[PATCH v3] kasan: infer allocation size by scanning metadata

[Kuan-Ying Lee]

From: Andrey Konovalov <andre...@google.com>

Make KASAN scan metadata to infer the requested allocation size instead of
printing cache->object_size.

This patch fixes confusing slab-out-of-bounds reports as reported in:

https://bugzilla.kernel.org/show_bug.cgi?id=216457

As an example of the confusing behavior, the report below hints that the
allocation size was 192, while the kernel actually called kmalloc(184):

==================================================================
BUG: KASAN: slab-out-of-bounds in _find_next_bit+0x143/0x160 lib/find_bit.c:109
Read of size 8 at addr ffff8880175766b8 by task kworker/1:1/26
...
The buggy address belongs to the object at ffff888017576600
which belongs to the cache kmalloc-192 of size 192
The buggy address is located 184 bytes inside of
192-byte region [ffff888017576600, ffff8880175766c0)
...
Memory state around the buggy address:
ffff888017576580: fb fb fb fb fb fb fb fb fc fc fc fc fc fc fc fc
ffff888017576600: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
>ffff888017576680: 00 00 00 00 00 00 00 fc fc fc fc fc fc fc fc fc
^
ffff888017576700: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
ffff888017576780: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
==================================================================

With this patch, the report shows:

==================================================================
...
The buggy address belongs to the object at ffff888017576600
which belongs to the cache kmalloc-192 of size 192
The buggy address is located 0 bytes to the right of
allocated 184-byte region [ffff888017576600, ffff8880175766b8)
...
==================================================================

Also report slab use-after-free bugs as "slab-use-after-free" and print
"freed" instead of "allocated" in the report when describing the accessed
memory region.

Also improve the metadata-related comment in kasan_find_first_bad_addr
and use addr_has_metadata across KASAN code instead of open-coding
KASAN_SHADOW_START checks.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=216457
Co-developed-by: Andrey Konovalov <andre...@gmail.com>
Signed-off-by: Kuan-Ying Lee <Kuan-Y...@mediatek.com>

---

Changes v2->v3:

- Rename obj_size to alloc_size and change its type to size_t.
- Add comments into kasan_get_alloc_size.
- Infer and report alloc_size for all report types.
- Update metadata-related comment in kasan_find_first_bad_addr for HW_TAGS.
- Use addr_has_metadata for Generic and SW_TAGS modes instead of
open-coding KASAN_SHADOW_START checks.
- Introduce slab-use-after-free report type.
- Print "freed" when describing memory region for slab-use-after-free bugs.
- Only print memory region state for Generic mode.

Changes v1->v2:

- Implement getting allocated size of object for tag-based kasan.
- Refine the kasan report.
- Check if it is slab-out-of-bounds report type.
- Thanks for Andrey and Dmitry suggestion.

---
mm/kasan/generic.c | 4 +---
mm/kasan/kasan.h | 2 ++
mm/kasan/report.c | 41 ++++++++++++++++++++++++++++-----------
mm/kasan/report_generic.c | 32 +++++++++++++++++++++++++++++-
mm/kasan/report_hw_tags.c | 35 ++++++++++++++++++++++++++++++++-
mm/kasan/report_sw_tags.c | 26 +++++++++++++++++++++++++
mm/kasan/report_tags.c | 2 +-
mm/kasan/sw_tags.c | 6 ++----
8 files changed, 127 insertions(+), 21 deletions(-)

diff --git a/mm/kasan/generic.c b/mm/kasan/generic.c
index cb762982c8ba..e5eef670735e 100644
--- a/mm/kasan/generic.c
+++ b/mm/kasan/generic.c
@@ -172,10 +172,8 @@ static __always_inline bool check_region_inline(unsigned long addr,
if (unlikely(addr + size < addr))
return !kasan_report(addr, size, write, ret_ip);

- if (unlikely((void *)addr <
- kasan_shadow_to_mem((void *)KASAN_SHADOW_START))) {
+ if (unlikely(!addr_has_metadata((void *)addr)))
return !kasan_report(addr, size, write, ret_ip);
- }

if (likely(!memory_is_poisoned(addr, size)))
return true;
diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
index dcc2a88e8121..3231314e071f 100644
--- a/mm/kasan/kasan.h
+++ b/mm/kasan/kasan.h
@@ -207,6 +207,7 @@ struct kasan_report_info {
void *first_bad_addr;
struct kmem_cache *cache;
void *object;
+ size_t alloc_size;

