FAILED: patch "[PATCH] futex/requeue: Prevent NULL pointer dereference in" failed to apply to 5.15-stable tree

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm tree, then please email the backport, including the original git commit id to <[email protected]>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x 74e144274af39935b0f410c0ee4d2b91c3730414
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<[email protected]>' --in-reply-to '2026072141-ravage-display-0696@gregkh' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Possible dependencies:

thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 74e144274af39935b0f410c0ee4d2b91c3730414 Mon Sep 17 00:00:00 2001
From: Ji'an Zhou <[email protected]>
Date: Tue, 2 Jun 2026 09:12:04 +0000
Subject: [PATCH] futex/requeue: Prevent NULL pointer dereference in
remove_waiter() on self-deadlock

When FUTEX_CMP_REQUEUE_PI requeues a non-top waiter that already owns the target PI futex, task_blocks_on_rt_mutex() returns -EDEADLK before setting waiter->task.

The subsequent remove_waiter() in rt_mutex_start_proxy_lock() dereferences the NULL waiter->task, causing a kernel crash.

Add a self-deadlock check for non-top waiters before calling rt_mutex_start_proxy_lock(), analogous to the top-waiter check in futex_lock_pi_atomic().

index b597cb3d17fc..1d99a84dc9ad 100644
@@ -643,6 +643,12 @@ int futex_requeue(u32 __user *uaddr1, unsigned int flags1,
 				continue;
 			}
 
+			/* Self-deadlock: non-top waiter already owns the PI futex. */
+			if (rt_mutex_owner(&pi_state->pi_mutex) == this->task) {
+				ret = -EDEADLK;
+				break;
+			}
+
 			ret = rt_mutex_start_proxy_lock(&pi_state->pi_mutex,
 							this->rt_waiter,
 							this->task);

As promised the backports of:

3bfdc63936dd ("rtmutex: Use waiter::task instead of current in remove_waiter()") 40a25d59e85b ("locking/rtmutex: Skip remove_waiter() when waiter is not enqueued")

for 5.15.y.

Thanks,

tglx

From: Keenan Dong <[email protected]>

commit 3bfdc63936dd4773109b7b8c280c0f3b5ae7d349 upstream.

remove_waiter() is used by the slowlock paths, but it is also used for proxy-lock rollback in rt_mutex_start_proxy_lock() when invoked from futex_requeue().

In the latter case waiter::task is not current, but remove_waiter() operates on current for the dequeue operation. That results in several problems:

1) the rbtree dequeue happens without waiter::task::pi_lock being held

2) the waiter task's pi_blocked_on state is not cleared, which leaves a dangling pointer primed for UAF around.

3) rt_mutex_adjust_prio_chain() operates on the wrong top priority waiter task

Use waiter::task instead of current in all related operations in remove_waiter() to cure those problems.

[ tglx: Fixup rt_mutex_adjust_prio_chain(), add a comment and amend the
changelog ]

[ tglx: Backport for 5.10.y and 5.15.y. The commit has not been backported
yet because the Fixes tag is wrong. Fixes: points now to the real culprit. ]

---
 kernel/locking/rtmutex.c |   11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

---

@@ -1500,20 +1500,23 @@ static bool rtmutex_spin_on_owner(struct
  *
  * Must be called with lock->wait_lock held and interrupts disabled. It must
  * have just failed to try_to_take_rt_mutex().
+ *
+ * When invoked from rt_mutex_start_proxy_lock() waiter::task != current !
  */
 static void __sched remove_waiter(struct rt_mutex_base *lock,
 				  struct rt_mutex_waiter *waiter)
 {
 	bool is_top_waiter = (waiter == rt_mutex_top_waiter(lock));
 	struct task_struct *owner = rt_mutex_owner(lock);
+	struct task_struct *waiter_task = waiter->task;
 	struct rt_mutex_base *next_lock;
 
 	lockdep_assert_held(&lock->wait_lock);
 
-	raw_spin_lock(&current->pi_lock);
+	raw_spin_lock(&waiter_task->pi_lock);
 	rt_mutex_dequeue(lock, waiter);
-	current->pi_blocked_on = NULL;
-	raw_spin_unlock(&current->pi_lock);
+	waiter_task->pi_blocked_on = NULL;
+	raw_spin_lock(&waiter_task->pi_lock);
 
 	/*
 	 * Only update priority if the waiter was the highest priority

@@ -1549,7 +1552,7 @@ static void __sched remove_waiter(struct
 	raw_spin_unlock_irq(&lock->wait_lock);
 
 	rt_mutex_adjust_prio_chain(owner, RT_MUTEX_MIN_CHAINWALK, lock,
-				   next_lock, NULL, current);
+				   next_lock, NULL, waiter_task);
 
 	raw_spin_lock_irq(&lock->wait_lock);
 }

From: Davidlohr Bueso <[email protected]>

commit 40a25d59e85b3c8709ac2424d44f65610467871e upstream.

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().

---
 kernel/locking/rtmutex.c     |    3 +++
 kernel/locking/rtmutex_api.c |    2 +-
 2 files changed, 4 insertions(+), 1 deletion(-)

---

@@ -1513,6 +1513,9 @@ static void __sched remove_waiter(struct
 
 	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;

@@ -322,7 +322,7 @@ int __sched rt_mutex_start_proxy_lock(st
 
 	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);