces for use-after-free bugs without
  using quarantine for freed memory. (Currently, if the object was
  reallocated multiple times, the report contains the latest alloc/free
  stack traces, not necessarily the ones relevant to the buggy allocation.)
- Allows to better identify and mark use-after-free bugs, effectively
  making the CONFIG_KASAN_TAGS_IDENTIFY functionality always-on.
- Has fixed memory overhead.

The disadvantage:

- If the affected object was allocated/freed long before the bug happened
  and the stack trace events were purged from the stack ring, the report
  will have no stack traces.

Discussion
==========

The proposed implementation of the stack ring uses a single ring buffer
for the whole kernel.  This might lead to contention due to atomic
accesses to the ring buffer index on multicore systems.

At this point, it is unknown whether the performance impact from this
contention would be significant compared to the slowdown introduced by
collecting stack traces due to the planned changes to the latter part, see
the section below.

For now, the proposed implementation is deemed to be good enough, but this
might need to be revisited once the stack collection becomes faster.

A considered alternative is to keep a separate ring buffer for each CPU
and then iterate over all of them when printing a bug report.  This
approach requires somehow figuring out which of the stack rings has the
freshest stack traces for an object if multiple stack rings have them.

Further plans
=============

This series is a part of an effort to make KASAN stack trace collection
suitable for production.  This requires stack trace collection to be fast
and memory-bounded.

The planned steps are:

1. Speed up stack trace collection (potentially, by using SCS;
   patches on-hold until steps  and  are completed).
2. Keep stack trace handles in the stack ring (this series).
3. Add a memory-bounded mode to stack depot or provide an alternative
   memory-bounded stack storage.
4. Potentially, implement stack trace collection sampling to minimize
   the performance impact.


This patch (of 34):

__kasan_metadata_size() calculates the size of the redzone for objects in
a slab cache.

When accounting for presence of kasan_free_meta in the redzone, this
function only compares free_meta_offset with 0.  But free_meta_offset
could also be equal to KASAN_NO_FREE_META, which indicates that
kasan_free_meta is not present at all.

Add a comparison with KASAN_NO_FREE_META into __kasan_metadata_size().

Link: https://lkml.kernel.org/r/cover.1662411799.git.andreyknvl@google.com
Link: https://lkml.kernel.org/r/c7b316d30d90e5947eb8280f4dc78856a49298cf.1662411799.git.andreyknvl@google.com
Signed-off-by: Andrey Konovalov <andreyknvl@google.com>
Reviewed-by: Marco Elver <elver@google.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Evgenii Stepanov <eugenis@google.com>
Cc: Peter Collingbourne <pcc@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
2022-10-03 14:02:56 -07:00