/* Filled in by the mode-specific reporting code. */
const char *bug_type;
@@ -323,6 +324,7 @@ static inline bool addr_has_metadata(const void *addr)
#endif /* CONFIG_KASAN_GENERIC || CONFIG_KASAN_SW_TAGS */

void *kasan_find_first_bad_addr(void *addr, size_t size);
+size_t kasan_get_alloc_size(void *object, struct kmem_cache *cache);
void kasan_complete_mode_report_info(struct kasan_report_info *info);
void kasan_metadata_fetch_row(char *buffer, void *row);

diff --git a/mm/kasan/report.c b/mm/kasan/report.c
index 22598b20c7b7..e0492124e90a 100644
--- a/mm/kasan/report.c
+++ b/mm/kasan/report.c
@@ -231,33 +231,46 @@ static inline struct page *addr_to_page(const void *addr)
return NULL;
}

-static void describe_object_addr(const void *addr, struct kmem_cache *cache,
- void *object)
+static void describe_object_addr(const void *addr, struct kasan_report_info *info)
{
unsigned long access_addr = (unsigned long)addr;
- unsigned long object_addr = (unsigned long)object;
- const char *rel_type;
+ unsigned long object_addr = (unsigned long)info->object;
+ const char *rel_type, *region_state = "";
int rel_bytes;

pr_err("The buggy address belongs to the object at %px\n"
" which belongs to the cache %s of size %d\n",
- object, cache->name, cache->object_size);
+ info->object, info->cache->name, info->cache->object_size);

if (access_addr < object_addr) {
rel_type = "to the left";
rel_bytes = object_addr - access_addr;
- } else if (access_addr >= object_addr + cache->object_size) {
+ } else if (access_addr >= object_addr + info->alloc_size) {
rel_type = "to the right";
- rel_bytes = access_addr - (object_addr + cache->object_size);
+ rel_bytes = access_addr - (object_addr + info->alloc_size);
} else {
rel_type = "inside";
rel_bytes = access_addr - object_addr;
}

+ /*
+ * Tag-Based modes use the stack ring to infer the bug type, but the
+ * memory region state description is generated based on the metadata.
+ * Thus, defining the region state as below can contradict the metadata.
+ * Fixing this requires further improvements, so only infer the state
+ * for the Generic mode.
+ */
+ if (IS_ENABLED(CONFIG_KASAN_GENERIC)) {
+ if (strcmp(info->bug_type, "slab-out-of-bounds") == 0)
+ region_state = "allocated ";
+ else if (strcmp(info->bug_type, "slab-use-after-free") == 0)
+ region_state = "freed ";
+ }
+
pr_err("The buggy address is located %d bytes %s of\n"
- " %d-byte region [%px, %px)\n",
- rel_bytes, rel_type, cache->object_size, (void *)object_addr,
- (void *)(object_addr + cache->object_size));
+ " %s%lu-byte region [%px, %px)\n",
+ rel_bytes, rel_type, region_state, info->alloc_size,
+ (void *)object_addr, (void *)(object_addr + info->alloc_size));
}

static void describe_object_stacks(struct kasan_report_info *info)
@@ -279,7 +292,7 @@ static void describe_object(const void *addr, struct kasan_report_info *info)
{
if (kasan_stack_collection_enabled())
describe_object_stacks(info);
- describe_object_addr(addr, info->cache, info->object);
+ describe_object_addr(addr, info);
}

static inline bool kernel_or_module_addr(const void *addr)
@@ -436,6 +449,12 @@ static void complete_report_info(struct kasan_report_info *info)
if (slab) {
info->cache = slab->slab_cache;
info->object = nearest_obj(info->cache, slab, addr);
+
+ /* Try to determine allocation size based on the metadata. */
+ info->alloc_size = kasan_get_alloc_size(info->object, info->cache);
+ /* Fallback to the object size if failed. */
+ if (!info->alloc_size)
+ info->alloc_size = info->cache->object_size;
} else
info->cache = info->object = NULL;

diff --git a/mm/kasan/report_generic.c b/mm/kasan/report_generic.c
index 043c94b04605..87d39bc0a673 100644
--- a/mm/kasan/report_generic.c
+++ b/mm/kasan/report_generic.c
@@ -43,6 +43,34 @@ void *kasan_find_first_bad_addr(void *addr, size_t size)
return p;
}

