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futex: Fix a race condition between REQUEUE_PI and task death

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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>
This commit is contained in:
Brian Silverman 2014-10-25 20:20:37 -04:00 committed by Thomas Gleixner
parent 993b2ff221
commit 30a6b8031f
1 changed files with 11 additions and 11 deletions
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22
kernel/futex.c
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@ -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;
}