[PATCH] kernel/futex: handle the case where we got a "late" waiter

[Sebastian Andrzej Siewior]

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 <bigeasy@xxxxxxxxxxxxx>
---
 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) {
 		raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
 		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)
 	spin_unlock(&hb->lock);
 	put_futex_key(&key);
 
+	if (ret == -EAGAIN)
+		goto retry;
 	ret = fault_in_user_writeable(uaddr);
 	if (!ret)
 		goto retry;
-- 
2.8.0.rc3