linux-kernel - Re: [PATCH] ipc: explicitly clear stack memory for shminfo

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Message-ID: <20101116201620.GA11278@albatros>
Date:	Tue, 16 Nov 2010 23:16:20 +0300
From:	Vasiliy Kulikov <segoon@...nwall.com>
To:	Kees Cook <kees.cook@...onical.com>
Cc:	linux-kernel@...r.kernel.org, Pekka Enberg <penberg@...nel.org>,
	Joe Perches <joe@...ches.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Jiri Slaby <jslaby@...e.cz>,
	"David S. Miller" <davem@...emloft.net>,
	Hugh Dickins <hughd@...gle.com>,
	Manfred Spraul <manfred@...orfullife.com>
Subject: Re: [PATCH] ipc: explicitly clear stack memory for shminfo

Hi Kees,

On Tue, Nov 16, 2010 at 11:58 -0800, Kees Cook wrote:
> This fixes a kernel stack memory contents leak by explicitly clearing
> the shminfo structure on the kernel stack before it is populated and
> copied back to userspace.
> 
> Signed-off-by: Kees Cook <kees.cook@...onical.com>
> Acked-by: Pekka Enberg <penberg@...nel.org>
> ---
>  ipc/shm.c |    1 +
>  1 files changed, 1 insertions(+), 0 deletions(-)
> 
> diff --git a/ipc/shm.c b/ipc/shm.c
> index 7d3bb22..1d3d41f 100644
> --- a/ipc/shm.c
> +++ b/ipc/shm.c
> @@ -531,6 +531,7 @@ static inline unsigned long copy_shminfo_to_user(void __user *buf, struct shminf
>  	    {
>  		struct shminfo out;
>  
> +		memset(&out, 0, sizeof(out));
>  		if(in->shmmax > INT_MAX)
>  			out.shmmax = INT_MAX;
>  		else
> -- 
> 1.7.2.3

Can you please clarify what fields (padding bytes?) are uninitialized
here?  I see this struct shminfo definition:

    /* Obsolete, used only for backwards compatibility */
    struct	shminfo {
        int shmmax;
        int shmmin;
        int shmmni;
        int shmseg;
        int shmall;
    };

And this filling:

		if(in->shmmax > INT_MAX)
			out.shmmax = INT_MAX;
		else
			out.shmmax = (int)in->shmmax;

		out.shmmin	= in->shmmin;
		out.shmmni	= in->shmmni;
		out.shmseg	= in->shmseg;
		out.shmall	= in->shmall; 

		return copy_to_user(buf, &out, sizeof(out));

As I see all five fields are set anyway, no padding bytes here, correct?


Thanks,

-- 
Vasiliy Kulikov
http://www.openwall.com - bringing security into open computing environments
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