596 lines
16 KiB
C

// SPDX-License-Identifier: GPL-2.0
/*
* This file contains common KASAN code.
*
* Copyright (c) 2014 Samsung Electronics Co., Ltd.
* Author: Andrey Ryabinin <ryabinin.a.a@gmail.com>
*
* Some code borrowed from https://github.com/xairy/kasan-prototype by
* Andrey Konovalov <andreyknvl@gmail.com>
*/
#include <linux/export.h>
#include <linux/init.h>
#include <linux/kasan.h>
#include <linux/kernel.h>
#include <linux/linkage.h>
#include <linux/memblock.h>
#include <linux/memory.h>
#include <linux/mm.h>
#include <linux/module.h>
#include <linux/printk.h>
#include <linux/sched.h>
#include <linux/sched/task_stack.h>
#include <linux/slab.h>
#include <linux/stacktrace.h>
#include <linux/string.h>
#include <linux/types.h>
#include <linux/bug.h>
#include "kasan.h"
#include "../slab.h"
depot_stack_handle_t kasan_save_stack(gfp_t flags, bool can_alloc)
{
unsigned long entries[KASAN_STACK_DEPTH];
unsigned int nr_entries;
nr_entries = stack_trace_save(entries, ARRAY_SIZE(entries), 0);
return __stack_depot_save(entries, nr_entries, flags, can_alloc);
}
void kasan_set_track(struct kasan_track *track, gfp_t flags)
{
track->pid = current->pid;
track->stack = kasan_save_stack(flags, true);
}
#if defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS)
void kasan_enable_current(void)
{
current->kasan_depth++;
}
EXPORT_SYMBOL(kasan_enable_current);
void kasan_disable_current(void)
{
current->kasan_depth--;
}
EXPORT_SYMBOL(kasan_disable_current);
#endif /* CONFIG_KASAN_GENERIC || CONFIG_KASAN_SW_TAGS */
void __kasan_unpoison_range(const void *address, size_t size)
{
kasan_unpoison(address, size, false);
}
#ifdef CONFIG_KASAN_STACK
/* Unpoison the entire stack for a task. */
void kasan_unpoison_task_stack(struct task_struct *task)
{
void *base = task_stack_page(task);
kasan_unpoison(base, THREAD_SIZE, false);
}
/* Unpoison the stack for the current task beyond a watermark sp value. */
asmlinkage void kasan_unpoison_task_stack_below(const void *watermark)
{
/*
* Calculate the task stack base address. Avoid using 'current'
* because this function is called by early resume code which hasn't
* yet set up the percpu register (%gs).
*/
void *base = (void *)((unsigned long)watermark & ~(THREAD_SIZE - 1));
kasan_unpoison(base, watermark - base, false);
}
#endif /* CONFIG_KASAN_STACK */
/*
* Only allow cache merging when stack collection is disabled and no metadata
* is present.
*/
slab_flags_t __kasan_never_merge(void)
{
if (kasan_stack_collection_enabled())
return SLAB_KASAN;
return 0;
}
void __kasan_unpoison_pages(struct page *page, unsigned int order, bool init)
{
u8 tag;
unsigned long i;
if (unlikely(PageHighMem(page)))
return;
tag = kasan_random_tag();
kasan_unpoison(set_tag(page_address(page), tag),
PAGE_SIZE << order, init);
for (i = 0; i < (1 << order); i++)
page_kasan_tag_set(page + i, tag);
}
void __kasan_poison_pages(struct page *page, unsigned int order, bool init)
{
if (likely(!PageHighMem(page)))
kasan_poison(page_address(page), PAGE_SIZE << order,
KASAN_PAGE_FREE, init);
}
/*
* Adaptive redzone policy taken from the userspace AddressSanitizer runtime.
* For larger allocations larger redzones are used.
*/
static inline unsigned int optimal_redzone(unsigned int object_size)
{
return
object_size <= 64 - 16 ? 16 :
object_size <= 128 - 32 ? 32 :
object_size <= 512 - 64 ? 64 :
object_size <= 4096 - 128 ? 128 :
object_size <= (1 << 14) - 256 ? 256 :
object_size <= (1 << 15) - 512 ? 512 :
object_size <= (1 << 16) - 1024 ? 1024 : 2048;
}
void __kasan_cache_create(struct kmem_cache *cache, unsigned int *size,
slab_flags_t *flags)
{
unsigned int ok_size;
unsigned int optimal_size;
/*
* SLAB_KASAN is used to mark caches as ones that are sanitized by
* KASAN. Currently this flag is used in two places:
* 1. In slab_ksize() when calculating the size of the accessible
* memory within the object.
* 2. In slab_common.c to prevent merging of sanitized caches.
*/
*flags |= SLAB_KASAN;
if (!kasan_stack_collection_enabled())
return;
ok_size = *size;
