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

long.wanglong
2015-09-09T06:46:48Z

On 2015/9/8 20:12, Xishi Qiu wrote:
> 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@huawei.com>
> ---
> 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);
>

Hi,
I also notice this problem, how about another method to fix it:

diff --git a/mm/kasan/kasan.c b/mm/kasan/kasan.c
index 5d65d06..6a20dda 100644
--- a/mm/kasan/kasan.c
+++ b/mm/kasan/kasan.c
@@ -140,7 +140,7 @@ static __always_inline bool memory_is_poisoned_16(unsigned long addr)
if (unlikely(shadow_first_bytes))
return true;

- if (likely(!last_byte))
+ if (likely(last_byte >= 7))
return false;

return memory_is_poisoned_1(addr + 15);

This method can ensure consistency of code, for example, in memory_is_poisoned_8:

static __always_inline bool memory_is_poisoned_8(unsigned long addr)
{
u16 *shadow_addr = (u16 *)kasan_mem_to_shadow((void *)addr);

if (unlikely(*shadow_addr)) {
if (memory_is_poisoned_1(addr + 7))
return true;

if (likely(((addr + 7) & KASAN_SHADOW_MASK) >= 7))
return false;

return unlikely(*(u8 *)shadow_addr);
}

return false;
}

Otherwise, we also should use IS_ALIGNED macro in memory_is_poisoned_8!

Best Regards
Wang Long