Re: open_by_handle_at() in userns

[Christian Brauner <christian.brauner@ubuntu.com>]

On Thu, Apr 08, 2021 at 06:34:01PM +0300, Amir Goldstein wrote:
> On Thu, Apr 8, 2021 at 3:55 PM Christian Brauner
> <christian.brauner@ubuntu.com> 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
> >
> 
> The check is needed for slightly different reasons.
> nfsd "subtree_check" feature explicitly meant to forbid access in case
> file was moved "out of reach", for example, out of the export path.
> Note the nfsd "subtree_check" affects both file handle encoding
> (i.e. "connectable") and file handle decoding (i.e. nfsd_acceptable()).
> 
> open_by_handle_at() in idmapped mount needs to verify that the ancestry
> inode owners can be mapped to the userns, because we already checked
> that user has CAP_DAC_READ_SEARCH in userns, but it's nicer to do
> the full inode_permission() check IMO.
> 
> > Afaict, an nfs server can't be mounted inside of userns right now. That
> > is something that folks from Netflix and from Kinvolk have been
> > interested in enabling. They also want the ability to use idmapped
> > mounts + nfs. Understandable that you don't want to drive this of
> > course. I'll sync with them about this.
> >
> > Independent of that, I thought our last understanding was that you
> > wouldn't need to handle open_by_handle_at() for now.
> >
> 
> I don't need it. But I realized that the fanotify_userns demo branch
> I provided you is buggy in terms of security, so I wanted to give you
> (or whoever wants to pursue this) a better reference.
> It was one of those things that are easier to code than to explain ;-)

And I highly appreciate it! :)
Please keep those branches for reference somewhere. I've noted them down
to make sure to look at them before I do anything!

Christian