/* Add alloc meta into redzone. */
cache->kasan_info.alloc_meta_offset = *size;
*size += sizeof(struct kasan_alloc_meta);
/*
* If alloc meta doesn't fit, don't add it.
* This can only happen with SLAB, as it has KMALLOC_MAX_SIZE equal
* to KMALLOC_MAX_CACHE_SIZE and doesn't fall back to page_alloc for
* larger sizes.
*/
if (*size > KMALLOC_MAX_SIZE) {
cache->kasan_info.alloc_meta_offset = 0;
*size = ok_size;
/* Continue, since free meta might still fit. */
}
/* Only the generic mode uses free meta or flexible redzones. */
if (!IS_ENABLED(CONFIG_KASAN_GENERIC)) {
cache->kasan_info.free_meta_offset = KASAN_NO_FREE_META;
return;
}
/*
* Add free meta into redzone when it's not possible to store
* it in the object. This is the case when:
* 1. Object is SLAB_TYPESAFE_BY_RCU, which means that it can
* be touched after it was freed, or
* 2. Object has a constructor, which means it's expected to
* retain its content until the next allocation, or
* 3. Object is too small.
* Otherwise cache->kasan_info.free_meta_offset = 0 is implied.
*/
if ((cache->flags & SLAB_TYPESAFE_BY_RCU) || cache->ctor ||
cache->object_size < sizeof(struct kasan_free_meta)) {
ok_size = *size;
cache->kasan_info.free_meta_offset = *size;
*size += sizeof(struct kasan_free_meta);
/* If free meta doesn't fit, don't add it. */
if (*size > KMALLOC_MAX_SIZE) {
cache->kasan_info.free_meta_offset = KASAN_NO_FREE_META;
*size = ok_size;
}
}
/* Calculate size with optimal redzone. */
optimal_size = cache->object_size + optimal_redzone(cache->object_size);
/* Limit it with KMALLOC_MAX_SIZE (relevant for SLAB only). */
if (optimal_size > KMALLOC_MAX_SIZE)
optimal_size = KMALLOC_MAX_SIZE;
/* Use optimal size if the size with added metas is not large enough. */
if (*size < optimal_size)
*size = optimal_size;
}
void __kasan_cache_create_kmalloc(struct kmem_cache *cache)
{
cache->kasan_info.is_kmalloc = true;
}
size_t __kasan_metadata_size(struct kmem_cache *cache)
{
if (!kasan_stack_collection_enabled())
return 0;
return (cache->kasan_info.alloc_meta_offset ?
sizeof(struct kasan_alloc_meta) : 0) +
((cache->kasan_info.free_meta_offset &&
cache->kasan_info.free_meta_offset != KASAN_NO_FREE_META) ?
sizeof(struct kasan_free_meta) : 0);
}
struct kasan_alloc_meta *kasan_get_alloc_meta(struct kmem_cache *cache,
const void *object)
{
if (!cache->kasan_info.alloc_meta_offset)
return NULL;
return kasan_reset_tag(object) + cache->kasan_info.alloc_meta_offset;
}
#ifdef CONFIG_KASAN_GENERIC
struct kasan_free_meta *kasan_get_free_meta(struct kmem_cache *cache,
const void *object)
{
BUILD_BUG_ON(sizeof(struct kasan_free_meta) > 32);
if (cache->kasan_info.free_meta_offset == KASAN_NO_FREE_META)
return NULL;
return kasan_reset_tag(object) + cache->kasan_info.free_meta_offset;
}
#endif
void __kasan_poison_slab(struct slab *slab)
{
struct page *page = slab_page(slab);
unsigned long i;
for (i = 0; i < compound_nr(page); i++)
page_kasan_tag_reset(page + i);
kasan_poison(page_address(page), page_size(page),
KASAN_SLAB_REDZONE, false);
}
void __kasan_unpoison_object_data(struct kmem_cache *cache, void *object)
{
kasan_unpoison(object, cache->object_size, false);
}
void __kasan_poison_object_data(struct kmem_cache *cache, void *object)
{
kasan_poison(object, round_up(cache->object_size, KASAN_GRANULE_SIZE),
KASAN_SLAB_REDZONE, false);
}
/*
* This function assigns a tag to an object considering the following:
* 1. A cache might have a constructor, which might save a pointer to a slab
* object somewhere (e.g. in the object itself). We preassign a tag for
* each object in caches with constructors during slab creation and reuse
* the same tag each time a particular object is allocated.
* 2. A cache might be SLAB_TYPESAFE_BY_RCU, which means objects can be
* accessed after being freed. We preassign tags for objects in these
* caches as well.
* 3. For SLAB allocator we can't preassign tags randomly since the freelist
* is stored as an array of indexes instead of a linked list. Assign tags
* based on objects indexes, so that objects that are next to each other
