[PATCH RT 3/6] sched: Prevent task state corruption by spurious lock wakeup

[Julia Cartwright]

4.1.42-rt50-rc1 stable review patch.
If you have any objection to the inclusion of this patch, let me know.

--- 8< --- 8< --- 8< ---
From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

Mathias and others reported GDB failures on RT.

The following scenario leads to task state corruption:

CPU0						CPU1

T1->state = TASK_XXX;
spin_lock(&lock)
  rt_spin_lock_slowlock(&lock->rtmutex)
    raw_spin_lock(&rtm->wait_lock);
    T1->saved_state = current->state;
    T1->state = TASK_UNINTERRUPTIBLE;
						spin_unlock(&lock)
    task_blocks_on_rt_mutex(rtm)  		  rt_spin_lock_slowunlock(&lock->rtmutex)
      queue_waiter(rtm)				    raw_spin_lock(&rtm->wait_lock);
      pi_chain_walk(rtm)
        raw_spin_unlock(&rtm->wait_lock);
						    wake_top_waiter(T1)

      raw_spin_lock(&rtm->wait_lock);

    for (;;) {
      if (__try_to_take_rt_mutex())  <- Succeeds
        break;
      ...
    }

    T1->state = T1->saved_state;
						     try_to_wake_up(T1)
						       ttwu_do_wakeup(T1)
						         T1->state = TASK_RUNNING;

In most cases this is harmless because waiting for some event, which is the
usual reason for TASK_[UN]INTERRUPTIBLE has to be safe against other forms
of spurious wakeups anyway.

But in case of TASK_TRACED this is actually fatal, because the task loses
the TASK_TRACED state. In consequence it fails to consume SIGSTOP which was
sent from the debugger and actually delivers SIGSTOP to the task which
breaks the ptrace mechanics and brings the debugger into an unexpected
state.

The TASK_TRACED state should prevent getting there due to the state
matching logic in try_to_wake_up(). But that's not true because
wake_up_lock_sleeper() uses TASK_ALL as state mask. That's bogus because
lock sleepers always use TASK_UNINTERRUPTIBLE, so the wakeup should use
that as well.

The cure is way simpler as figuring it out:

Change the mask used in wake_up_lock_sleeper() from TASK_ALL to
TASK_UNINTERRUPTIBLE.

Cc: stable-rt@xxxxxxxxxxxxxxx
Reported-by: Mathias Koehrer <mathias.koehrer@xxxxxxxx>
Reported-by: David Hauck <davidh@xxxxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
(cherry picked from commit 2f9f24e15088d2ef3244d088a9604d7e98c9c625)
Signed-off-by: Julia Cartwright <julia@xxxxxx>
---
 kernel/sched/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 0d3a40b24304..ee11a59e53ff 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1876,7 +1876,7 @@ EXPORT_SYMBOL(wake_up_process);
  */
 int wake_up_lock_sleeper(struct task_struct *p)
 {
-	return try_to_wake_up(p, TASK_ALL, WF_LOCK_SLEEPER);
+	return try_to_wake_up(p, TASK_UNINTERRUPTIBLE, WF_LOCK_SLEEPER);
 }
 
 int wake_up_state(struct task_struct *p, unsigned int state)
-- 
2.13.1