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Subject[tip:locking/urgent] futex: Fix a race condition between REQUEUE_PI and task death
Commit-ID:  30a6b8031fe14031ab27c1fa3483cb9780e7f63c
Gitweb: http://git.kernel.org/tip/30a6b8031fe14031ab27c1fa3483cb9780e7f63c
Author: Brian Silverman <bsilver16384@gmail.com>
AuthorDate: Sat, 25 Oct 2014 20:20:37 -0400
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitDate: Sun, 26 Oct 2014 16:16:18 +0100

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>
---
kernel/futex.c | 22 +++++++++++-----------
1 file changed, 11 insertions(+), 11 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index bbf071f..63678b5 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;
}


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