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

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Message-ID: <1290471649.2704.24.camel@edumazet-laptop>
Date:	Tue, 23 Nov 2010 01:20:48 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Boaz Harrosh <bharrosh@...asas.com>,
	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

Le lundi 22 novembre 2010 à 15:50 -0800, Andrew Morton a écrit :

> Well.  We certainly assume in many places that
> 
> 	struct foo {
> 		int a;
> 		int b;
> 	} f = {
> 		.a = 1,
> 	};
> 
> will initialise b to zero.  But I doubt if much code at all assumes
> that this initialisation patterm will reliably zero out *holes* in the
> struct.
> 

We did such assertions in the past, we were wrong.

Check commit 1c40be12f7d8ca1d387510d39787b12e512a7ce8 for an example
(net sched: fix some kernel memory leaks)

I guess we must make a full audit of all C99 initializers or structures
copied to userspace, giving a name to hidden holes, to force gcc to init
them to 0.

# cat try.c
struct s {
	char c;
	long l;
	};

void bar(void *v)
{
	unsigned long *p = v;

	printf("%lx %lx\n", p[0], p[1]);
}

int main()
{
	struct s s1 = {
		.c = 1,
		.l = 2,
	};

	bar(&s1);
	return 0;
}

# gcc -O2 -o try try.c
# ./try
8049401 2

Strangely, if we remove ".l = 2," line, gcc emits code to clear al the
fields

main:
	pushl	%ebp
	movl	%esp, %ebp
	andl	$-16, %esp
	subl	$32, %esp
	leal	24(%esp), %eax
	movl	$0, 24(%esp)
	movl	%eax, (%esp)
	movl	$0, 28(%esp)
	movb	$1, 24(%esp)
	call	bar
	xorl	%eax, %eax
	leave
	ret


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