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

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Date:	Tue, 16 Nov 2010 13:19:36 +0200
From:	Boaz Harrosh <bharrosh@...asas.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	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

On 11/15/2010 09:12 PM, 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.
> 
> This kind of construct is widely used in networking tree.
> 
> Maybe we should ask to gcc experts if this behavior is guaranteed by
> gcc, or if we must review our code ;(
> 
> CC Jakub
> 
> Thanks !
> 

This is what I thought too. If it is not there are tones of bugs I wrote
of code that relays on this behaviour.

It would be interesting to know for sure

Thanks
Boaz
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