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From: Sebastian Andrzej Siewior <>
To: Thomas Gleixner <>
Cc: Davidlohr Bueso <>,,
	Sebastian Andrzej Siewior <>
Subject: [PATCH] kernel/futex: handle the case where we got a "late" waiter
Date: Fri, 15 Apr 2016 14:35:39 +0200
Message-ID: <> (raw)

futex_unlock_pi() gets uval before taking the hb lock. Now imagine
someone in futex_lock_pi() took the lock. While futex_unlock_pi() waits
for the hb lock, the LOCK_PI sets FUTEX_WAITERS and drops the lock.
Now, futex_unlock_pi() figures out that there is waiter and invokes
wake_futex_pi() with the old uval which does not yet have FUTEX_WAITERS
set. This flaw lets cmpxchg_futex_value_locked() fail and return -EINVAL.

To address this race we could either use get_futex_value_locked() after
we obtained the lock but then we would need to handle the EFAULT case
(which I think means get_user() without the lock).
To avoid this I let wake_futex_pi() detect this and return -EAGAIN so it
can read the new value and try again.

As bad as this may sound: FUTEX_OWNER_DIED is not set while the owner is
unlocking the lock. FUTEX_WAITERS gets set by another task *but* this
means that the owner invoked the futex() syscall rather than doing a
cmpxchg() in userland (and libc does not do that).

Fixes: ccf9e6a80d9e ("futex: Make unlock_pi more robust")
Signed-off-by: Sebastian Andrzej Siewior <>
 kernel/futex.c | 16 ++++++++++++++--
 1 file changed, 14 insertions(+), 2 deletions(-)

diff --git a/kernel/futex.c b/kernel/futex.c
index 0d557e13823a..2911f54a6dde 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -1496,8 +1496,12 @@ static int wake_futex_pi(u32 __user *uaddr, u32 uval, struct futex_q *this,
 	if (cmpxchg_futex_value_locked(&curval, uaddr, uval, newval))
 		ret = -EFAULT;
-	else if (curval != uval)
-		ret = -EINVAL;
+	else if (curval != uval) {
+		if ((FUTEX_TID_MASK & curval) == uval)
+			ret = -EAGAIN;
+		else
+			ret = -EINVAL;
+	}
 	if (ret) {
 		return ret;
@@ -2852,6 +2856,12 @@ static int futex_unlock_pi(u32 __user *uaddr, unsigned int flags)
 		if (ret == -EFAULT)
 			goto pi_faulted;
+		 * Between get_user() and obtaining the hb->lock the uval
+		 * gained a flag. Retry with the new value.
+		 */
+		if (ret == -EAGAIN)
+			goto pi_faulted;
+		/*
 		 * wake_futex_pi has detected invalid state. Tell user
 		 * space.
@@ -2883,6 +2893,8 @@ static int futex_unlock_pi(u32 __user *uaddr, unsigned int flags)
+	if (ret == -EAGAIN)
+		goto retry;
 	ret = fault_in_user_writeable(uaddr);
 	if (!ret)
 		goto retry;

             reply index

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-04-15 12:35 Sebastian Andrzej Siewior [this message]
2016-04-19 22:27 ` Davidlohr Bueso
2016-04-20  7:09   ` Sebastian Andrzej Siewior
2016-04-20  7:36   ` Thomas Gleixner
2016-04-20 11:51 ` [tip:locking/urgent] futex: Handle unlock_pi race gracefully tip-bot for Sebastian Andrzej Siewior

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