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

[Joe Perches <joe@perches.com>]

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.