[PATCH V2] kasan: fix last shadow judgement in memory_is_poisoned_16()

[Xishi Qiu]

The shadow which correspond 16 bytes memory may span 2 or 3 bytes. If the
memory is aligned on 8, then the shadow takes only 2 bytes. So we check
"shadow_first_bytes" is enough, and need not to call "memory_is_poisoned_1(addr + 15);".
But the code "if (likely(!last_byte))" is wrong judgement.

e.g. addr=0, so last_byte = 15 & KASAN_SHADOW_MASK = 7, then the code will
continue to call "memory_is_poisoned_1(addr + 15);"

Signed-off-by: Xishi Qiu <qiuxishi@xxxxxxxxxx>
---
 mm/kasan/kasan.c |    3 +--
 1 files changed, 1 insertions(+), 2 deletions(-)

diff --git a/mm/kasan/kasan.c b/mm/kasan/kasan.c
index 7b28e9c..8da2114 100644
--- a/mm/kasan/kasan.c
+++ b/mm/kasan/kasan.c
@@ -135,12 +135,11 @@ static __always_inline bool memory_is_poisoned_16(unsigned long addr)
 
 	if (unlikely(*shadow_addr)) {
 		u16 shadow_first_bytes = *(u16 *)shadow_addr;
-		s8 last_byte = (addr + 15) & KASAN_SHADOW_MASK;
 
 		if (unlikely(shadow_first_bytes))
 			return true;
 
-		if (likely(!last_byte))
+		if (likely(IS_ALIGNED(addr, 8)))
 			return false;
 
 		return memory_is_poisoned_1(addr + 15);
-- 
1.7.1


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