kasan: fix last shadow judgement in memory_is_poisoned_16()

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 <[e-mail]>
Acked-by: Andrey Ryabinin <[e-mail]>
Cc: Andrey Konovalov <[e-mail]>
Cc: Rusty Russell <[e-mail]>
Cc: Michal Marek <[e-mail]>
Cc: <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

mm/kasan/kasan.c

···
 
 	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);