futex: Fix potential use-after-free in FUTEX_REQUEUE_PI

While working on the futex code, I stumbled over this potential
use-after-free scenario. Dmitry triggered it later with syzkaller.

pi_mutex is a pointer into pi_state, which we drop the reference on in
unqueue_me_pi(). So any access to that pointer after that is bad.

Since other sites already do rt_mutex_unlock() with hb->lock held, see
for example futex_lock_pi(), simply move the unlock before
unqueue_me_pi().

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Darren Hart <dvhart@linux.intel.com>
Cc: juri.lelli@arm.com
Cc: bigeasy@linutronix.de
Cc: xlpang@redhat.com
Cc: rostedt@goodmis.org
Cc: mathieu.desnoyers@efficios.com
Cc: jdesfossez@efficios.com
Cc: dvhart@infradead.org
Cc: bristot@redhat.com
Cc: stable@vger.kernel.org
Link: http://lkml.kernel.org/r/20170304093558.801744246@infradead.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 11 insertions, 9 deletions

diff --git a/kernel/futex.c b/kernel/futex.c
index 229a744b1781..3a4775fd7468 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -2815,7 +2815,6 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
 {
         struct hrtimer_sleeper timeout, *to = NULL;
         struct rt_mutex_waiter rt_waiter;
-        struct rt_mutex *pi_mutex = NULL;
         struct futex_hash_bucket *hb;
         union futex_key key2 = FUTEX_KEY_INIT;
         struct futex_q q = futex_q_init;
@@ -2907,6 +2906,8 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
                         spin_unlock(q.lock_ptr);
                 }
         } else {
+                struct rt_mutex *pi_mutex;
+
                 /*
                  * We have been woken up by futex_unlock_pi(), a timeout, or a
                  * signal.  futex_unlock_pi() will not destroy the lock_ptr nor
@@ -2930,18 +2931,19 @@ static int futex_wait_requeue_pi(u32 __user *uaddr, unsigned int flags,
                 if (res)
                         ret = (res < 0) ? res : 0;
 
+                /*
+                 * If fixup_pi_state_owner() faulted and was unable to handle
+                 * the fault, unlock the rt_mutex and return the fault to
+                 * userspace.
+                 */
+                if (ret && rt_mutex_owner(pi_mutex) == current)
+                        rt_mutex_unlock(pi_mutex);
+
                 /* Unqueue and drop the lock. */
                 unqueue_me_pi(&q);
         }
 
-        /*
-         * If fixup_pi_state_owner() faulted and was unable to handle the
-         * fault, unlock the rt_mutex and return the fault to userspace.
-         */
-        if (ret == -EFAULT) {
-                if (pi_mutex && rt_mutex_owner(pi_mutex) == current)
-                        rt_mutex_unlock(pi_mutex);
-        } else if (ret == -EINTR) {
+        if (ret == -EINTR) {
                 /*
                  * We've already been requeued, but cannot restart by calling
                  * futex_lock_pi() directly. We could restart this syscall, but