futex: Fix a race condition between REQUEUE_PI and task death

free_pi_state and exit_pi_state_list both clean up futex_pi_state's.
exit_pi_state_list takes the hb lock first, and most callers of
free_pi_state do too. requeue_pi doesn't, which means free_pi_state
can free the pi_state out from under exit_pi_state_list. For example:

task A                            |  task B
exit_pi_state_list                |
  pi_state =                      |
      curr->pi_state_list->next   |
                                  |  futex_requeue(requeue_pi=1)
                                  |    // pi_state is the same as
                                  |    // the one in task A
                                  |    free_pi_state(pi_state)
                                  |      list_del_init(&pi_state->list)
                                  |      kfree(pi_state)
  list_del_init(&pi_state->list)  |

Move the free_pi_state calls in requeue_pi to before it drops the hb
locks which it's already holding.

[ tglx: Removed a pointless free_pi_state() call and the hb->lock held
  	debugging. The latter comes via a seperate patch ]

Signed-off-by: Brian Silverman <bsilver16384@gmail.com>
Cc: austin.linux@gmail.com
Cc: darren@dvhart.com
Cc: peterz@infradead.org
Cc: stable@vger.kernel.org
Link: http://lkml.kernel.org/r/1414282837-23092-1-git-send-email-bsilver16384@gmail.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 11 insertions, 11 deletions

diff --git a/kernel/futex.c b/kernel/futex.c
index bbf071f325b8..63678b573d61 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -647,8 +647,14 @@ static struct futex_pi_state * alloc_pi_state(void)
         return pi_state;
 }
 
+/*
+ * Must be called with the hb lock held.
+ */
 static void free_pi_state(struct futex_pi_state *pi_state)
 {
+        if (!pi_state)
+                return;
+
         if (!atomic_dec_and_test(&pi_state->refcount))
                 return;
 
@@ -1527,15 +1533,6 @@ static int futex_requeue(u32 __user *uaddr1, unsigned int flags,
         }
 
 retry:
-        if (pi_state != NULL) {
-                /*
-                 * We will have to lookup the pi_state again, so free this one
-                 * to keep the accounting correct.
-                 */
-                free_pi_state(pi_state);
-                pi_state = NULL;
-        }
-
         ret = get_futex_key(uaddr1, flags & FLAGS_SHARED, &key1, VERIFY_READ);
         if (unlikely(ret != 0))
                 goto out;
@@ -1625,6 +1622,8 @@ retry_private:
                 case 0:
                         break;
                 case -EFAULT:
+                        free_pi_state(pi_state);
+                        pi_state = NULL;
                         double_unlock_hb(hb1, hb2);
                         hb_waiters_dec(hb2);
                         put_futex_key(&key2);
@@ -1640,6 +1639,8 @@ retry_private:
                          *   exit to complete.
                          * - The user space value changed.
                          */
+                        free_pi_state(pi_state);
+                        pi_state = NULL;
                         double_unlock_hb(hb1, hb2);
                         hb_waiters_dec(hb2);
                         put_futex_key(&key2);
@@ -1716,6 +1717,7 @@ retry_private:
         }
 
 out_unlock:
+        free_pi_state(pi_state);
         double_unlock_hb(hb1, hb2);
         hb_waiters_dec(hb2);
 
@@ -1733,8 +1735,6 @@ out_put_keys:
 out_put_key1:
         put_futex_key(&key1);
 out:
-        if (pi_state != NULL)
-                free_pi_state(pi_state);
         return ret ? ret : task_count;
 }