btrfs: do not allow non subvolume root targets for snapshot

commit a8892fd71933126ebae3d60aec5918d4dceaae76 upstream.

Our btrfs subvolume snapshot <source> <destination> utility enforces
that <source> is the root of the subvolume, however this isn't enforced
in the kernel.  Update the kernel to also enforce this limitation to
avoid problems with other users of this ioctl that don't have the
appropriate checks in place.

Reported-by: Martin Michaelis <code@mgjm.de>
CC: stable@vger.kernel.org # 4.14+
Reviewed-by: Neal Gompa <neal@gompa.dev>
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>

diff --git a/fs/btrfs/ioctl.c b/fs/btrfs/ioctl.c
index dc6e3cce747c1ec76b5e12b957e64705c9397ffe..e8e4781c48a50dc48a40680ddb98cea719f4833e 100644 (file)
--- a/fs/btrfs/ioctl.c
+++ b/fs/btrfs/ioctl.c
@@ -2182,6 +2182,15 @@ static noinline int __btrfs_ioctl_snap_create(struct file *file,
                         * are limited to own subvolumes only
                         */
                        ret = -EPERM;
+               } else if (btrfs_ino(BTRFS_I(src_inode)) != BTRFS_FIRST_FREE_OBJECTID) {
+                       /*
+                        * Snapshots must be made with the src_inode referring
+                        * to the subvolume inode, otherwise the permission
+                        * checking above is useless because we may have
+                        * permission on a lower directory but not the subvol
+                        * itself.
+                        */
+                       ret = -EINVAL;
                } else {
                        ret = btrfs_mksnapshot(&file->f_path, mnt_userns,
                                               name, namelen,