linux-kernel - [PATCH AUTOSEL 4.14 18/25] md: protect md_unregister

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Date:   Tue,  7 Jun 2022 14:02:19 -0400
From:   Sasha Levin <sashal@...nel.org>
To:     linux-kernel@...r.kernel.org, stable@...r.kernel.org
Cc:     Guoqing Jiang <guoqing.jiang@...ud.ionos.com>,
        Donald Buczek <buczek@...gen.mpg.de>,
        Guoqing Jiang <guoqing.jiang@...ux.dev>,
        Song Liu <song@...nel.org>, Sasha Levin <sashal@...nel.org>,
        linux-raid@...r.kernel.org
Subject: [PATCH AUTOSEL 4.14 18/25] md: protect md_unregister_thread from reentrancy

From: Guoqing Jiang <guoqing.jiang@...ud.ionos.com>

[ Upstream commit 1e267742283a4b5a8ca65755c44166be27e9aa0f ]

Generally, the md_unregister_thread is called with reconfig_mutex, but
raid_message in dm-raid doesn't hold reconfig_mutex to unregister thread,
so md_unregister_thread can be called simulitaneously from two call sites
in theory.

Then after previous commit which remove the protection of reconfig_mutex
for md_unregister_thread completely, the potential issue could be worse
than before.

Let's take pers_lock at the beginning of function to ensure reentrancy.

Reported-by: Donald Buczek <buczek@...gen.mpg.de>
Signed-off-by: Guoqing Jiang <guoqing.jiang@...ux.dev>
Signed-off-by: Song Liu <song@...nel.org>
Signed-off-by: Sasha Levin <sashal@...nel.org>
---
 drivers/md/md.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/drivers/md/md.c b/drivers/md/md.c
index 85bccc53fdd1..bdbd62007ee5 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -7588,17 +7588,22 @@ EXPORT_SYMBOL(md_register_thread);
 
 void md_unregister_thread(struct md_thread **threadp)
 {
-	struct md_thread *thread = *threadp;
-	if (!thread)
-		return;
-	pr_debug("interrupting MD-thread pid %d\n", task_pid_nr(thread->tsk));
-	/* Locking ensures that mddev_unlock does not wake_up a
+	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);
+		return;
+	}
 	*threadp = NULL;
 	spin_unlock(&pers_lock);
 
+	pr_debug("interrupting MD-thread pid %d\n", task_pid_nr(thread->tsk));
 	kthread_stop(thread->tsk);
 	kfree(thread);
 }
-- 
2.35.1