Commit 30a6b803 authored by Brian Silverman's avatar Brian Silverman Committed by Thomas Gleixner
Browse files

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: default avatarBrian 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: default avatarThomas Gleixner <tglx@linutronix.de>
parent 993b2ff2

kernel/futex.c

+11 −11
Original line number Diff line number Diff line
@@ -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 @@ static int futex_requeue(u32 __user *uaddr1, unsigned int flags, 
		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 @@ static int futex_requeue(u32 __user *uaddr1, unsigned int flags, 
			 *   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 @@ static int futex_requeue(u32 __user *uaddr1, unsigned int flags, 
	}



out_unlock:

	free_pi_state(pi_state);

	double_unlock_hb(hb1, hb2);

	hb_waiters_dec(hb2);
@@ -1733,8 +1735,6 @@ static int futex_requeue(u32 __user *uaddr1, unsigned int flags, 
out_put_key1:

	put_futex_key(&key1);

out:

	if (pi_state != NULL)

		free_pi_state(pi_state);

	return ret ? ret : task_count;

}