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From:   Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To:     linux-kernel@vger.kernel.org
Cc:     Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        stable@vger.kernel.org,
        syzbot+fadc0aaf497e6a493b9f@syzkaller.appspotmail.com,
        Christoph Hellwig <hch@lst.de>, NeilBrown <neilb@suse.de>,
        Song Liu <songliubraving@fb.com>,
        Sasha Levin <sashal@kernel.org>
Subject: [PATCH 5.10 068/103] md: fix a lock order reversal in md_alloc
Date:   Mon, 27 Sep 2021 19:02:40 +0200
Message-Id: <20210927170228.128635855@linuxfoundation.org>
In-Reply-To: <20210927170225.702078779@linuxfoundation.org>
References: <20210927170225.702078779@linuxfoundation.org>
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From: Christoph Hellwig <hch@lst.de>

[ Upstream commit 7df835a32a8bedf7ce88efcfa7c9b245b52ff139 ]

Commit b0140891a8cea3 ("md: Fix race when creating a new md device.")
not only moved assigning mddev->gendisk before calling add_disk, which
fixes the races described in the commit log, but also added a
mddev->open_mutex critical section over add_disk and creation of the
md kobj.  Adding a kobject after add_disk is racy vs deleting the gendisk
right after adding it, but md already prevents against that by holding
a mddev->active reference.

On the other hand taking this lock added a lock order reversal with what
is not disk->open_mutex (used to be bdev->bd_mutex when the commit was
added) for partition devices, which need that lock for the internal open
for the partition scan, and a recent commit also takes it for
non-partitioned devices, leading to further lockdep splatter.

Fixes: b0140891a8ce ("md: Fix race when creating a new md device.")
Fixes: d62633873590 ("block: support delayed holder registration")
Reported-by: syzbot+fadc0aaf497e6a493b9f@syzkaller.appspotmail.com
Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: syzbot+fadc0aaf497e6a493b9f@syzkaller.appspotmail.com
Reviewed-by: NeilBrown <neilb@suse.de>
Signed-off-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 drivers/md/md.c | 5 -----
 1 file changed, 5 deletions(-)

diff --git a/drivers/md/md.c b/drivers/md/md.c
index 288d26013de2..f16f190546ef 100644
--- a/drivers/md/md.c
+++ b/drivers/md/md.c
@@ -5759,10 +5759,6 @@ static int md_alloc(dev_t dev, char *name)
 	disk->flags |= GENHD_FL_EXT_DEVT;
 	disk->events |= DISK_EVENT_MEDIA_CHANGE;
 	mddev->gendisk = disk;
-	/* As soon as we call add_disk(), another thread could get
-	 * through to md_open, so make sure it doesn't get too far
-	 */
-	mutex_lock(&mddev->open_mutex);
 	add_disk(disk);
 
 	error = kobject_add(&mddev->kobj, &disk_to_dev(disk)->kobj, "%s", "md");
@@ -5777,7 +5773,6 @@ static int md_alloc(dev_t dev, char *name)
 	if (mddev->kobj.sd &&
 	    sysfs_create_group(&mddev->kobj, &md_bitmap_group))
 		pr_debug("pointless warning\n");
-	mutex_unlock(&mddev->open_mutex);
  abort:
 	mutex_unlock(&disks_mutex);
 	if (!error && mddev->kobj.sd) {
-- 
2.33.0