diff options
author | Sebastian Andrzej Siewior <bigeasy@linutronix.de> | 2016-04-15 14:35:39 +0200 |
---|---|---|
committer | Sasha Levin <sasha.levin@oracle.com> | 2016-05-10 12:17:31 -0400 |
commit | 28f83d2daecd97392c9a0580c00853a19a76a8bc (patch) | |
tree | 3936018bfd9dc6dd3d29d6f7bbb3b1ed21dce404 | |
parent | 6024877a19556baa28a422ff555c1e1e6058504d (diff) |
futex: Handle unlock_pi race gracefully
[ Upstream commit 89e9e66ba1b3bde9d8ea90566c2aee20697ad681 ]
If userspace calls UNLOCK_PI unconditionally without trying the TID -> 0
transition in user space first then the user space value might not have the
waiters bit set. This opens the following race:
CPU0 CPU1
uval = get_user(futex)
lock(hb)
lock(hb)
futex |= FUTEX_WAITERS
....
unlock(hb)
cmpxchg(futex, uval, newval)
So the cmpxchg fails and returns -EINVAL to user space, which is wrong because
the futex value is valid.
To handle this (yes, yet another) corner case gracefully, check for a flag
change and retry.
[ tglx: Massaged changelog and slightly reworked implementation ]
Fixes: ccf9e6a80d9e ("futex: Make unlock_pi more robust")
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Cc: stable@vger.kernel.org
Cc: Davidlohr Bueso <dave@stgolabs.net>
Cc: Darren Hart <dvhart@linux.intel.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1460723739-5195-1-git-send-email-bigeasy@linutronix.de
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
-rw-r--r-- | kernel/futex.c | 25 |
1 files changed, 22 insertions, 3 deletions
diff --git a/kernel/futex.c b/kernel/futex.c index b75fbddacf0e..46b168e19c98 100644 --- a/kernel/futex.c +++ b/kernel/futex.c @@ -1157,10 +1157,20 @@ static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q *this) */ newval = FUTEX_WAITERS | task_pid_vnr(new_owner); - if (cmpxchg_futex_value_locked(&curval, uaddr, uval, newval)) + if (cmpxchg_futex_value_locked(&curval, uaddr, uval, newval)) { ret = -EFAULT; - else if (curval != uval) - ret = -EINVAL; + } else if (curval != uval) { + /* + * If a unconditional UNLOCK_PI operation (user space did not + * try the TID->0 transition) raced with a waiter setting the + * FUTEX_WAITERS flag between get_user() and locking the hash + * bucket lock, retry the operation. + */ + if ((FUTEX_TID_MASK & curval) == uval) + ret = -EAGAIN; + else + ret = -EINVAL; + } if (ret) { raw_spin_unlock(&pi_state->pi_mutex.wait_lock); return ret; @@ -2419,6 +2429,15 @@ retry: */ if (ret == -EFAULT) goto pi_faulted; + /* + * A unconditional UNLOCK_PI op raced against a waiter + * setting the FUTEX_WAITERS bit. Try again. + */ + if (ret == -EAGAIN) { + spin_unlock(&hb->lock); + put_futex_key(&key); + goto retry; + } goto out_unlock; } |