Re: [PATCH v11 07/12] seccomp: add SECCOMP_RET_ERRNO

[Kees Cook <keescook@chromium.org>]

On Fri, Feb 24, 2012 at 7:21 PM, Will Drewry <wad@chromium.org> wrote:
> This change adds the SECCOMP_RET_ERRNO as a valid return value from a
> seccomp filter.  Additionally, it makes the first use of the lower
> 16-bits for storing a filter-supplied errno.  16-bits is more than
> enough for the errno-base.h calls.
>
> Returning errors instead of immediately terminating processes that
> violate seccomp policy allow for broader use of this functionality
> for kernel attack surface reduction.  For example, a linux container
> could maintain a whitelist of pre-existing system calls but drop
> all new ones with errnos.  This would keep a logically static attack
> surface while providing errnos that may allow for graceful failure
> without the downside of do_exit() on a bad call.
>
> v11: - check for NULL filter (keescook@chromium.org)
> v10: - change loaders to fn
>  v9: - n/a
>  v8: - update Kconfig to note new need for syscall_set_return_value.
>     - reordered such that TRAP behavior follows on later.
>     - made the for loop a little less indent-y
>  v7: - introduced
>
> Signed-off-by: Will Drewry <wad@chromium.org>

Reviewed-by: Kees Cook <keescook@chromium.org>

> +       /* Ensure unexpected behavior doesn't result in failing open. */
> +       if (unlikely(current->seccomp.filter == NULL))
> +               ret = SECCOMP_RET_KILL;

Any reason to not just immediately return in this case?

-Kees

-- 
Kees Cook
ChromeOS Security