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

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 ]

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

index c6a2dafd4a3b..2ab2d6cce004 100644
@@ -1061,20 +1061,23 @@ static void mark_wakeup_next_waiter(struct wake_q_head *wake_q,
  *
  * Must be called with lock->wait_lock held and interrupts disabled. I must
  * have just failed to try_to_take_rt_mutex().
+ *
+ * When invoked from rt_mutex_start_proxy_lock() waiter::task != current !
  */
 static void remove_waiter(struct rt_mutex *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 *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

@@ -1110,7 +1113,7 @@ static void remove_waiter(struct rt_mutex *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);
 }

2.55.0.229.g6434b31f56-goog

From: Davidlohr Bueso <[email protected]>

[ 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().

---
 kernel/locking/rtmutex.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

index 2ab2d6cce004..9cfe7f0aa04e 100644
@@ -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