[PATCH] kasan: infer the requested size by scanning shadow memory

[Kuan-Ying Lee]

We scan the shadow memory to infer the requested size instead of
printing cache->object_size directly.

This patch will fix the confusing generic kasan report like below. [1]
Report shows "cache kmalloc-192 of size 192", but user
actually 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
==================================================================

After this patch, report will show "cache kmalloc-192 of size 184".

Link: https://bugzilla.kernel.org/show_bug.cgi?id=216457 [1]

Signed-off-by: Kuan-Ying Lee <Kuan-Ying.Lee@xxxxxxxxxxxx>
---
 mm/kasan/kasan.h          |  5 +++++
 mm/kasan/report.c         |  3 ++-
 mm/kasan/report_generic.c | 18 ++++++++++++++++++
 3 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/mm/kasan/kasan.h b/mm/kasan/kasan.h
index 32413f22aa82..7bb627d21580 100644
--- a/mm/kasan/kasan.h
+++ b/mm/kasan/kasan.h
@@ -340,8 +340,13 @@ static inline void kasan_print_address_stack_frame(const void *addr) { }
 
 #ifdef CONFIG_KASAN_GENERIC
 void kasan_print_aux_stacks(struct kmem_cache *cache, const void *object);
+int kasan_get_alloc_size(void *object_addr, struct kmem_cache *cache);
 #else
 static inline void kasan_print_aux_stacks(struct kmem_cache *cache, const void *object) { }
+static inline int kasan_get_alloc_size(void *object_addr, struct kmem_cache *cache)
+{
+	return cache->object_size;
+}
 #endif
 
 bool kasan_report(unsigned long addr, size_t size,
diff --git a/mm/kasan/report.c b/mm/kasan/report.c
index 1d02757e90a3..6de454bb2cad 100644
--- a/mm/kasan/report.c
+++ b/mm/kasan/report.c
@@ -236,12 +236,13 @@ static void describe_object_addr(const void *addr, struct kmem_cache *cache,
 {
 	unsigned long access_addr = (unsigned long)addr;
 	unsigned long object_addr = (unsigned long)object;
+	int real_size = kasan_get_alloc_size((void *)object_addr, cache);
 	const char *rel_type;
 	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);
+		object, cache->name, real_size);
 
 	if (access_addr < object_addr) {
 		rel_type = "to the left";
diff --git a/mm/kasan/report_generic.c b/mm/kasan/report_generic.c
index 043c94b04605..01b38e459352 100644
--- a/mm/kasan/report_generic.c
+++ b/mm/kasan/report_generic.c
@@ -43,6 +43,24 @@ void *kasan_find_first_bad_addr(void *addr, size_t size)
 	return p;
 }
 
+int kasan_get_alloc_size(void *addr, struct kmem_cache *cache)
+{
+	int size = 0;
+	u8 *shadow = (u8 *)kasan_mem_to_shadow(addr);
+
+	while (size < cache->object_size) {
+		if (*shadow == 0)
+			size += KASAN_GRANULE_SIZE;
+		else if (*shadow >= 1 && *shadow <= KASAN_GRANULE_SIZE - 1)
+			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";
-- 
2.18.0