timer: fix event destruction race

In current timer implementation, each event has an absolute time at which
it will be fired. When the first timer of the queue has not elapsed yet,
a pthread_cond_timedwait() is used to wait until the expected time.

Apparently that's fine. However the time passed to that function was a
pointer to the timespec structure contained in the event itself. This is
problematic because of how pthread_cond_timedwait() works internally.

Simplifying a bit, pthread_cond_timedwait() basically queues itself as a
waiter for the given condition variable and releases the mutex. Then it
does the timed wait using the passed value.

With that in mind, the follwing case is possible:

   Timer Thread                            Other Thread
   ------------                            ------------

					   gf_timer_call_cancel()
   pthread_mutex_lock()                    |
                                           + pthread_mutex_lock()
   event = current_event()                 |
   pthread_cond_timedwait(&event->at)      |
   + pthread_mutex_unlock()                |
   |                                       + remove_event()
   |                                       + destroy_event()
   + timed_wait(&event->at)

As we can see, the time is used after it has been destroyed, which means
we have a use-after-free problem.

This patch fixes the problem by copying the time to a local variable
before calling pthread_cond_timedwait()

Backport of:
> Change-Id: I0f4e8eded24fe3a1276dc75c6cf093bae973d26b
> Signed-off-by: Xavi Hernandez <xhernandez@redhat.com>
> Fixes: bz#1785208

Change-Id: I0f4e8eded24fe3a1276dc75c6cf093bae973d26b
Signed-off-by: Xavi Hernandez <xhernandez@redhat.com>
Fixes: bz#1767264

1 files changed, 2 insertions, 1 deletions

diff --git a/libglusterfs/src/timer.c b/libglusterfs/src/timer.c
index 1e19ffdff22..66c861b04cd 100644
--- a/libglusterfs/src/timer.c
+++ b/libglusterfs/src/timer.c
@@ -137,7 +137,8 @@ gf_timer_proc(void *data)
             timespec_now(&now);
             event = list_first_entry(&reg->active, gf_timer_t, list);
             if (TS(now) < TS(event->at)) {
-                pthread_cond_timedwait(&reg->cond, &reg->lock, &event->at);
+                now = event->at;
+                pthread_cond_timedwait(&reg->cond, &reg->lock, &now);
             } else {
                 event->fired = _gf_true;
                 list_del_init(&event->list);