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

[Tycho Andersen]

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;
+
 	/* 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) {
 			listener_f->private_data = NULL;
 			fput(listener_f);
 			put_unused_fd(listener);
-- 
2.19.1