[PATCH 18/24] btrfs/ioctl: relax restrictions for BTRFS_IOC_SNAP_DESTROY_V2 with subvolids

[Christian Brauner <brauner@kernel.org>]

From: Christian Brauner <christian.brauner@ubuntu.com>

So far we prevented the deletion of subvolumes and snapshots using subvolume
ids possible with the BTRFS_SUBVOL_SPEC_BY_ID flag.
This restriction is necessary on idmapped mounts as this allows filesystem wide
subvolume and snapshot deletions and thus can escape the scope of what's
exposed under the mount identified by the fd passed with the ioctl.

Deletion by subvolume id works by looking for an alias of the parent of the
subvolume or snapshot to be deleted. The parent alias can be anywhere in the
filesystem. However, as long as the alias of the parent that is found is the
same as the one identified by the file descriptor passed through the ioctl we
can allow the deletion.

Cc: Chris Mason <clm@fb.com>
Cc: Josef Bacik <josef@toxicpanda.com>
Cc: Christoph Hellwig <hch@infradead.org>
Cc: David Sterba <dsterba@suse.com>
Cc: linux-btrfs@vger.kernel.org
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>
---
 fs/btrfs/ioctl.c | 27 ++++++++++++++++-----------
 1 file changed, 16 insertions(+), 11 deletions(-)

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index dd0fabdbeeeb..586ec4af9777 100644
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -2943,17 +2943,7 @@ static noinline int btrfs_ioctl_snap_destroy(struct file *file,
 			if (err)
 				goto out;
 		} else {
-			/*
-			 * Deleting by subvolume id can be used to delete
-			 * subvolumes/snapshots anywhere in the filesystem.
-			 * Ensure that users can't abuse idmapped mounts of
-			 * btrfs subvolumes/snapshots to perform operations in
-			 * the whole filesystem.
-			 */
-			if (mnt_userns != &init_user_ns) {
-				err = -EINVAL;
-				goto out;
-			}
+			struct inode *old_dir;
 
 			if (vol_args2->subvolid < BTRFS_FIRST_FREE_OBJECTID) {
 				err = -EINVAL;
@@ -2991,6 +2981,7 @@ static noinline int btrfs_ioctl_snap_destroy(struct file *file,
 				err = PTR_ERR(parent);
 				goto out_drop_write;
 			}
+			old_dir = dir;
 			dir = d_inode(parent);
 
 			/*
@@ -3001,6 +2992,20 @@ static noinline int btrfs_ioctl_snap_destroy(struct file *file,
 			 */
 			destroy_parent = true;
 
+			/*
+			 * On idmapped mounts, deletion via subvolid is
+			 * restricted to subvolumes that are immediate
+			 * ancestors of the inode referenced by the file
+			 * descriptor in the ioctl. Otherwise the idmapping
+			 * could potentially be abused to delete subvolumes
+			 * anywhere in the filesystem the user wouldn't be able
+			 * to delete without an idmapped mount.
+			 */
+			if (old_dir != dir && mnt_userns != &init_user_ns) {
+				err = -EINVAL;
+				goto free_parent;
+			}
+
 			subvol_name_ptr = btrfs_get_subvol_name_from_objectid(
 						fs_info, vol_args2->subvolid);
 			if (IS_ERR(subvol_name_ptr)) {
-- 
2.30.2