[PATCH 6.1.y] locking/rtmutex: Skip remove_waiter() when waiter is not enqueued

[Sasha Levin <sashal@kernel.org>]

From: Davidlohr Bueso <dave@stgolabs.net>

[ 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@syzkaller.appspotmail.com
Signed-off-by: Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Cc: stable@vger.kernel.org
Closes: https://lore.kernel.org/all/69f114ac.050a0220.ac8b.0003.GAE@google.com/
Link: https://patch.msgid.link/20260507112913.1019537-1-dave@stgolabs.net
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/locking/rtmutex.c     | 3 +++
 kernel/locking/rtmutex_api.c | 2 +-
 2 files changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index def1f8fbc85cda..6df2f674712a47 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -1524,6 +1524,9 @@ static void __sched remove_waiter(struct rt_mutex_base *lock,
 
 	lockdep_assert_held(&lock->wait_lock);
 
+	if (!waiter_task) /* never enqueued */
+		return;
+
 	scoped_guard(raw_spinlock, &waiter_task->pi_lock) {
 		rt_mutex_dequeue(lock, waiter);
 		waiter_task->pi_blocked_on = NULL;
diff --git a/kernel/locking/rtmutex_api.c b/kernel/locking/rtmutex_api.c
index a6974d04459301..06c3db9f6cf1c5 100644
--- a/kernel/locking/rtmutex_api.c
+++ b/kernel/locking/rtmutex_api.c
@@ -344,7 +344,7 @@ int __sched rt_mutex_start_proxy_lock(struct rt_mutex_base *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);
 
-- 
2.53.0