Re: [PATCH 2/2] seccomp: disallow NEW_LISTENER and TSYNC flags

[Christian Brauner]

On Wed, Mar 06, 2019 at 01:14:13PM -0700, Tycho Andersen wrote:
> As the comment notes, the return codes for TSYNC and NEW_LISTENER conflict,
> because they both return positive values, one in the case of success and
> one in the case of error. So, let's disallow both of these flags together.
> 
> While this is technically a userspace break, all the users I know of are
> still waiting on me to land this feature in libseccomp, so I think it'll be
> safe. Also, at present my use case doesn't require TSYNC at all, so this
> isn't a big deal to disallow. If someone wanted to support this, a path
> forward would be to add a new flag like
> TSYNC_AND_LISTENER_YES_I_UNDERSTAND_THAT_TSYNC_WILL_JUST_RETURN_EAGAIN, but
> the use cases are so different I don't see it really happening.
> 
> Finally, it's worth noting that this does actually fix a UAF issue: at the end
> of seccomp_set_mode_filter(), we have:
> 
>         if (flags & SECCOMP_FILTER_FLAG_NEW_LISTENER) {
>                 if (ret < 0) {
>                         listener_f->private_data = NULL;
>                         fput(listener_f);
>                         put_unused_fd(listener);
>                 } else {
>                         fd_install(listener, listener_f);
>                         ret = listener;
>                 }
>         }
> out_free:
>         seccomp_filter_free(prepared);
> 
> But if ret > 0 because TSYNC raced, we'll install the listener fd and then free
> the filter out from underneath it, causing a UAF when the task closes it or
> dies. This patch also switches the condition to be simply if (ret), so that
> if someone does add the flag mentioned above, they won't have to remember
> to fix this too.
> 
> Signed-off-by: Tycho Andersen <tycho@xxxxxxxx>
> Fixes: 6a21cc50f0c7 ("seccomp: add a return code to trap to userspace")
> CC: stable@xxxxxxxxxxxxxxx # v5.0+
> ---
>  kernel/seccomp.c | 17 +++++++++++++++--
>  1 file changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index d0d355ded2f4..79bada51091b 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -500,7 +500,10 @@ seccomp_prepare_user_filter(const char __user *user_filter)
>   *
>   * Caller must be holding current->sighand->siglock lock.
>   *
> - * Returns 0 on success, -ve on error.
> + * Returns 0 on success, -ve on error, or
> + *   - in TSYNC mode: the pid of a thread which was either not in the correct
> + *     seccomp mode or did not have an ancestral seccomp filter
> + *   - in NEW_LISTENER mode: the fd of the new listener
>   */
>  static long seccomp_attach_filter(unsigned int flags,
>  				  struct seccomp_filter *filter)
> @@ -1256,6 +1259,16 @@ static long seccomp_set_mode_filter(unsigned int flags,
>  	if (flags & ~SECCOMP_FILTER_FLAG_MASK)
>  		return -EINVAL;
>  
> +	/*
> +	 * In the successful case, NEW_LISTENER returns the new listener fd.
> +	 * But in the failure case, TSYNC returns the thread that died. If you
> +	 * combine these two flags, there's no way to tell whether something
> +	 * succeded or failed. So, let's disallow this combination.
> +	 */
> +	if ((flags & SECCOMP_FILTER_FLAG_TSYNC) &&
> +	    (flags && SECCOMP_FILTER_FLAG_NEW_LISTENER))
> +		return -EINVAL;

May license a manpage entry that this makes it potentially unsafe to use
with multiple threads. But I don't see a use-case for this right now so
it looks sane to me. :)

(Though one simple question below.)

Acked-by: Christian Brauner <christian.brauner@xxxxxxxxxx>

> +
>  	/* Prepare the new filter before holding any locks. */
>  	prepared = seccomp_prepare_user_filter(filter);
>  	if (IS_ERR(prepared))
> @@ -1302,7 +1315,7 @@ static long seccomp_set_mode_filter(unsigned int flags,
>  		mutex_unlock(&current->signal->cred_guard_mutex);
>  out_put_fd:
>  	if (flags & SECCOMP_FILTER_FLAG_NEW_LISTENER) {
> -		if (ret < 0) {
> +		if (ret) {

Why that change but keep checking if (ret < 0) further up?

>  			listener_f->private_data = NULL;
>  			fput(listener_f);
>  			put_unused_fd(listener);
> -- 
> 2.19.1
>