[PATCH] futex: Fix handling of bad requeue syscall pairing

[Darren Hart <dvhltc@us.ibm.com>]

From: Darren Hart <dvhltc@us.ibm.com>

If futex_requeue(requeue_pi=1) finds a futex_q that was created by a call
other than futex_wait_requeue_pi(), the q.rt_waiter will be null.  If so,
this will result in an oops from the following call graph:

futex_requeue()
  rt_mutex_start_proxy_lock()
    task_blocks_on_rt_mutex()
      waiter->task dereference
        OOPS

We currently WARN_ON() if this is detected, clearly this is inadequate.
If we detect a mis-pairing in futex_requeue(), bail out, sending -EINVAL
to user-space.

Signed-off-by: Darren Hart <dvhltc@us.ibm.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Ingo Molnar <mingo@elte.hu>
CC: Eric Dumazet <eric.dumazet@gmail.com>
CC: John Kacur <jkacur@redhat.com>
CC: Dinakar Guniguntala <dino@in.ibm.com>
CC: John Stultz <johnstul@us.ibm.com>
---

 kernel/futex.c |   11 +++++++++--
 1 files changed, 9 insertions(+), 2 deletions(-)


diff --git a/kernel/futex.c b/kernel/futex.c
index 578c7b7..d806d6c 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1306,8 +1306,15 @@ retry_private:
 		if (!match_futex(&this->key, &key1))
 			continue;
 
-		WARN_ON(!requeue_pi && this->rt_waiter);
-		WARN_ON(requeue_pi && !this->rt_waiter);
+		/*
+		 * FUTEX_WAIT_REQEUE_PI and FUTEX_CMP_REQUEUE_PI should always
+		 * be paired with each other and no other futex ops.
+		 */
+		if (requeue_pi && !this->rt_waiter ||
+		    !requeue_pi && this->rt_waiter) {
+			ret = -EINVAL;
+			break;
+		}
 
 		/*
 		 * Wake nr_wake waiters.  For requeue_pi, if we acquired the
-- 
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team