In the Linux kernel, the following vulnerability has been resolved:
btrfs: don't free qgroup space unless specified
Boris noticed in his simple quotas testing that he was getting a leak with Sweet Tea's change to subvol create that stopped doing a transaction commit. This was just a side effect of that change.
In the delayed inode code we have an optimization that will free extra reservations if we think we can pack a dir item into an already modified leaf. Previously this wouldn't be triggered in the subvolume create case because we'd commit the transaction, it was still possible but much harder to trigger. It could actually be triggered if we did a mkdir && subvol create with qgroups enabled.
This occurs because in btrfs_insert_delayed_dir_index(), which gets called when we're adding the dir item, we do the following:
btrfs_block_rsv_release(fs_info, trans->block_rsv, bytes, NULL);
if we're able to skip reserving space.
The problem here is that trans->block_rsv points at the temporary block rsv for the subvolume create, which has qgroup reservations in the block rsv.
This is a problem because btrfs_block_rsv_release() will do the following:
if (block_rsv->qgroup_rsv_reserved >= block_rsv->qgroup_rsv_size) { qgroup_to_release = block_rsv->qgroup_rsv_reserved - block_rsv->qgroup_rsv_size; block_rsv->qgroup_rsv_reserved = block_rsv->qgroup_rsv_size; }
The temporary block rsv just has ->qgroup_rsv_reserved set, ->qgroup_rsv_size == 0. The optimization in btrfs_insert_delayed_dir_index() sets ->qgroup_rsv_reserved = 0. Then later on when we call btrfs_subvolume_release_metadata() which has
btrfs_block_rsv_release(fs_info, rsv, (u64)-1, &qgroup_to_release); btrfs_qgroup_convert_reserved_meta(root, qgroup_to_release);
qgroup_to_release is set to 0, and we do not convert the reserved metadata space.
The problem here is that the block rsv code has been unconditionally messing with ->qgroup_rsv_reserved, because the main place this is...