Commit 561c943e authored by Sebastian Andrzej Siewior's avatar Sebastian Andrzej Siewior Committed by Sasha Levin

futex: Handle unlock_pi race gracefully

[ Upstream commit 89e9e66b ]

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: ccf9e6a8 ("futex: Make unlock_pi more robust")
Signed-off-by: default avatarSebastian 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.deSigned-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Signed-off-by: default avatarSasha Levin <sasha.levin@oracle.com>
parent 0a3c8132
......@@ -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 @@ static int futex_unlock_pi(u32 __user *uaddr, unsigned int flags)
*/
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;
}
......
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