linux-kernel - [PATCH 4.4 01/30] futex: Ensure the correct return value from futex_lock

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Date:   Fri,  5 Mar 2021 13:22:30 +0100
From:   Greg Kroah-Hartman <gregkh@...uxfoundation.org>
To:     linux-kernel@...r.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        stable@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Sharan Turlapati <sturlapati@...are.com>
Subject: [PATCH 4.4 01/30] futex: Ensure the correct return value from futex_lock_pi()

From: Thomas Gleixner <tglx@...utronix.de>

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@...utronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@...radead.org>
[Sharan: Backported patch for kernel 4.4.y. Also folded in is a part
 of the cleanup patch d7c5ed73b19c("futex: Remove needless goto's")]
Signed-off-by: Sharan Turlapati <sturlapati@...are.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@...uxfoundation.org>
---
 kernel/futex.c |   24 ++++++++++++------------
 1 file changed, 12 insertions(+), 12 deletions(-)

--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -2283,7 +2283,7 @@ retry:
 		}
 
 		if (__rt_mutex_futex_trylock(&pi_state->pi_mutex)) {
-			/* We got the lock after all, nothing to fix. */
+			/* We got the lock. pi_state is correct. Tell caller */
 			return 1;
 		}
 
@@ -2328,7 +2328,7 @@ retry:
 	 */
 	pi_state_update_owner(pi_state, newowner);
 
-	return 0;
+	return argowner == current;
 
 	/*
 	 * To handle the page fault we need to drop the hash bucket
@@ -2411,8 +2411,6 @@ 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
@@ -2423,8 +2421,8 @@ 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;
 	}
 
 	/*
@@ -2435,10 +2433,8 @@ 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
@@ -2447,8 +2443,7 @@ static int fixup_owner(u32 __user *uaddr
 	if (WARN_ON_ONCE(rt_mutex_owner(&q->pi_state->pi_mutex) == current))
 		return fixup_pi_state_owner(uaddr, q, current);
 
-out:
-	return ret ? ret : locked;
+	return 0;
 }
 
 /**
@@ -3070,6 +3065,11 @@ static int futex_wait_requeue_pi(u32 __u
 			 */
 			free_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;