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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, Nikolay Borisov <nborisov@suse.com>,
        Anand Jain <anand.jain@oracle.com>,
        David Sterba <dsterba@suse.com>
Subject: [PATCH 4.9 237/241] btrfs: Fix use-after-free when cleaning up fs_devs with a single stale device
Date:   Mon, 19 Mar 2018 19:08:22 +0100
Message-Id: <20180319180800.989583878@linuxfoundation.org>
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4.9-stable review patch.  If anyone has any objections, please let me know.

------------------

From: Nikolay Borisov <nborisov@suse.com>

commit fd649f10c3d21ee9d7542c609f29978bdf73ab94 upstream.

Commit 4fde46f0cc71 ("Btrfs: free the stale device") introduced
btrfs_free_stale_device which iterates the device lists for all
registered btrfs filesystems and deletes those devices which aren't
mounted. In a btrfs_devices structure has only 1 device attached to it
and it is unused then btrfs_free_stale_devices will proceed to also free
the btrfs_fs_devices struct itself. Currently this leads to a use after
free since list_for_each_entry will try to perform a check on the
already freed memory to see if it has to terminate the loop.

The fix is to use 'break' when we know we are freeing the current
fs_devs.

Fixes: 4fde46f0cc71 ("Btrfs: free the stale device")
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
 fs/btrfs/volumes.c |    1 +
 1 file changed, 1 insertion(+)

--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -583,6 +583,7 @@ void btrfs_free_stale_device(struct btrf
 				btrfs_sysfs_remove_fsid(fs_devs);
 				list_del(&fs_devs->list);
 				free_fs_devices(fs_devs);
+				break;
 			} else {
 				fs_devs->num_devices--;
 				list_del(&dev->dev_list);