linux-kernel - Re: [PATCH] samples/seccomp: Zero out datastructures based on seccomp_notif

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Date:   Mon, 30 Dec 2019 14:02:29 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Sargun Dhillon <sargun@...gun.me>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Aleksa Sarai <cyphar@...har.com>,
        Tycho Andersen <tycho@...ho.ws>
Subject: Re: [PATCH] samples/seccomp: Zero out datastructures based on
 seccomp_notif_sizes

On Mon, Dec 30, 2019 at 12:35:03PM -0800, Sargun Dhillon wrote:
> The sizes by which seccomp_notif and seccomp_notif_resp are allocated are
> based on the SECCOMP_GET_NOTIF_SIZES ioctl. This allows for graceful
> extension of these datastructures. If userspace zeroes out the
> datastructure based on its version, and it is lagging behind the kernel's
> version, it will end up sending trailing garbage. On the other hand,
> if it is ahead of the kernel version, it will write extra zero space,
> and potentially cause corruption.
> 
> Signed-off-by: Sargun Dhillon <sargun@...gun.me>
> Suggested-by: Tycho Andersen <tycho@...ho.ws>
> Cc: Kees Cook <keescook@...omium.org>

Ah, cool. I've rearranged things and split the other patch's samples
changes into this one, etc etc :)

Thanks!

-Kees

> ---
>  samples/seccomp/user-trap.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/samples/seccomp/user-trap.c b/samples/seccomp/user-trap.c
> index 3e31ec0cf4a5..20291ec6489f 100644
> --- a/samples/seccomp/user-trap.c
> +++ b/samples/seccomp/user-trap.c
> @@ -302,10 +302,10 @@ int main(void)
>  		resp = malloc(sizes.seccomp_notif_resp);
>  		if (!resp)
>  			goto out_req;
> -		memset(resp, 0, sizeof(*resp));
> +		memset(resp, 0, sizes.seccomp_notif_resp);
>  
>  		while (1) {
> -			memset(req, 0, sizeof(*req));
> +			memset(req, 0, sizes.seccomp_notif);
>  			if (ioctl(listener, SECCOMP_IOCTL_NOTIF_RECV, req)) {
>  				perror("ioctl recv");
>  				goto out_resp;
> -- 
> 2.20.1
> 

-- 
Kees Cook