[PATCH v1 0/9] fuse: basic support for idmapped mounts

[Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>]

Dear friends,

This patch series aimed to provide support for idmapped mounts
for fuse. We already have idmapped mounts support for almost all
widely-used filesystems:
* local (ext4, btrfs, xfs, fat, vfat, ntfs3, squashfs, f2fs, erofs, ZFS (out-of-tree))
* network (ceph)

Git tree (based on https://git.kernel.org/pub/scm/linux/kernel/git/mszeredi/fuse.git/log/?h=for-next):
v1: https://github.com/mihalicyn/linux/commits/fuse_idmapped_mounts.v1
current: https://github.com/mihalicyn/linux/commits/fuse_idmapped_mounts

Having fuse supported looks like a good next step. At the same time
fuse conceptually close to the network filesystems and supporting it is
a quite challenging task.

Let me briefly explain what was done in this series and which obstacles we have.

With this series, you can use idmapped mounts with fuse if the following
conditions are met:
1. The filesystem daemon declares idmap support (new FUSE_INIT response feature
flags FUSE_OWNER_UID_GID_EXT and FUSE_ALLOW_IDMAP)
2. The filesystem superblock was mounted with the "default_permissions" parameter
3. The filesystem fuse daemon does not perform any UID/GID-based checks internally
and fully trusts the kernel to do that (yes, it's almost the same as 2.)

I have prepared a bunch of real-world examples of the user space modifications
that can be done to use this extension:
- libfuse support
https://github.com/mihalicyn/libfuse/commits/idmap_support
- fuse-overlayfs support:
https://github.com/mihalicyn/fuse-overlayfs/commits/idmap_support
- cephfs-fuse conversion example
https://github.com/mihalicyn/ceph/commits/fuse_idmap
- glusterfs conversion example (there is a conceptual issue)
https://github.com/mihalicyn/glusterfs/commits/fuse_idmap

The glusterfs is a bit problematic, unfortunately, because even if the glusterfs
superblock was mounted with the "default_permissions" parameter (1 and 2 conditions
are satisfied), it fails to satisfy the 3rd condition. The glusterfs fuse daemon sends
caller UIDs/GIDs over the wire and all the permission checks are done twice (first
on the client side (in the fuse kernel module) and second on the glusterfs server side).
Just for demonstration's sake, I found a hacky (but working) solution for glusterfs
that disables these server-side permission checks (see [1]). This allows you to play
with the filesystem and idmapped mounts and it works just fine.

The problem described above is the main problem that we can meet when
working on idmapped mounts support for network-based filesystems (or network-like filesystems
like fuse). When people look at the idmapped mounts feature at first they tend to think
that idmaps are for faking caller UIDs/GIDs, but that's not the case. There was a big
discussion about this in the "ceph: support idmapped mounts" patch series [2], [3].
The brief outcome from this discussion is that we don't want and don't have to fool
filesystem code and map a caller's UID/GID everywhere, but only in VFS i_op's
which are provided with a "struct mnt_idmap *idmap"). For example ->lookup()
callback is not provided with it and that's on purpose! We don't expect the low-level
filesystem code to do any permissions checks inside this callback because everything
was already checked on the higher level (see may_lookup() helper). For local filesystems
this assumption works like a charm, but for network-based, unfortunately, not.
For example, the cephfs kernel client *always* send called UID/GID with *any* request
(->lookup included!) and then *may* (depending on the MDS configuration) perform any
permissions checks on the server side based on these values, which obviously leads
to issues/inconsistencies if VFS idmaps are involved.

Fuse filesystem very-very close to cephfs example, because we have req->in.h.uid/req->in.h.gid
and these values are present in all fuse requests and userspace may use them as it wants.

All of the above explains why we have a "default_permissions" requirement. If filesystem
does not use it, then permission checks will be widespread across all the i_op's like
->lookup, ->unlink, ->readlink instead of being consolidated in the one place (->permission callback).

In this series, my approach is the same as in cephfs [4], [5]. Don't touch req->in.h.uid/req->in.h.gid values
at all (because we can't properly idmap them as we don't have "struct mnt_idmap *idmap" everywhere),
instead, provide the userspace with a new optional (FUSE_OWNER_UID_GID_EXT) UID/GID suitable
only for ->mknod, ->mkdir, ->symlink, ->atomic_open and these values have to be used as the
owner UID and GID for newly created inodes.

