linux-kernel - Re: [PATCH V2] kasan: fix last shadow judgement in memory_is_poisoned

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Date:	Wed, 9 Sep 2015 15:43:21 +0800
From:	Xishi Qiu <qiuxishi@...wei.com>
To:	"long.wanglong" <long.wanglong@...wei.com>
CC:	<ryabinin.a.a@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrey Konovalov <adech.fo@...il.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Michal Marek <mmarek@...e.cz>, <zhongjiang@...wei.com>,
	Linux MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [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@...wei.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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