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

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Message-Id: <20101114180643.593d19ac.akpm@linux-foundation.org>
Date:	Sun, 14 Nov 2010 18:06:43 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Vasiliy Kulikov <segoon@...nwall.com>
Cc:	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
Subject: Re: [PATCH v2] fs: select: fix information leak to userspace

On Sun, 14 Nov 2010 12:25:33 +0300 Vasiliy Kulikov <segoon@...nwall.com> wrote:

> On some architectures __kernel_suseconds_t is int.  On these archs
> struct timeval has padding bytes at the end.  This struct is copied to
> userspace with these padding bytes uninitialized.  This leads to leaking
> of contents of kernel stack memory.
> 
> Signed-off-by: Vasiliy Kulikov <segoon@...nwall.com>
> ---
>  Patch v1 used memset(), it was waste of cycles on almost all archs.
> 
>  Compile tested.
> 
>  fs/select.c |    7 ++++---
>  1 files changed, 4 insertions(+), 3 deletions(-)
> 
> diff --git a/fs/select.c b/fs/select.c
> index b7b10aa..43d4805 100644
> --- a/fs/select.c
> +++ b/fs/select.c
> @@ -288,7 +288,6 @@ static int poll_select_copy_remaining(struct timespec *end_time, void __user *p,
>  				      int timeval, int ret)
>  {
>  	struct timespec rts;
> -	struct timeval rtv;
>  
>  	if (!p)
>  		return ret;
> @@ -306,8 +305,10 @@ static int poll_select_copy_remaining(struct timespec *end_time, void __user *p,
>  		rts.tv_sec = rts.tv_nsec = 0;
>  
>  	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.

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