Patch "dm: fix race between dm_get_from_kobject() and __dm_destroy()" has been added to the 3.18-stable tree

This is a note to let you know that I've just added the patch titled

dm: fix race between dm_get_from_kobject() and __dm_destroy()

to the 3.18-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
dm-fix-race-between-dm_get_from_kobject-and-__dm_destroy.patch
and it can be found in the queue-3.18 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: Hou Tao <[email protected]>
Date: Wed, 1 Nov 2017 15:42:36 +0800
Subject: dm: fix race between dm_get_from_kobject() and __dm_destroy()

From: Hou Tao <[email protected]>

commit b9a41d21dceadf8104812626ef85dc56ee8a60ed upstream.

The following BUG_ON was hit when testing repeat creation and removal of DM devices:

    kernel BUG at drivers/md/dm.c:2919!
    CPU: 7 PID: 750 Comm: systemd-udevd Not tainted 4.1.44
    Call Trace:
     [<ffffffff81649e8b>] dm_get_from_kobject+0x34/0x3a
     [<ffffffff81650ef1>] dm_attr_show+0x2b/0x5e
     [<ffffffff817b46d1>] ? mutex_lock+0x26/0x44
     [<ffffffff811df7f5>] sysfs_kf_seq_show+0x83/0xcf
     [<ffffffff811de257>] kernfs_seq_show+0x23/0x25
     [<ffffffff81199118>] seq_read+0x16f/0x325
     [<ffffffff811de994>] kernfs_fop_read+0x3a/0x13f
     [<ffffffff8117b625>] __vfs_read+0x26/0x9d
     [<ffffffff8130eb59>] ? security_file_permission+0x3c/0x44
     [<ffffffff8117bdb8>] ? rw_verify_area+0x83/0xd9
     [<ffffffff8117be9d>] vfs_read+0x8f/0xcf
     [<ffffffff81193e34>] ? __fdget_pos+0x12/0x41
     [<ffffffff8117c686>] SyS_read+0x4b/0x76
     [<ffffffff817b606e>] system_call_fastpath+0x12/0x71

The bug can be easily triggered, if an extra delay (e.g. 10ms) is added between the test of DMF_FREEING & DMF_DELETING and dm_get() in dm_get_from_kobject().

To fix it, we need to ensure the test of DMF_FREEING & DMF_DELETING and dm_get() are done in an atomic way, so _minor_lock is used.

The other callers of dm_get() have also been checked to be OK: some callers invoke dm_get() under _minor_lock, some callers invoke it under _hash_lock, and dm_start_request() invoke it after increasing md->open_count.

---
 drivers/md/dm.c |   12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

@@ -3033,11 +3033,15 @@ struct mapped_device *dm_get_from_kobjec
 
 	md = container_of(kobj, struct mapped_device, kobj_holder.kobj);
 
-	if (test_bit(DMF_FREEING, &md->flags) ||
-	    dm_deleting_md(md))
-		return NULL;
-
+	spin_lock(&_minor_lock);
+	if (test_bit(DMF_FREEING, &md->flags) || dm_deleting_md(md)) {
+		md = NULL;
+		goto out;
+	}
 	dm_get(md);
+out:
+	spin_unlock(&_minor_lock);
+
 	return md;
 }
 

Patches currently in stable-queue which might be from [email protected] are

queue-3.18/dm-fix-race-between-dm_get_from_kobject-and-__dm_destroy.patch