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

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Message-ID: <20101123140111.GA3816@hack>
Date:	Tue, 23 Nov 2010 22:01:11 +0800
From:	Américo Wang <xiyou.wangcong@...il.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 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.

-- 
Live like a child, think like the god.
 
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