[tip: locking/urgent] rtmutex: Use waiter::task instead of current in remove_waiter()

[tip-bot2 for Keenan Dong]

The following commit has been merged into the locking/urgent branch of tip:

Commit-ID:     3bfdc63936dd4773109b7b8c280c0f3b5ae7d349
Gitweb:        https://git.kernel.org/tip/3bfdc63936dd4773109b7b8c280c0f3b5ae7d349
Author:        Keenan Dong <keenanat2000@xxxxxxxxx>
AuthorDate:    Wed, 08 Apr 2026 16:46:00 +08:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxx>
CommitterDate: Tue, 21 Apr 2026 00:22:31 +02:00

rtmutex: Use waiter::task instead of current in remove_waiter()

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 ]

Fixes: 8161239a8bcc ("rtmutex: Simplify PI algorithm and make highest prio task get lock")
Reported-by: Yuan Tan <yuantan098@xxxxxxxxx>
Reported-by: Yifan Wu <yifanwucs@xxxxxxxxx>
Reported-by: Juefei Pu <tomapufckgml@xxxxxxxxx>
Reported-by: Xin Liu <bird@xxxxxxxxxx>
Signed-off-by: Keenan Dong <keenanat2000@xxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
---
 kernel/locking/rtmutex.c | 13 ++++++++-----
 1 file changed, 8 insertions(+), 5 deletions(-)

diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
index ccaba61..4f386ea 100644
--- a/kernel/locking/rtmutex.c
+++ b/kernel/locking/rtmutex.c
@@ -1544,6 +1544,8 @@ 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)
@@ -1551,14 +1553,15 @@ static void __sched remove_waiter(struct rt_mutex_base *lock,
 {
 	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);
-	rt_mutex_dequeue(lock, waiter);
-	current->pi_blocked_on = NULL;
-	raw_spin_unlock(&current->pi_lock);
+	scoped_guard(raw_spinlock, &waiter_task->pi_lock) {
+		rt_mutex_dequeue(lock, waiter);
+		waiter_task->pi_blocked_on = NULL;
+	}
 
 	/*
 	 * Only update priority if the waiter was the highest priority
@@ -1594,7 +1597,7 @@ 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);
 }