[PATCH 5.4 084/222] btrfs: fix invalid removal of root ref

Greg Kroah-Hartman
2020-01-22T13:20:43Z

From: Josef Bacik <josef@toxicpanda.com>

commit d49d3287e74ffe55ae7430d1e795e5f9bf7359ea upstream.

If we have the following sequence of events

btrfs sub create A
btrfs sub create A/B
btrfs sub snap A C
mkdir C/foo
mv A/B C/foo
rm -rf *

We will end up with a transaction abort.

The reason for this is because we create a root ref for B pointing to A.
When we create a snapshot of C we still have B in our tree, but because
the root ref points to A and not C we will make it appear to be empty.

The problem happens when we move B into C. This removes the root ref
for B pointing to A and adds a ref of B pointing to C. When we rmdir C
we'll see that we have a ref to our root and remove the root ref,
despite not actually matching our reference name.

Now btrfs_del_root_ref() allowing this to work is a bug as well, however
we know that this inode does not actually point to a root ref in the
first place, so we shouldn't be calling btrfs_del_root_ref() in the
first place and instead simply look up our dir index for this item and
do the rest of the removal.

CC: stable@vger.kernel.org # 4.4+
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
fs/btrfs/inode.c | 27 +++++++++++++++++++--------
1 file changed, 19 insertions(+), 8 deletions(-)

--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -4260,13 +4260,16 @@ static int btrfs_unlink_subvol(struct bt
}
btrfs_release_path(path);

- ret = btrfs_del_root_ref(trans, objectid, root->root_key.objectid,
- dir_ino, &index, name, name_len);
- if (ret < 0) {
- if (ret != -ENOENT) {
- btrfs_abort_transaction(trans, ret);
- goto out;
- }
+ /*
+ * This is a placeholder inode for a subvolume we didn't have a
+ * reference to at the time of the snapshot creation. In the meantime
+ * we could have renamed the real subvol link into our snapshot, so
+ * depending on btrfs_del_root_ref to return -ENOENT here is incorret.
+ * Instead simply lookup the dir_index_item for this entry so we can
+ * remove it. Otherwise we know we have a ref to the root and we can
+ * call btrfs_del_root_ref, and it _shouldn't_ fail.
+ */
+ if (btrfs_ino(inode) == BTRFS_EMPTY_SUBVOL_DIR_OBJECTID) {
di = btrfs_search_dir_index_item(root, path, dir_ino,
name, name_len);
if (IS_ERR_OR_NULL(di)) {
@@ -4281,8 +4284,16 @@ static int btrfs_unlink_subvol(struct bt
leaf = path->nodes[0];
btrfs_item_key_to_cpu(leaf, &key, path->slots[0]);
index = key.offset;
+ btrfs_release_path(path);
+ } else {
+ ret = btrfs_del_root_ref(trans, objectid,
+ root->root_key.objectid, dir_ino,
+ &index, name, name_len);
+ if (ret) {
+ btrfs_abort_transaction(trans, ret);
+ goto out;
+ }
}
- btrfs_release_path(path);

ret = btrfs_delete_delayed_dir_index(trans, BTRFS_I(dir), index);
if (ret) {