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

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