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

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Message-ID: <CAPAsAGzeRz5hrvJiO94vFy3SEjcHW2vvPdA0DM3LF0HOtfLhyA@mail.gmail.com>
Date:	Tue, 8 Sep 2015 13:05:34 +0300
From:	Andrey Ryabinin <ryabinin.a.a@...il.com>
To:	Xishi Qiu <qiuxishi@...wei.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Andrey Konovalov <adech.fo@...il.com>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Michal Marek <mmarek@...e.cz>, Linux MM <linux-mm@...ck.org>,
	LKML <linux-kernel@...r.kernel.org>, zhongjiang@...wei.com
Subject: Re: [PATCH] kasan: fix last shadow judgement in memory_is_poisoned_16()

2015-09-08 12:52 GMT+03:00 Xishi Qiu <qiuxishi@...wei.com>:
> On 2015/9/8 17:49, Xishi Qiu wrote:
>
>> On 2015/9/8 17:36, Andrey Ryabinin wrote:
>>
>>> 2015-09-08 4:42 GMT+03:00 Xishi Qiu <qiuxishi@...wei.com>:
>>>> The shadow which correspond 16 bytes may span 2 or 3 bytes. If shadow
>>>> only take 2 bytes, we can return in "if (likely(!last_byte)) ...", but
>>>> it calculates wrong, so fix it.
>>>>
>>>
>>> Please, be more specific. Describe what is wrong with the current code and why,
>>> what's the effect of this bug and how you fixed it.
>>>
>>>
>>
>> If the 16 bytes 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);".
>> The code "if (likely(IS_ALIGNED(addr, 8)))" is wrong judgement.
>
> Sorry, a mistake, 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 "return memory_is_poisoned_1(addr + 15);"
>>

Right, put this into changelog please.

>> Thanks,
>> Xishi Qiu
>>
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