kasan: infer allocation size by scanning metadata

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.

[akpm@linux-foundation.org: fix printk warning]
Link: https://bugzilla.kernel.org/show_bug.cgi?id=216457
Link: https://lkml.kernel.org/r/20230129021437.18812-1-Kuan-Ying.Lee@mediatek.com
Signed-off-by: Kuan-Ying Lee <Kuan-Ying.Lee@mediatek.com>
Co-developed-by: Andrey Konovalov <andreyknvl@gmail.com>
Cc: Alexander Potapenko <glider@google.com>
Cc: Andrey Ryabinin <ryabinin.a.a@gmail.com>
Cc: Chinwen Chang <chinwen.chang@mediatek.com>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Matthias Brugger <matthias.bgg@gmail.com>
Cc: Qun-Wei Lin <qun-wei.lin@mediatek.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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;

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);
 

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%zu-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;
 

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:

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;

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);

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)

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));