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

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Message-ID: <4A7CA06E.1090904@us.ibm.com>
Date:	Fri, 07 Aug 2009 14:45:18 -0700
From:	Darren Hart <dvhltc@...ibm.com>
To:	"lkml, " <linux-kernel@...r.kernel.org>,
	linux-rt-users <linux-rt-users@...r.kernel.org>
CC:	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Ingo Molnar <mingo@...e.hu>, John Kacur <jkacur@...hat.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Dinakar Guniguntala <dino@...ibm.com>,
	John Stultz <johnstul@...ux.vnet.ibm.com>
Subject: [PATCH] futex: Fix handling of bad requeue syscall pairing

From: Darren Hart <dvhltc@...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@...ibm.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Ingo Molnar <mingo@...e.hu>
CC: Eric Dumazet <eric.dumazet@...il.com>
CC: John Kacur <jkacur@...hat.com>
CC: Dinakar Guniguntala <dino@...ibm.com>
CC: John Stultz <johnstul@...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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