futex: Fix handling of bad requeue syscall pairing

If futex_requeue(requeue_pi=1) finds a futex_q that was created by a call
other the futex_wait_requeue_pi(), the q.rt_waiter may 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 mispairing in futex_requeue(), bail out, seding -EINVAL to
user-space.

V2: Fix parenthesis warnings.

Signed-off-by: Darren Hart <dvhltc@us.ibm.com>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: John Kacur <jkacur@redhat.com>
Cc: Eric Dumazet <eric.dumazet@gmail.com>
Cc: Dinakar Guniguntala <dino@in.ibm.com>
Cc: John Stultz <johnstul@linux.vnet.ibm.com>
LKML-Reference: <4A7CA8C0.7010809@us.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 9 insertions, 2 deletions

diff --git a/kernel/futex.c b/kernel/futex.c
index 8cc3ee1363a0..e18cfbdc7190 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1256,8 +1256,15 @@ 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