Things to discuss:
- we enable idmapped mounts support only if "default_permissions" mode is enabled,
because otherwise, we would need to deal with UID/GID mappings on the userspace side OR
provide the userspace with idmapped req->in.h.uid/req->in.h.gid values which is not
something that we probably want to do. Idmapped mounts philosophy is not about faking
caller uid/gid.

- We have a small offlist discussion with Christian about adding fs_type->allow_idmap
hook. Christian pointed out that it would be nice to have a superblock flag instead like
SB_I_NOIDMAP and we can set this flag during mount time if we see that the filesystem does not
support idmappings. But, unfortunately, I didn't succeed here because the kernel will
know if the filesystem supports idmapping or not after FUSE_INIT request, but FUSE_INIT request
is being sent at the end of the mounting process, so the mount and superblock will exist and
visible by the userspace in that time. It seems like setting SB_I_NOIDMAP flag, in this
case, is too late as a user may do the trick by creating an idmapped mount while it wasn't
restricted by SB_I_NOIDMAP. Alternatively, we can introduce a "positive" version SB_I_ALLOWIDMAP
and a "weak" version of FS_ALLOW_IDMAP like FS_MAY_ALLOW_IDMAP. So if FS_MAY_ALLOW_IDMAP is set,
then SB_I_ALLOWIDMAP has to be set on the superblock to allow the creation of an idmapped mount.
But that's a matter of our discussion.

How to play with it:
1. take any patched filesystem from the list (fuse-overlayfs, cephfs-fuse, glusterfs) and mount it
2. ./mount-idmapped --map-mount b:1000:0:2 /mnt/my_fuse_mount /mnt/my_fuse_mount_idmapped
(maps UID/GIDs as 1000 -> 0, 1001 -> 1)
[ taken from https://raw.githubusercontent.com/brauner/mount-idmapped/master/mount-idmapped.c ]

[1] https://github.com/mihalicyn/glusterfs/commit/ab3ec2c7cbe22618cba9cc94a52a492b1904d0b2
[2] https://lore.kernel.org/lkml/20230608154256.562906-1-aleksandr.mikhalitsyn@canonical.com/
[3] https://lore.kernel.org/lkml/CAEivzxfw1fHO2TFA4dx3u23ZKK6Q+EThfzuibrhA3RKM=ZOYLg@mail.gmail.com/
[4] https://github.com/ceph/ceph/pull/52575
[5] https://lore.kernel.org/all/20230807132626.182101-4-aleksandr.mikhalitsyn@canonical.com/

Thanks!
Alex

Cc: Christian Brauner <brauner@kernel.org>
Cc: Seth Forshee <sforshee@kernel.org>
Cc: Miklos Szeredi <miklos@szeredi.hu>
Cc: Amir Goldstein <amir73il@gmail.com>
Cc: Bernd Schubert <bschubert@ddn.com>
Cc: <linux-fsdevel@vger.kernel.org>
Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>

Alexander Mikhalitsyn (9):
  fs/namespace: introduce fs_type->allow_idmap hook
  fs/fuse: add FUSE_OWNER_UID_GID_EXT extension
  fs/fuse: support idmap for mkdir/mknod/symlink/create
  fs/fuse: support idmapped getattr inode op
  fs/fuse: support idmapped ->permission inode op
  fs/fuse: support idmapped ->setattr op
  fs/fuse: drop idmap argument from __fuse_get_acl
  fs/fuse: support idmapped ->set_acl
  fs/fuse: allow idmapped mounts

 fs/fuse/acl.c             |  10 ++-
 fs/fuse/dir.c             | 143 +++++++++++++++++++++++++-------------
 fs/fuse/file.c            |   2 +-
 fs/fuse/fuse_i.h          |  10 ++-
 fs/fuse/inode.c           |  24 ++++++-
 fs/namespace.c            |   3 +-
 include/linux/fs.h        |   1 +
 include/uapi/linux/fuse.h |  24 ++++++-
 8 files changed, 153 insertions(+), 64 deletions(-)

-- 
2.34.1