[STABLE v5.10 2/2] locking/rtmutex: Skip remove_waiter() when waiter is not enqueued

[Lee Jones]

From: Davidlohr Bueso <dave@xxxxxxxxxxxx>

[ Upstream commit 40a25d59e85b3c8709ac2424d44f65610467871e ]

syzbot triggered the following splat in remove_waiter() via
FUTEX_CMP_REQUEUE_PI:

  KASAN: null-ptr-deref in range [0x0000000000000a88-0x0000000000000a8f]
   class_raw_spinlock_constructor
   remove_waiter+0x159/0x1200 kernel/locking/rtmutex.c:1561
   rt_mutex_start_proxy_lock+0x103/0x120
   futex_requeue+0x10e4/0x20d0
   __x64_sys_futex+0x34f/0x4d0

task_blocks_on_rt_mutex() does not arm the waiter upon deadlock detection,
leaving waiter->task nil, where 3bfdc63936dd ("rtmutex: Use waiter::task instead
of current in remove_waiter()") made this fatal.

Furthermore, rt_mutex_start_proxy_lock() should not be calling into remove_waiter()
upon a successfully grabbing the rtmutex. 1a1fb985f2e2 ("futex: Handle early deadlock
return correctly"), moved the remove_waiter() out of __rt_mutex_start_proxy_lock()
(where 'ret' was only ever 0 or < 0) into the wrapper. Tighten this check to
account for try_to_take_rt_mutex().

Fixes: 3bfdc63936dd ("rtmutex: Use waiter::task instead of current in remove_waiter()")
Reported-by: syzbot+78147abe6c524f183ee9@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Davidlohr Bueso <dave@xxxxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Closes: https://lore.kernel.org/all/69f114ac.050a0220.ac8b.0003.GAE@xxxxxxxxxx/
Link: https://patch.msgid.link/20260507112913.1019537-1-dave@xxxxxxxxxxxx
Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
(cherry picked from commit 4afda3a1da02129568a3a2f1898aa13e6763bcba)
[Lee: Backported to linux-5.10.y from linux-6.1.y]
Signed-off-by: Lee Jones <lee@xxxxxxxxxx>
---
 kernel/locking/rtmutex.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index 2ab2d6cce004..9cfe7f0aa04e 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -1074,6 +1074,9 @@ static void remove_waiter(struct rt_mutex *lock,
 
 	lockdep_assert_held(&lock->wait_lock);
 
+	if (!waiter_task) /* never enqueued */
+		return;
+
 	raw_spin_lock(&waiter_task->pi_lock);
 	rt_mutex_dequeue(lock, waiter);
 	waiter_task->pi_blocked_on = NULL;
@@ -1803,7 +1806,7 @@ int rt_mutex_start_proxy_lock(struct rt_mutex *lock,
 
 	raw_spin_lock_irq(&lock->wait_lock);
 	ret = __rt_mutex_start_proxy_lock(lock, waiter, task);
-	if (unlikely(ret))
+	if (unlikely(ret < 0))
 		remove_waiter(lock, waiter);
 	raw_spin_unlock_irq(&lock->wait_lock);
 
-- 
2.55.0.229.g6434b31f56-goog