* get different tags.
*/
static inline u8 assign_tag(struct kmem_cache *cache,
const void *object, bool init)
{
if (IS_ENABLED(CONFIG_KASAN_GENERIC))
return 0xff;
/*
* If the cache neither has a constructor nor has SLAB_TYPESAFE_BY_RCU
* set, assign a tag when the object is being allocated (init == false).
*/
if (!cache->ctor && !(cache->flags & SLAB_TYPESAFE_BY_RCU))
return init ? KASAN_TAG_KERNEL : kasan_random_tag();
/* For caches that either have a constructor or SLAB_TYPESAFE_BY_RCU: */
#ifdef CONFIG_SLAB
/* For SLAB assign tags based on the object index in the freelist. */
return (u8)obj_to_index(cache, virt_to_slab(object), (void *)object);
#else
/*
* For SLUB assign a random tag during slab creation, otherwise reuse
* the already assigned tag.
*/
return init ? kasan_random_tag() : get_tag(object);
#endif
}
void * __must_check __kasan_init_slab_obj(struct kmem_cache *cache,
const void *object)
{
struct kasan_alloc_meta *alloc_meta;
if (kasan_stack_collection_enabled()) {
alloc_meta = kasan_get_alloc_meta(cache, object);
if (alloc_meta)
__memset(alloc_meta, 0, sizeof(*alloc_meta));
}
/* Tag is ignored in set_tag() without CONFIG_KASAN_SW/HW_TAGS */
object = set_tag(object, assign_tag(cache, object, true));
return (void *)object;
}
static inline bool ____kasan_slab_free(struct kmem_cache *cache, void *object,
unsigned long ip, bool quarantine, bool init)
{
u8 tag;
void *tagged_object;
if (!kasan_arch_is_ready())
return false;
tag = get_tag(object);
tagged_object = object;
object = kasan_reset_tag(object);
if (is_kfence_address(object))
return false;
if (unlikely(nearest_obj(cache, virt_to_slab(object), object) !=
object)) {
kasan_report_invalid_free(tagged_object, ip, KASAN_REPORT_INVALID_FREE);
return true;
}
/* RCU slabs could be legally used after free within the RCU period */
if (unlikely(cache->flags & SLAB_TYPESAFE_BY_RCU))
return false;
if (!kasan_byte_accessible(tagged_object)) {
kasan_report_invalid_free(tagged_object, ip, KASAN_REPORT_DOUBLE_FREE);
return true;
}
kasan_poison(object, round_up(cache->object_size, KASAN_GRANULE_SIZE),
KASAN_SLAB_FREE, init);
if ((IS_ENABLED(CONFIG_KASAN_GENERIC) && !quarantine))
return false;
if (kasan_stack_collection_enabled())
kasan_set_free_info(cache, object, tag);
return kasan_quarantine_put(cache, object);
}
bool __kasan_slab_free(struct kmem_cache *cache, void *object,
unsigned long ip, bool init)
{
return ____kasan_slab_free(cache, object, ip, true, init);
}
static inline bool ____kasan_kfree_large(void *ptr, unsigned long ip)
{
if (ptr != page_address(virt_to_head_page(ptr))) {
kasan_report_invalid_free(ptr, ip, KASAN_REPORT_INVALID_FREE);
return true;
}
if (!kasan_byte_accessible(ptr)) {
kasan_report_invalid_free(ptr, ip, KASAN_REPORT_DOUBLE_FREE);
return true;
}
/*
* The object will be poisoned by kasan_poison_pages() or
* kasan_slab_free_mempool().
*/
return false;
}
void __kasan_kfree_large(void *ptr, unsigned long ip)
{
____kasan_kfree_large(ptr, ip);
}
void __kasan_slab_free_mempool(void *ptr, unsigned long ip)
{
struct folio *folio;
folio = virt_to_folio(ptr);
/*
* Even though this function is only called for kmem_cache_alloc and
* kmalloc backed mempool allocations, those allocations can still be
* !PageSlab() when the size provided to kmalloc is larger than
* KMALLOC_MAX_SIZE, and kmalloc falls back onto page_alloc.
*/
if (unlikely(!folio_test_slab(folio))) {
if (____kasan_kfree_large(ptr, ip))
return;
kasan_poison(ptr, folio_size(folio), KASAN_PAGE_FREE, false);
} else {
struct slab *slab = folio_slab(folio);
____kasan_slab_free(slab->slab_cache, ptr, ip, false, false);
}
}
static void set_alloc_info(struct kmem_cache *cache, void *object,
gfp_t flags, bool is_kmalloc)
{
struct kasan_alloc_meta *alloc_meta;
/* Don't save alloc info for kmalloc caches in kasan_slab_alloc(). */
if (cache->kasan_info.is_kmalloc && !is_kmalloc)
return;
alloc_meta = kasan_get_alloc_meta(cache, object);
if (alloc_meta)
kasan_set_track(&alloc_meta->alloc_track, flags);
}
