Subject: futex: Ensure the correct return value from futex_lock_pi()
From: Thomas Gleixner <tglx@linutronix.de>
Date: Wed Jan 20 16:00:24 2021 +0100
Git-commit: 12bb3f7f1b03d5913b3f9d4236a488aa7774dfe9
Patch-mainline: v5.11-rc6
References: bsc#1181349 bsc#1149032 CVE-2021-3347

commit 12bb3f7f1b03d5913b3f9d4236a488aa7774dfe9 upstream

In case that futex_lock_pi() was aborted by a signal or a timeout and the
task returned without acquiring the rtmutex, but is the designated owner of
the futex due to a concurrent futex_unlock_pi() fixup_owner() is invoked to
establish consistent state. In that case it invokes fixup_pi_state_owner()
which in turn tries to acquire the rtmutex again. If that succeeds then it
does not propagate this success to fixup_owner() and futex_lock_pi()
returns -EINTR or -ETIMEOUT despite having the futex locked.

Return success from fixup_pi_state_owner() in all cases where the current
task owns the rtmutex and therefore the futex and propagate it correctly
through fixup_owner(). Fixup the other callsite which does not expect a
positive return value.

Fixes: c1e2f0eaf015 ("futex: Avoid violating the 10th rule of futex")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: stable@vger.kernel.org
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>

---
 kernel/futex.c |   32 ++++++++++++++++----------------
 1 file changed, 16 insertions(+), 16 deletions(-)

--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -2507,8 +2507,8 @@ static int fixup_pi_state_owner(u32 __us
 		}
 
 		if (__rt_mutex_futex_trylock(&pi_state->pi_mutex)) {
-			/* We got the lock after all, nothing to fix. */
-			ret = 0;
+			/* We got the lock. pi_state is correct. Tell caller. */
+			ret = 1;
 			goto out_unlock;
 		}
 
@@ -2536,7 +2536,7 @@ static int fixup_pi_state_owner(u32 __us
 			 * We raced against a concurrent self; things are
 			 * already fixed up. Nothing to do.
 			 */
-			ret = 0;
+			ret = 1;
 			goto out_unlock;
 		}
 		newowner = argowner;
@@ -2582,7 +2582,7 @@ static int fixup_pi_state_owner(u32 __us
 	raw_spin_unlock(&newowner->pi_lock);
 	raw_spin_unlock_irq(&pi_state->pi_mutex.wait_lock);
 
-	return 0;
+	return argowner == current;
 
 	/*
 	 * In order to reschedule or handle a page fault, we need to drop the
@@ -2624,7 +2624,7 @@ static int fixup_pi_state_owner(u32 __us
 	 * Check if someone else fixed it for us:
 	 */
 	if (pi_state->owner != oldowner) {
-		ret = 0;
+		ret = argowner == current;
 		goto out_unlock;
 	}
 
@@ -2657,8 +2657,6 @@ static long futex_wait_restart(struct re
  */
 static int fixup_owner(u32 __user *uaddr, struct futex_q *q, int locked)
 {
-	int ret = 0;
-
 	if (locked) {
 		/*
 		 * Got the lock. We might not be the anticipated owner if we
@@ -2669,8 +2667,8 @@ static int fixup_owner(u32 __user *uaddr
 		 * stable state, anything else needs more attention.
 		 */
 		if (q->pi_state->owner != current)
-			ret = fixup_pi_state_owner(uaddr, q, current);
-		goto out;
+			return fixup_pi_state_owner(uaddr, q, current);
+		return 1;
 	}
 
 	/*
@@ -2681,10 +2679,8 @@ static int fixup_owner(u32 __user *uaddr
 	 * Another speculative read; pi_state->owner == current is unstable
 	 * but needs our attention.
 	 */
-	if (q->pi_state->owner == current) {
-		ret = fixup_pi_state_owner(uaddr, q, NULL);
-		goto out;
-	}
+	if (q->pi_state->owner == current)
+		return fixup_pi_state_owner(uaddr, q, NULL);
 
 	/*
 	 * Paranoia check. If we did not take the lock, then we should not be
@@ -2697,8 +2693,7 @@ static int fixup_owner(u32 __user *uaddr
 				q->pi_state->owner);
 	}
 
-out:
-	return ret ? ret : locked;
+	return 0;
 }
 
 /**
@@ -3429,7 +3424,7 @@ static int futex_wait_requeue_pi(u32 __u
 		if (q.pi_state && (q.pi_state->owner != current)) {
 			spin_lock(q.lock_ptr);
 			ret = fixup_pi_state_owner(uaddr2, &q, current);
-			if (ret && rt_mutex_owner(&q.pi_state->pi_mutex) == current) {
+			if (ret < 0 && rt_mutex_owner(&q.pi_state->pi_mutex) == current) {
 				pi_state = q.pi_state;
 				get_pi_state(pi_state);
 			}
@@ -3439,6 +3434,11 @@ static int futex_wait_requeue_pi(u32 __u
 			 */
 			put_pi_state(q.pi_state);
 			spin_unlock(q.lock_ptr);
+			/*
+			 * Adjust the return value. It's either -EFAULT or
+			 * success (1) but the caller expects 0 for success.
+			 */
+			ret = ret < 0 ? ret : 0;
 		}
 	} else {
 		struct rt_mutex *pi_mutex;