1 files changed, 24 insertions, 3 deletions

diff --git a/kernel/locking/spinlock_rt.c b/kernel/locking/spinlock_rt.c
index db1e11b45de6..1d5e1b3c60bf 100644
--- a/kernel/locking/spinlock_rt.c
+++ b/kernel/locking/spinlock_rt.c
@@ -79,10 +79,27 @@ void __sched rt_spin_unlock(spinlock_t *lock) __releases(RCU)
 {
 	spin_release(&lock->dep_map, _RET_IP_);
 	migrate_enable();
-	rcu_read_unlock();
 
 	if (unlikely(!rt_mutex_cmpxchg_release(&lock->lock, current, NULL)))
 		rt_mutex_slowunlock(&lock->lock);
+
+	/*
+	 * This must be last to prevent the following UAF:
+	 *
+	 * T1					T2
+	 * spin_lock(&p->lock);			rcu_read_lock();
+	 * invalidate(p);			p = rcu_dereference(ptr);
+	 * rcu_assign_pointer(ptr, NULL);	if (!p) return;
+	 * spin_unlock(&p->lock);		spin_lock(&p->lock);
+	 * kfree_rcu(p);			rcu_read_unlock();
+	 *					....
+	 *					spin_unlock(&p->lock)
+	 *					  rcu_read_unlock(); // Ends grace period
+	 * rcu_do_batch()
+	 *   kfree(p);
+	 *			    UAF ->	  rt_mutex_cmpxchg_release(&p->lock.lock...)
+	 */
+	rcu_read_unlock();
 }
 EXPORT_SYMBOL(rt_spin_unlock);
 
@@ -262,17 +279,21 @@ void __sched rt_read_unlock(rwlock_t *rwlock) __releases(RCU)
 {
 	rwlock_release(&rwlock->dep_map, _RET_IP_);
 	migrate_enable();
-	rcu_read_unlock();
 	rwbase_read_unlock(&rwlock->rwbase, TASK_RTLOCK_WAIT);
+
+	/* This must be last. See comment in rt_spin_unlock() */
+	rcu_read_unlock();
 }
 EXPORT_SYMBOL(rt_read_unlock);
 
 void __sched rt_write_unlock(rwlock_t *rwlock) __releases(RCU)
 {
 	rwlock_release(&rwlock->dep_map, _RET_IP_);
-	rcu_read_unlock();
 	migrate_enable();
 	rwbase_write_unlock(&rwlock->rwbase);
+
+	/* This must be last. See comment in rt_spin_unlock() */
+	rcu_read_unlock();
 }
 EXPORT_SYMBOL(rt_write_unlock);