void * __must_check __kasan_slab_alloc(struct kmem_cache *cache,
void *object, gfp_t flags, bool init)
{
u8 tag;
void *tagged_object;
if (gfpflags_allow_blocking(flags))
kasan_quarantine_reduce();
if (unlikely(object == NULL))
return NULL;
if (is_kfence_address(object))
return (void *)object;
/*
* Generate and assign random tag for tag-based modes.
* Tag is ignored in set_tag() for the generic mode.
*/
tag = assign_tag(cache, object, false);
tagged_object = set_tag(object, tag);
/*
* Unpoison the whole object.
* For kmalloc() allocations, kasan_kmalloc() will do precise poisoning.
*/
kasan_unpoison(tagged_object, cache->object_size, init);
/* Save alloc info (if possible) for non-kmalloc() allocations. */
if (kasan_stack_collection_enabled())
set_alloc_info(cache, (void *)object, flags, false);
return tagged_object;
}
static inline void *____kasan_kmalloc(struct kmem_cache *cache,
const void *object, size_t size, gfp_t flags)
{
unsigned long redzone_start;
unsigned long redzone_end;
if (gfpflags_allow_blocking(flags))
kasan_quarantine_reduce();
if (unlikely(object == NULL))
return NULL;
if (is_kfence_address(kasan_reset_tag(object)))
return (void *)object;
/*
* The object has already been unpoisoned by kasan_slab_alloc() for
* kmalloc() or by kasan_krealloc() for krealloc().
*/
/*
* The redzone has byte-level precision for the generic mode.
* Partially poison the last object granule to cover the unaligned
* part of the redzone.
*/
if (IS_ENABLED(CONFIG_KASAN_GENERIC))
kasan_poison_last_granule((void *)object, size);
/* Poison the aligned part of the redzone. */
redzone_start = round_up((unsigned long)(object + size),
KASAN_GRANULE_SIZE);
redzone_end = round_up((unsigned long)(object + cache->object_size),
KASAN_GRANULE_SIZE);
kasan_poison((void *)redzone_start, redzone_end - redzone_start,
KASAN_SLAB_REDZONE, false);
/*
* Save alloc info (if possible) for kmalloc() allocations.
* This also rewrites the alloc info when called from kasan_krealloc().
*/
if (kasan_stack_collection_enabled())
set_alloc_info(cache, (void *)object, flags, true);
/* Keep the tag that was set by kasan_slab_alloc(). */
return (void *)object;
}
void * __must_check __kasan_kmalloc(struct kmem_cache *cache, const void *object,
size_t size, gfp_t flags)
{
return ____kasan_kmalloc(cache, object, size, flags);
}
EXPORT_SYMBOL(__kasan_kmalloc);
void * __must_check __kasan_kmalloc_large(const void *ptr, size_t size,
gfp_t flags)
{
unsigned long redzone_start;
unsigned long redzone_end;
if (gfpflags_allow_blocking(flags))
kasan_quarantine_reduce();
if (unlikely(ptr == NULL))
return NULL;
/*
* The object has already been unpoisoned by kasan_unpoison_pages() for
* alloc_pages() or by kasan_krealloc() for krealloc().
*/
/*
* The redzone has byte-level precision for the generic mode.
* Partially poison the last object granule to cover the unaligned
* part of the redzone.
*/
if (IS_ENABLED(CONFIG_KASAN_GENERIC))
kasan_poison_last_granule(ptr, size);
/* Poison the aligned part of the redzone. */
redzone_start = round_up((unsigned long)(ptr + size),
KASAN_GRANULE_SIZE);
redzone_end = (unsigned long)ptr + page_size(virt_to_page(ptr));
kasan_poison((void *)redzone_start, redzone_end - redzone_start,
KASAN_PAGE_REDZONE, false);
return (void *)ptr;
}
void * __must_check __kasan_krealloc(const void *object, size_t size, gfp_t flags)
{
struct slab *slab;
if (unlikely(object == ZERO_SIZE_PTR))
return (void *)object;
/*
* Unpoison the object's data.
* Part of it might already have been unpoisoned, but it's unknown
* how big that part is.
*/
kasan_unpoison(object, size, false);
slab = virt_to_slab(object);
/* Piggy-back on kmalloc() instrumentation to poison the redzone. */
if (unlikely(!slab))
return __kasan_kmalloc_large(object, size, flags);
else
return ____kasan_kmalloc(slab->slab_cache, object, size, flags);
}
bool __kasan_check_byte(const void *address, unsigned long ip)
{
if (!kasan_byte_accessible(address)) {
kasan_report((unsigned long)address, 1, false, ip);
return false;
}
return true;
}