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

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Date:	Wed, 9 Sep 2015 16:10:01 +0800
From:	"long.wanglong" <long.wanglong@...wei.com>
To:	Xishi Qiu <qiuxishi@...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 15:43, Xishi Qiu wrote:
> 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.
> 

I think we can use  IS_ALIGNED macro in memory_is_poisoned_8 and memory_is_poisoned_16.
but for functions memory_is_poisoned_2 and memory_is_poisoned_4, we can't.

Wang Long


> Thanks,
> Xishi Qiu
> 
>>
>> Best Regards
>> Wang Long
>>
>>
>>
>>
>>
>> .
>>
> 
> 
> 
> 
> .
> 


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