+size_t kasan_get_alloc_size(void *object, struct kmem_cache *cache)
+{
+ size_t size = 0;
+ u8 *shadow;
+
+ /*
+ * Skip the addr_has_metadata check, as this function only operates on
+ * slab memory, which must have metadata.
+ */
+
+ /*
+ * The loop below returns 0 for freed objects, for which KASAN cannot
+ * calculate the allocation size based on the metadata.
+ */
+ shadow = (u8 *)kasan_mem_to_shadow(object);
+ while (size < cache->object_size) {
+ if (*shadow == 0)
+ size += KASAN_GRANULE_SIZE;
+ else if (*shadow >= 1 && *shadow <= KASAN_GRANULE_SIZE - 1)
+ return size + *shadow;
+ else
+ return size;
+ shadow++;
+ }
+
+ return cache->object_size;
+}
+
static const char *get_shadow_bug_type(struct kasan_report_info *info)
{
const char *bug_type = "unknown-crash";
@@ -79,9 +107,11 @@ static const char *get_shadow_bug_type(struct kasan_report_info *info)
bug_type = "stack-out-of-bounds";
break;
case KASAN_PAGE_FREE:
+ bug_type = "use-after-free";
+ break;
case KASAN_SLAB_FREE:
case KASAN_SLAB_FREETRACK:
- bug_type = "use-after-free";
+ bug_type = "slab-use-after-free";
break;
case KASAN_ALLOCA_LEFT:
case KASAN_ALLOCA_RIGHT:
diff --git a/mm/kasan/report_hw_tags.c b/mm/kasan/report_hw_tags.c
index f3d3be614e4b..32e80f78de7d 100644
--- a/mm/kasan/report_hw_tags.c
+++ b/mm/kasan/report_hw_tags.c
@@ -17,10 +17,43 @@

void *kasan_find_first_bad_addr(void *addr, size_t size)
{
- /* Return the same value regardless of whether addr_has_metadata(). */
+ /*
+ * Hardware Tag-Based KASAN only calls this function for normal memory
+ * accesses, and thus addr points precisely to the first bad address
+ * with an invalid (and present) memory tag. Therefore:
+ * 1. Return the address as is without walking memory tags.
+ * 2. Skip the addr_has_metadata check.
+ */
return kasan_reset_tag(addr);
}

+size_t kasan_get_alloc_size(void *object, struct kmem_cache *cache)
+{
+ size_t size = 0;
+ int i = 0;
+ u8 memory_tag;
+
+ /*
+ * Skip the addr_has_metadata check, as this function only operates on
+ * slab memory, which must have metadata.
+ */
+
+ /*
+ * The loop below returns 0 for freed objects, for which KASAN cannot
+ * calculate the allocation size based on the metadata.
+ */
+ while (size < cache->object_size) {
+ memory_tag = hw_get_mem_tag(object + i * KASAN_GRANULE_SIZE);
+ if (memory_tag != KASAN_TAG_INVALID)
+ size += KASAN_GRANULE_SIZE;
+ else
+ return size;
+ i++;
+ }
+
+ return cache->object_size;
+}
+
void kasan_metadata_fetch_row(char *buffer, void *row)
{
int i;
diff --git a/mm/kasan/report_sw_tags.c b/mm/kasan/report_sw_tags.c
index 7a26397297ed..8b1f5a73ee6d 100644
--- a/mm/kasan/report_sw_tags.c
+++ b/mm/kasan/report_sw_tags.c
@@ -45,6 +45,32 @@ void *kasan_find_first_bad_addr(void *addr, size_t size)
return p;
}

+size_t kasan_get_alloc_size(void *object, struct kmem_cache *cache)
+{
+ size_t size = 0;
+ u8 *shadow;
+
+ /*
+ * Skip the addr_has_metadata check, as this function only operates on
+ * slab memory, which must have metadata.
+ */
+
+ /*
+ * The loop below returns 0 for freed objects, for which KASAN cannot
+ * calculate the allocation size based on the metadata.
+ */
+ shadow = (u8 *)kasan_mem_to_shadow(object);
+ while (size < cache->object_size) {
+ if (*shadow != KASAN_TAG_INVALID)
+ size += KASAN_GRANULE_SIZE;
+ else
+ return size;
+ shadow++;
+ }
+
+ return cache->object_size;
+}
+
void kasan_metadata_fetch_row(char *buffer, void *row)
{
memcpy(buffer, kasan_mem_to_shadow(row), META_BYTES_PER_ROW);
diff --git a/mm/kasan/report_tags.c b/mm/kasan/report_tags.c
index ecede06ef374..8b8bfdb3cfdb 100644
--- a/mm/kasan/report_tags.c
+++ b/mm/kasan/report_tags.c
@@ -89,7 +89,7 @@ void kasan_complete_mode_report_info(struct kasan_report_info *info)
* a use-after-free.
*/
if (!info->bug_type)
- info->bug_type = "use-after-free";
+ info->bug_type = "slab-use-after-free";
} else {
/* Second alloc of the same object. Give up. */
if (alloc_found)
diff --git a/mm/kasan/sw_tags.c b/mm/kasan/sw_tags.c
index a3afaf2ad1b1..30da65fa02a1 100644
--- a/mm/kasan/sw_tags.c
+++ b/mm/kasan/sw_tags.c
@@ -106,10 +106,8 @@ bool kasan_check_range(unsigned long addr, size_t size, bool write,
return true;

