futex: Fix potential use-after-free in FUTEX_REQUEUE_PI

commit c236c8e95a3d395b0494e7108f0d41cf36ec107c upstream.

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
Link: http://lkml.kernel.org/r/20170304093558.801744246@infradead.org
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/kernel/futex.c b/kernel/futex.c
index d82fdbb..ad69658 100644 (file)
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -2410,7 +2410,6 @@ 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;
@@ -2500,6 +2499,8 @@ 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
@@ -2523,18 +2524,19 @@ 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