[PATCH] btrfs: btrfs_shrink_device should call commit transaction

[Anand Jain <anand.jain@oracle.com>]

test case btrfs/164 reported UAF..

[ 6712.084324] general protection fault: 0000 [#1] PREEMPT SMP
::
[ 6712.195423]  btrfs_update_commit_device_size+0x75/0xf0 [btrfs]
[ 6712.201424]  btrfs_commit_transaction+0x57d/0xa90 [btrfs]
[ 6712.206999]  btrfs_rm_device+0x627/0x850 [btrfs]
[ 6712.211800]  btrfs_ioctl+0x2b03/0x3120 [btrfs]
::

reason for this is that btrfs_shrink_device() adds the device resized to
the fs_devices::resized_devices after it has called the last commit
transaction.
So the list fs_devices::resized_devices is not empty when
btrfs_shrink_device() returns.
Now the parent function btrfs_rm_device() calls
        btrfs_close_bdev(device);
        call_rcu(&device->rcu, free_device_rcu);
and then does the commit transaction.
The commit transaction goes through the fs_devices::resized_devices
in btrfs_update_commit_device_size() and leads to UAF.

Fix this by making sure btrfs_shrink_device() calls the last needed
btrfs_commit_transaction() before the return.

Signed-off-by: Anand Jain <anand.jain@oracle.com>
---
I am bit skeptical about the condition when btrfs_commit_transaction()
fails, can you pls be wary about it when reviewing. Thanks. 

Lu/David,
As this issues isn't reproducible at my end, can you pls test this
patch on top of the patch
  btrfs: fix missing superblock update in the device delete commit transaction
Thanks.

 fs/btrfs/volumes.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/fs/btrfs/volumes.c b/fs/btrfs/volumes.c
index 418843b8f8e8..344083cc811c 100644
--- a/fs/btrfs/volumes.c
+++ b/fs/btrfs/volumes.c
@@ -4464,7 +4464,10 @@ int btrfs_shrink_device(struct btrfs_device *device, u64 new_size)
 
 	/* Now btrfs_update_device() will change the on-disk size. */
 	ret = btrfs_update_device(trans, device);
-	btrfs_end_transaction(trans);
+	if (ret)
+		btrfs_end_transaction(trans);
+	else
+		ret = btrfs_commit_transaction(trans);
 done:
 	btrfs_free_path(path);
 	if (ret) {
-- 
2.7.0