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SubjectRe: [PATCH V2] kasan: fix last shadow judgement in memory_is_poisoned_16()
On 2015/9/9 14:40, long.wanglong wrote:

> 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!
>

Hi Wanglong,

How about use IS_ALIGNED instead of those code in memory_is_poisoned_8()
and other functions? I think the current code looks a bit ugly.

Thanks,
Xishi Qiu

>
> Best Regards
> Wang Long
>
>
>
>
>
> .
>





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