locking/rtmutex: Dont dereference waiter lockless

The new rt_mutex_spin_on_onwer() loop checks whether the spinning waiter is
still the top waiter on the lock by utilizing rt_mutex_top_waiter(), which
is broken because that function contains a sanity check which dereferences
the top waiter pointer to check whether the waiter belongs to the
lock. That's wrong in the lockless spinwait case:

 CPU 0							CPU 1
 rt_mutex_lock(lock)					rt_mutex_lock(lock);
   queue(waiter0)
   waiter0 == rt_mutex_top_waiter(lock)
   rt_mutex_spin_on_onwer(lock, waiter0) {		queue(waiter1)
   					 		waiter1 == rt_mutex_top_waiter(lock)
   							...
     top_waiter = rt_mutex_top_waiter(lock)
       leftmost = rb_first_cached(&lock->waiters);
							-> signal
							dequeue(waiter1)
							destroy(waiter1)
       w = rb_entry(leftmost, ....)
       BUG_ON(w->lock != lock)	 <- UAF

The BUG_ON() is correct for the case where the caller holds lock->wait_lock
which guarantees that the leftmost waiter entry cannot vanish. For the
lockless spinwait case it's broken.

Create a new helper function which avoids the pointer dereference and just
compares the leftmost entry pointer with current's waiter pointer to
validate that currrent is still elegible for spinning.

Fixes: 992caf7f ("locking/rtmutex: Add adaptive spinwait mechanism")
Reported-by: Sebastian Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210825102453.981720644@linutronix.de

kernel/locking/rtmutex.c

@@ -1329,8 +1329,9 @@ static bool rtmutex_spin_on_owner(struct rt_mutex_base *lock,
 		 *    for CONFIG_PREEMPT_RCU=y)
 		 *  - the VCPU on which owner runs is preempted
 		 */
-		if (!owner->on_cpu || waiter != rt_mutex_top_waiter(lock) ||
-		    need_resched() || vcpu_is_preempted(task_cpu(owner))) {
+		if (!owner->on_cpu || need_resched() ||
+		    rt_mutex_waiter_is_top_waiter(lock, waiter) ||
+		    vcpu_is_preempted(task_cpu(owner))) {
 			res = false;
 			break;
 		}

kernel/locking/rtmutex_common.h

@@ -95,6 +95,19 @@ static inline int rt_mutex_has_waiters(struct rt_mutex_base *lock)
 	return !RB_EMPTY_ROOT(&lock->waiters.rb_root);
 }
 
+/*
+ * Lockless speculative check whether @waiter is still the top waiter on
+ * @lock. This is solely comparing pointers and not derefencing the
+ * leftmost entry which might be about to vanish.
+ */
+static inline bool rt_mutex_waiter_is_top_waiter(struct rt_mutex_base *lock,
+						 struct rt_mutex_waiter *waiter)
+{
+	struct rb_node *leftmost = rb_first_cached(&lock->waiters);
+
+	return rb_entry(leftmost, struct rt_mutex_waiter, tree_entry) == waiter;
+}
+
 static inline struct rt_mutex_waiter *rt_mutex_top_waiter(struct rt_mutex_base *lock)
 {
 	struct rb_node *leftmost = rb_first_cached(&lock->waiters);