[PATCH 5.15.y v2 1/2] rtmutex: Use waiter::task instead of current in remove_waiter()

[PATCH 5.15.y v2 1/2] rtmutex: Use waiter::task instead of current in remove_waiter()

[Sid Kumar]

From: Keenan Dong <keenanat2000@gmail.com>

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 ]

[ sidk: Replace scoped_guard() macro with raw spinlock operations for
	5.15]

Fixes: 8161239a8bcc ("rtmutex: Simplify PI algorithm and make highest prio task get lock")
Reported-by: Yuan Tan <yuantan098@gmail.com>
Reported-by: Yifan Wu <yifanwucs@gmail.com>
Reported-by: Juefei Pu <tomapufckgml@gmail.com>
Reported-by: Xin Liu <bird@lzu.edu.cn>
Signed-off-by: Keenan Dong <keenanat2000@gmail.com>
Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Cc: stable@vger.kernel.org
(cherry picked from commit 3bfdc63936dd4773109b7b8c280c0f3b5ae7d349)
Signed-off-by: Sid Kumar <sidkumar1@gmail.com>
---

v1: https://lore.kernel.org/stable/20260708150527.3212183-1-sidkumar1@gmail.com/

v1 -> v2:
	add a backport of 40a25d59e85b3c which is a fix
	for this commit.

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

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index cf72ef77bfe7..f79c9286c7c0 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -1500,20 +1500,23 @@ static bool rtmutex_spin_on_owner(struct rt_mutex_base *lock,
  *
  * 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_unlock(&waiter_task->pi_lock);
 
 	/*
 	 * Only update priority if the waiter was the highest priority
@@ -1549,7 +1552,7 @@ static void __sched remove_waiter(struct rt_mutex_base *lock,
 	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);
 }
-- 
2.39.2

[PATCH 5.15.y v2 2/2] locking/rtmutex: Skip remove_waiter() when waiter is not enqueued

[Sid Kumar]

From: Davidlohr Bueso <dave@stgolabs.net>

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
(cherry picked from commit 40a25d59e85b3c8709ac2424d44f65610467871e)
Signed-off-by: Sid Kumar <sidkumar1@gmail.com>
---
 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 f79c9286c7c0..2118f8928eb7 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -1513,6 +1513,9 @@ static void __sched remove_waiter(struct rt_mutex_base *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;
diff --git a/kernel/locking/rtmutex_api.c b/kernel/locking/rtmutex_api.c
index 56d1938cb52a..c4e191340c59 100644
--- a/kernel/locking/rtmutex_api.c
+++ b/kernel/locking/rtmutex_api.c
@@ -322,7 +322,7 @@ 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);
 
-- 
2.39.2

Re: [PATCH 5.15.y v2 1/2] rtmutex: Use waiter::task instead of current in remove_waiter()

[Sasha Levin]

On Thu, Jul 09, 2026 at 10:59:48AM -0400, Sid Kumar wrote:
> 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:

Queued the series for 5.15, thanks.

-- 
Thanks,
Sasha