Re: open_by_handle_at() in userns

[Amir Goldstein <amir73il@gmail.com>]

On Thu, Apr 8, 2021 at 5:15 PM J. Bruce Fields <bfields@fieldses.org> wrote:
>
> On Thu, Apr 08, 2021 at 02:55:30PM +0200, Christian Brauner wrote:
> > On Thu, Apr 08, 2021 at 02:44:47PM +0300, Amir Goldstein wrote:
> > > > One thing your patch
> > > >
> > > > commit ea31e84fda83c17b88851de399f76f5d9fc1abf4
> > > > Author: Amir Goldstein <amir73il@gmail.com>
> > > > Date:   Sat Mar 20 12:58:12 2021 +0200
> > > >
> > > >     fs: allow open by file handle inside userns
> > > >
> > > >     open_by_handle_at(2) requires CAP_DAC_READ_SEARCH in init userns,
> > > >     where most filesystems are mounted.
> > > >
> > > >     Relax the requirement to allow a user with CAP_DAC_READ_SEARCH
> > > >     inside userns to open by file handle in filesystems that were
> > > >     mounted inside that userns.
> > > >
> > > >     In addition, also allow open by handle in an idmapped mount, which is
> > > >     mapped to the userns while verifying that the returned open file path
> > > >     is under the root of the idmapped mount.
> > > >
> > > >     This is going to be needed for setting an fanotify mark on a filesystem
> > > >     and watching events inside userns.
> > > >
> > > >     Signed-off-by: Amir Goldstein <amir73il@gmail.com>
> > > >
> > > > Requires fs/exportfs/expfs.c to be made idmapped mounts aware.
> > > > open_by_handle_at() uses exportfs_decode_fh() which e.g. has the
> > > > following and other callchains:
> > > >
> > > > exportfs_decode_fh()
> > > > -> exportfs_decode_fh_raw()
> > > >    -> lookup_one_len()
> > > >       -> inode_permission(mnt_userns, ...)
> > > >
> > > > That's not a huge problem though I did all these changes for the
> > > > overlayfs support for idmapped mounts I have in a branch from an earlier
> > > > version of the idmapped mounts patchset. Basically lookup_one_len(),
> > > > lookup_one_len_unlocked(), and lookup_positive_unlocked() need to take
> > > > the mnt_userns into account. I can rebase my change and send it for
> > > > consideration next cycle. If you can live without the
> > > > open_by_handle_at() support for now in this patchset (Which I think you
> > > > said you could.) then it's not a blocker either. Sorry for the
> > > > inconvenience.
> > > >
> > >
> > > Christian,
> > >
> > > I think making exportfs_decode_fh() idmapped mount aware is not
> > > enough, because when a dentry alias is found in dcache, none of
> > > those lookup functions are called.
> > >
> > > I think we will also need something like this:
> > > https://github.com/amir73il/linux/commits/fhandle_userns
> > >
> > > I factored-out a helper from nfsd_apcceptable() which implements
> > > the "subtree_check" nfsd logic and uses it for open_by_handle_at().
> > >
> > > I've also added a small patch to name_to_handle_at() with a UAPI
> > > change that could make these changes usable by userspace nfs
> > > server inside userns, but I have no demo nor tests for that and frankly,
> > > I have little incentive to try and promote this UAPI change without
> > > anybody asking for it...
> >
> > Ah, at first I was confused about why this would matter but it matters
> > because nfsd already implements a check of that sort directly in nfsd
> > independent of idmapped mounts:
> > https://github.com/amir73il/linux/commit/4bef9ff1718935b7b42afbae71cfaab7770e8436
>
> Only in the NFSEXP_NOSUBTREECHECK case.  Taking a quick look, I think
> Amir's not proposing a check like that by default, so, fine.  (I assume
> problems with e.g. subtreechecking and cross-directory renames are
> understood....)
>

They are understood to me :) but I didn't want to get into it, because it is
complicated to explain and I wasn't sure if anyone cared...

I started working on open_by_handle_at() in userns for fanotify and fanotify
mostly reports directory fhandle, so no issues with cross-directory renames.
In any case, fanotify never reports "connectable" non-dir file handles.

Because my proposed change ALSO makes it possible to start talking about
userspace nfs server inside userns (in case anyone cares), I wanted to lay
out the path towards a userspace "subtree_check" like solution.

Another thing I am contemplating is, if and when idmapped mount support
is added to overlayfs, we can store an additional "connectable" file handle
in the overlayfs index (whose key is the non-connectable fhandle) and
fix ovl_acceptable() similar to nfsd_acceptable() and then we will be able
to mount an overlayfs inside userns with nfs_export support.

I've included a two liner patch on the fhandle_userns branch to allow
overlayfs inside userns with nfs_export support in the case that
underlying filesystem was mounted inside userns, but that is not such
an interesting use case IMO.

Thanks,
Amir.