[PATCH -next 5/5] md: protect md_thread with a new disk level spin lock

[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
			raid1d
			 md_check_recovery
			  md_reap_sync_thread
			   md_unregister_thread
			    kfree

 md_wakeup_thread
  wake_up
  ->sync_thread was freed

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 use a disk level spinlock to protect md_thread in relevant apis.

Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
---
 drivers/md/md.c | 23 ++++++++++-------------
 drivers/md/md.h |  1 +
 2 files changed, 11 insertions(+), 13 deletions(-)

diff --git a/drivers/md/md.c b/drivers/md/md.c
index ab9299187cfe..a952978884a5 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -663,6 +663,7 @@ void mddev_init(struct mddev *mddev)
 	atomic_set(&mddev->active, 1);
 	atomic_set(&mddev->openers, 0);
 	spin_lock_init(&mddev->lock);
+	spin_lock_init(&mddev->thread_lock);
 	atomic_set(&mddev->flush_pending, 0);
 	init_waitqueue_head(&mddev->sb_wait);
 	init_waitqueue_head(&mddev->recovery_wait);
@@ -801,13 +802,8 @@ 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, mddev);
 	wake_up(&mddev->sb_wait);
-	spin_unlock(&pers_lock);
 }
 EXPORT_SYMBOL_GPL(mddev_unlock);
 
@@ -7895,13 +7891,16 @@ static int md_thread(void *arg)
 
 void md_wakeup_thread(struct md_thread **threadp, struct mddev *mddev)
 {
-	struct md_thread *thread = *threadp;
+	struct md_thread *thread;
 
+	spin_lock(&mddev->thread_lock);
+	thread = *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);
 	}
+	spin_unlock(&mddev->thread_lock);
 }
 EXPORT_SYMBOL(md_wakeup_thread);
 
@@ -7929,7 +7928,9 @@ int md_register_thread(struct md_thread **threadp,
 		return err;
 	}
 
+	spin_lock(&mddev->thread_lock);
 	*threadp = thread;
+	spin_unlock(&mddev->thread_lock);
 	return 0;
 }
 EXPORT_SYMBOL(md_register_thread);
@@ -7938,18 +7939,14 @@ void md_unregister_thread(struct md_thread **threadp, struct mddev *mddev)
 {
 	struct md_thread *thread;
 
-	/*
-	 * Locking ensures that mddev_unlock does not wake_up a
-	 * non-existent thread
-	 */
-	spin_lock(&pers_lock);
+	spin_lock(&mddev->thread_lock);
 	thread = *threadp;
 	if (!thread) {
-		spin_unlock(&pers_lock);
+		spin_unlock(&mddev->thread_lock);
 		return;
 	}
 	*threadp = NULL;
-	spin_unlock(&pers_lock);
+	spin_unlock(&mddev->thread_lock);
 
 	pr_debug("interrupting MD-thread pid %d\n", task_pid_nr(thread->tsk));
 	kthread_stop(thread->tsk);
diff --git a/drivers/md/md.h b/drivers/md/md.h
index 8f4137ad2dde..ca182d21dd8d 100644
--- a/drivers/md/md.h
+++ b/drivers/md/md.h
@@ -367,6 +367,7 @@ struct mddev {
 	int				new_chunk_sectors;
 	int				reshape_backwards;
 
+	spinlock_t			thread_lock;
 	struct md_thread		*thread;	/* management thread */
 	struct md_thread		*sync_thread;	/* doing resync or reconstruct */
 
-- 
2.31.1