UBUNTU: SAUCE: sched: panic on corrupted stack end

BugLink: http://bugs.launchpad.net/bugs/1588871

Until now, hitting this BUG_ON caused a recursive oops (because oops
handling involves do_exit(), which calls into the scheduler, which in
turn raises an oops), which caused stuff below the stack to be
overwritten until a panic happened (e.g. via an oops in interrupt context,
caused by the overwritten CPU index in the thread_info).

Just panic directly.
Signed-off-by: Jann Horn <jannh@google.com>
CVE-2016-1583
Acked-by: Tyler Hicks <tyhicks@canonical.com>
Acked-by: Luis Henriques <luis.henriques@canonical.com>
Signed-off-by: Kamal Mostafa <kamal@canonical.com>

kernel/sched/core.c

@@ -3008,7 +3008,8 @@ static noinline void __schedule_bug(struct task_struct *prev)
 static inline void schedule_debug(struct task_struct *prev)
 {
 #ifdef CONFIG_SCHED_STACK_END_CHECK
-	BUG_ON(task_stack_end_corrupted(prev));
+	if (task_stack_end_corrupted(prev))
+		panic("corrupted stack end detected inside scheduler\n");
 #endif
 
 	if (unlikely(in_atomic_preempt_off())) {