signals: avoid random wakeups in sigsuspend()

A random wakeup can get us out of sigsuspend() without TIF_SIGPENDING
being set.

Avoid that by making sure we were signaled, like sys_pause() does.
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Dmitry Vyukov <dvyukov@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

kernel/signal.c

@@ -3508,8 +3508,10 @@ static int sigsuspend(sigset_t *set)
 	current->saved_sigmask = current->blocked;
 	set_current_blocked(set);
 
+	while (!signal_pending(current)) {
 		__set_current_state(TASK_INTERRUPTIBLE);
 		schedule();
+	}
 	set_restore_sigmask();
 	return -ERESTARTNOHAND;
 }