linux-kernel - Re: [PATCH v2] fs: select: fix information leak to userspace

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Date:	Tue, 23 Nov 2010 15:45:18 +0100
From:	walter harms <wharms@....de>
To:	Américo Wang <xiyou.wangcong@...il.com>
CC:	Eric Dumazet <eric.dumazet@...il.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Vasiliy Kulikov <segoon@...nwall.com>,
	Andreas Dilger <adilger.kernel@...ger.ca>,
	kernel-janitors@...r.kernel.org,
	Alexander Viro <viro@...iv.linux.org.uk>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	Jakub Jelinek <jakub@...hat.com>
Subject: Re: [PATCH v2] fs: select: fix information leak to userspace



Am 23.11.2010 15:01, schrieb Américo Wang:
> On Mon, Nov 15, 2010 at 08:12:21PM +0100, Eric Dumazet wrote:
>> Le dimanche 14 novembre 2010 à 18:06 -0800, Andrew Morton a écrit : 
>>> On Sun, 14 Nov 2010 12:25:33 +0300 Vasiliy Kulikov <segoon@...nwall.com> wrote:
>>>>  
>>>>  	if (timeval) {
>>>> -		rtv.tv_sec = rts.tv_sec;
>>>> -		rtv.tv_usec = rts.tv_nsec / NSEC_PER_USEC;
>>>> +		struct timeval rtv = {
>>>> +			.tv_sec = rts.tv_sec,
>>>> +			.tv_usec = rts.tv_nsec / NSEC_PER_USEC
>>>> +		};
>>>>  
>>>>  		if (!copy_to_user(p, &rtv, sizeof(rtv)))
>>>>  			return ret;
>>>
>>> Please check the assembly code - this will still leave four bytes of
>>> uninitalised stack data in 'rtv', surely.
>>
>> Thats a good question.
>>
>> In my understanding, gcc should initialize all holes (and other not
>> mentioned fields) with 0, even for automatic storage [C99 only mandates
>> this on static storage]
>>
>> I tested on x86_64 and this is the case, but could not find a definitive
>> answer in gcc documentation.
>>
> 
> Yeah, this is not clearly defined by C99 I think, but we can still
> find some clues in 6.2.6.1, Paragraph 6,
> 
> "
> When a value is stored in an object of structure or union type,
> including in a member object, the bytes of the object representation
> that correspond to any padding bytes take unspecified values.
> "
> 
> So we can't rely on the compiler to initialize the padding bytes
> too.
> 
hi all,
as we see this is not a question of c99.
Maybe we can convince the gcc people to make 0 padding default. That will not solve the
problems for other compilers but when they claim "works like gcc" we can press then to
support this also. I can imagine that this will close some other subtle leaks also.

People that still want a "undefined" (for what ever reason) can use an option to enable it
again (e.g.  --no-zero-padding).

do anyone have a contact so we can forward that request ?

re,
 wh
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