untagged_addr = kasan_reset_tag((const void *)addr);
- if (unlikely(untagged_addr <
- kasan_shadow_to_mem((void *)KASAN_SHADOW_START))) {
+ if (unlikely(!addr_has_metadata(untagged_addr)))
return !kasan_report(addr, size, write, ret_ip);
- }
shadow_first = kasan_mem_to_shadow(untagged_addr);
shadow_last = kasan_mem_to_shadow(untagged_addr + size - 1);
for (shadow = shadow_first; shadow <= shadow_last; shadow++) {
@@ -127,7 +125,7 @@ bool kasan_byte_accessible(const void *addr)
void *untagged_addr = kasan_reset_tag(addr);
u8 shadow_byte;

- if (untagged_addr < kasan_shadow_to_mem((void *)KASAN_SHADOW_START))
+ if (!addr_has_metadata(untagged_addr))
return false;

shadow_byte = READ_ONCE(*(u8 *)kasan_mem_to_shadow(untagged_addr));
--
2.18.0

[Andrey Konovalov]

/On Sat, Jan 28, 2023 at 4:00 PM Kuan-Ying Lee
<Kuan-Y...@mediatek.com> wrote:
>
> From: Andrey Konovalov <andre...@google.com>

Ah, I think you need to reset the commit author before sending, so
that the patch gets recorded as authored by you.

Or change the Co-developed-by/Signed-off-by tags.

I don't mind either approach.

Thanks!

[Kuan-Ying Lee (李冠穎)]

On Sat, 2023-01-28 at 23:37 +0100, Andrey Konovalov wrote:
> /On Sat, Jan 28, 2023 at 4:00 PM Kuan-Ying Lee
> <Kuan-Y...@mediatek.com> wrote:
> >
> > From: Andrey Konovalov <andre...@google.com>
>
> Ah, I think you need to reset the commit author before sending, so
> that the patch gets recorded as authored by you.

Got it.
Will do in v4.

> > Make KASAN scan metadata to infer the requested allocation size
> > instead of
> > printing cache->object_size.
> >
> > This patch fixes confusing slab-out-of-bounds reports as reported
> > in:
> >
> >

https://urldefense.com/v3/__https://bugzilla.kernel.org/show_bug.cgi?id=216457__;!!CTRNKA9wMg0ARbw!mXW4Z05dX9YXnBYWxw-OOBYutqBM0JFoaApK61lFCSldptsVi0JEtWNSU9uaSnXbGq5oiKCBfyHLFFtEmY5uFgDl$ 

> > https://urldefense.com/v3/__https://bugzilla.kernel.org/show_bug.cgi?id=216457__;!!CTRNKA9wMg0ARbw!mXW4Z05dX9YXnBYWxw-OOBYutqBM0JFoaApK61lFCSldptsVi0JEtWNSU9uaSnXbGq5oiKCBfyHLFFtEmY5uFgDl$ 

[Kuan-Ying Lee]

Make KASAN scan metadata to infer the requested allocation size instead of
printing cache->object_size.

This patch fixes confusing slab-out-of-bounds reports as reported in:

https://bugzilla.kernel.org/show_bug.cgi?id=216457

Link: https://bugzilla.kernel.org/show_bug.cgi?id=216457

Co-developed-by: Andrey Konovalov <andre...@gmail.com>
Signed-off-by: Kuan-Ying Lee <Kuan-Y...@mediatek.com>

---

Changes v3->v4:

- Change the author

Changes v2->v3:

- Rename obj_size to alloc_size and change its type to size_t.
- Add comments into kasan_get_alloc_size.
- Infer and report alloc_size for all report types.
- Update metadata-related comment in kasan_find_first_bad_addr for HW_TAGS.

- Use addr_has_metadata for Generic and SW_TAGS modes instead of
open-coding KASAN_SHADOW_START checks.