Commit 89e9e66b authored Apr 15, 2016 by Sebastian Andrzej Siewior Committed by Thomas Gleixner Apr 20, 2016

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: 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>

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 @@ 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;
		}
		/*
		* wake_futex_pi has detected invalid state. Tell user
		* space.

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