[PATCH v3 3/3] md: protect md_thread with rcu

[Yu Kuai <yukuai1@xxxxxxxxxxxxxxx>]

From: Yu Kuai <yukuai3@xxxxxxxxxx>

Our test reports a uaf for 'mddev->sync_thread':

T1                      T2
md_start_sync
 md_register_thread
 // mddev->sync_thread is set
			raid1d
			 md_check_recovery
			  md_reap_sync_thread
			   md_unregister_thread
			    kfree

 md_wakeup_thread
  wake_up
  ->sync_thread was freed

Root cause is that there is a small windown between register thread and
wake up thread, where the thread can be freed concurrently.

Currently, a global spinlock 'pers_lock' is borrowed to protect
'mddev->thread', this problem can be fixed likewise, however, there might
be similar problem for other md_thread, and I really don't like the idea to
borrow a global lock.

This patch protect md_thread with rcu.

Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
---
 drivers/md/md.c | 32 ++++++++++----------------------
 1 file changed, 10 insertions(+), 22 deletions(-)

diff --git a/drivers/md/md.c b/drivers/md/md.c
index 9e80c5491c9a..161231e01faa 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -70,11 +70,7 @@
 #include "md-bitmap.h"
 #include "md-cluster.h"
 
-/* pers_list is a list of registered personalities protected
- * by pers_lock.
- * pers_lock does extra service to protect accesses to
- * mddev->thread when the mutex cannot be held.
- */
+/* pers_list is a list of registered personalities protected by pers_lock. */
 static LIST_HEAD(pers_list);
 static DEFINE_SPINLOCK(pers_lock);
 
@@ -802,13 +798,8 @@ void mddev_unlock(struct mddev *mddev)
 	} else
 		mutex_unlock(&mddev->reconfig_mutex);
 
-	/* As we've dropped the mutex we need a spinlock to
-	 * make sure the thread doesn't disappear
-	 */
-	spin_lock(&pers_lock);
 	md_wakeup_thread(&mddev->thread);
 	wake_up(&mddev->sb_wait);
-	spin_unlock(&pers_lock);
 }
 EXPORT_SYMBOL_GPL(mddev_unlock);
 
@@ -7921,13 +7912,16 @@ static int md_thread(void *arg)
 
 void md_wakeup_thread(struct md_thread **threadp)
 {
-	struct md_thread *thread = *threadp;
+	struct md_thread *thread;
 
+	rcu_read_lock();
+	thread = rcu_dereference(*threadp);
 	if (thread) {
 		pr_debug("md: waking up MD thread %s.\n", thread->tsk->comm);
 		set_bit(THREAD_WAKEUP, &thread->flags);
 		wake_up(&thread->wqueue);
 	}
+	rcu_read_unlock();
 }
 EXPORT_SYMBOL(md_wakeup_thread);
 
@@ -7955,7 +7949,7 @@ int md_register_thread(struct md_thread **threadp,
 		return err;
 	}
 
-	*threadp = thread;
+	rcu_assign_pointer(*threadp, thread);
 	return 0;
 }
 EXPORT_SYMBOL(md_register_thread);
@@ -7964,18 +7958,12 @@ void md_unregister_thread(struct md_thread **threadp)
 {
 	struct md_thread *thread;
 
-	/*
-	 * Locking ensures that mddev_unlock does not wake_up a
-	 * non-existent thread
-	 */
-	spin_lock(&pers_lock);
 	thread = *threadp;
-	if (!thread) {
-		spin_unlock(&pers_lock);
+	if (!thread)
 		return;
-	}
-	*threadp = NULL;
-	spin_unlock(&pers_lock);
+
+	rcu_assign_pointer(*threadp, NULL);
+	synchronize_rcu();
 
 	pr_debug("interrupting MD-thread pid %d\n", task_pid_nr(thread->tsk));
 	kthread_stop(thread->tsk);
-- 
2.39.2