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Commit 89e9e66b authored by Sebastian Andrzej Siewior's avatar Sebastian Andrzej Siewior Committed by Thomas Gleixner
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futex: Handle unlock_pi race gracefully 



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.de


Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
parent 66876595

kernel/futex.c

+22 −3
Original line number Diff line number Diff line
@@ -1295,10 +1295,20 @@ static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q *this, 
	if (unlikely(should_fail_futex(true)))

		ret = -EFAULT;



	if (cmpxchg_futex_value_locked(&curval, uaddr, uval, newval))

	if (cmpxchg_futex_value_locked(&curval, uaddr, uval, newval)) {

		ret = -EFAULT;

	else if (curval != uval)

	} 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_irq(&pi_state->pi_mutex.wait_lock);

		return ret;
@@ -2622,6 +2632,15 @@ 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;

		}

		/*

		 * wake_futex_pi has detected invalid state. Tell user

		 * space.