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

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Date:	Tue, 16 Nov 2010 12:08:56 -0800
From:	Joe Perches <joe@...ches.com>
To:	Kees Cook <kees.cook@...onical.com>
Cc:	linux-kernel@...r.kernel.org, Pekka Enberg <penberg@...nel.org>,
	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>,
	Vasiliy Kulikov <segooon@...il.com>
Subject: Re: [PATCH] ipc: explicitly clear stack memory for shminfo

On Tue, 2010-11-16 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.
> 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

Hi Kees.

Trivial size optimization:

Perhaps it's better to use
	struct type var = {};
instead of
	struct type var;
	memset(&var, 0, sizeof(var));

At least for x86, gcc produces very slightly smaller code
when there are other automatic variables like:

Larger:
	struct type var;
	struct type var2;
	memset(&var, 0, sizeof(var));
Smaller:
	struct type var = {};
	struct type var2;

On the other hand, memset is more obvious.

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