[PATCH 5.16 235/285] btrfs: prevent subvol with swapfile from being deleted

Greg Kroah-Hartman
2022-04-12T08:26:00Z

From: Kaiwen Hu <kevinhu@synology.com>

commit 60021bd754c6ca0addc6817994f20290a321d8d6 upstream.

A subvolume with an active swapfile must not be deleted otherwise it
would not be possible to deactivate it.

After the subvolume is deleted, we cannot swapoff the swapfile in this
deleted subvolume because the path is unreachable. The swapfile is
still active and holding references, the filesystem cannot be unmounted.

The test looks like this:

mkfs.btrfs -f $dev > /dev/null
mount $dev $mnt

btrfs sub create $mnt/subvol
touch $mnt/subvol/swapfile
chmod 600 $mnt/subvol/swapfile
chattr +C $mnt/subvol/swapfile
dd if=/dev/zero of=$mnt/subvol/swapfile bs=1K count=4096
mkswap $mnt/subvol/swapfile
swapon $mnt/subvol/swapfile

btrfs sub delete $mnt/subvol
swapoff $mnt/subvol/swapfile # failed: No such file or directory
swapoff --all

unmount $mnt # target is busy.

To prevent above issue, we simply check that whether the subvolume
contains any active swapfile, and stop the deleting process. This
behavior is like snapshot ioctl dealing with a swapfile.

CC: stable@vger.kernel.org # 5.4+
Reviewed-by: Robbie Ko <robbieko@synology.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Kaiwen Hu <kevinhu@synology.com>
Signed-off-by: David Sterba <dsterba@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
fs/btrfs/inode.c | 24 +++++++++++++++++++++++-
1 file changed, 23 insertions(+), 1 deletion(-)

--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -4462,6 +4462,13 @@ int btrfs_delete_subvolume(struct inode
dest->root_key.objectid);
return -EPERM;
}
+ if (atomic_read(&dest->nr_swapfiles)) {
+ spin_unlock(&dest->root_item_lock);
+ btrfs_warn(fs_info,
+ "attempt to delete subvolume %llu with active swapfile",
+ root->root_key.objectid);
+ return -EPERM;
+ }
root_flags = btrfs_root_flags(&dest->root_item);
btrfs_set_root_flags(&dest->root_item,
root_flags | BTRFS_ROOT_SUBVOL_DEAD);
@@ -10764,8 +10771,23 @@ static int btrfs_swap_activate(struct sw
* set. We use this counter to prevent snapshots. We must increment it
* before walking the extents because we don't want a concurrent
* snapshot to run after we've already checked the extents.
- */
+ *
+ * It is possible that subvolume is marked for deletion but still not
+ * removed yet. To prevent this race, we check the root status before
+ * activating the swapfile.
+ */
+ spin_lock(&root->root_item_lock);
+ if (btrfs_root_dead(root)) {
+ spin_unlock(&root->root_item_lock);
+
+ btrfs_exclop_finish(fs_info);
+ btrfs_warn(fs_info,
+ "cannot activate swapfile because subvolume %llu is being deleted",
+ root->root_key.objectid);
+ return -EPERM;
+ }
atomic_inc(&root->nr_swapfiles);
+ spin_unlock(&root->root_item_lock);

isize = ALIGN_DOWN(inode->i_size, fs_info->sectorsize);