On 06/24, Kees Cook wrote:

+static void seccomp_sync_threads(void)
+{
+	struct task_struct *thread, *caller;
+
+	BUG_ON(!spin_is_locked(&current->sighand->siglock));
+
+	/* Synchronize all threads. */
+	caller = current;
+	for_each_thread(caller, thread) {
+		/* Get a task reference for the new leaf node. */
+		get_seccomp_filter(caller);
+		/*
+		 * Drop the task reference to the shared ancestor since
+		 * current's path will hold a reference.  (This also
+		 * allows a put before the assignment.)
+		 */
+		put_seccomp_filter(thread);
+		thread->seccomp.filter = caller->seccomp.filter;
+		/* Opt the other thread into seccomp if needed.
+		 * As threads are considered to be trust-realm
+		 * equivalent (see ptrace_may_access), it is safe to
+		 * allow one thread to transition the other.
+		 */
+		if (thread->seccomp.mode == SECCOMP_MODE_DISABLED) {
+			/*
+			 * Don't let an unprivileged task work around
+			 * the no_new_privs restriction by creating
+			 * a thread that sets it up, enters seccomp,
+			 * then dies.
+			 */
+			if (task_no_new_privs(caller))
+				task_set_no_new_privs(thread);
+
+			seccomp_assign_mode(thread, SECCOMP_MODE_FILTER);
+		}
+	}
+}

OK, personally I think this all make sense. I even think that perhaps
SECCOMP_FILTER_FLAG_TSYNC should allow filter == NULL, a thread might
want to "sync" without adding the new filter, but this is minor/offtopic.

But. Doesn't this change add the new security hole?

Obviously, we should not allow to install a filter and then (say) exec
a suid binary, that is why we have no_new_privs/LSM_UNSAFE_NO_NEW_PRIVS.

But what if "thread->seccomp.filter = caller->seccomp.filter" races with
any user of task_no_new_privs() ? Say, suppose this thread has already
passed check_unsafe_exec/etc and it is going to exec the suid binary?

Oleg.