Re: [PATCH RESEND v2 11/18] fs: Ensure the mounter of a filesystem is privileged towards its inodes

[Seth Forshee <seth.forshee@canonical.com>]

On Fri, Mar 04, 2016 at 04:43:06PM -0600, Eric W. Biederman wrote:
> Seth Forshee <seth.forshee@canonical.com> writes:
> 
> > On Mon, Jan 04, 2016 at 12:03:50PM -0600, Seth Forshee wrote:
> >> The mounter of a filesystem should be privileged towards the
> >> inodes of that filesystem. Extend the checks in
> >> inode_owner_or_capable() and capable_wrt_inode_uidgid() to
> >> permit access by users priviliged in the user namespace of the
> >> inode's superblock.
> >
> > Eric - I've discovered a problem related to this patch. The patches
> > you've already applied to your testing branch make it so that s_user_ns
> > can be an unprivileged user for proc and kernfs-based mounts. In some
> > cases DAC is the only thing protecting files in these mounts (ignoring
> > MAC), and with this patch an unprivileged user could bypass DAC.
> >
> > There's a simple solution - always set s_user_ns to &init_user_ns for
> > those filesystems. I think this is the right thing to do, since the
> > backing store behind these filesystems are really kernel objects.  But
> > this would break the assumption behind your patch "userns: Simpilify
> > MNT_NODEV handling" and cause a regression in mounting behavior.
> >
> > I've come up with several possible solutions for this conflict.
> >
> >  1. Drop this patch and keep on setting s_user_ns to unprivilged users.
> >     This would be unfortunate because I think this patch does make sense
> >     for most filesystems.
> >  2. Restrict this patch so that a user privileged towards s_user_ns is
> >     only privileged towards the super blocks inodes if s_user_ns has a
> >     mapping for both i_uid and i_gid. This is better than (1) but still
> >     not ideal in my mind.
> >  3. Drop your patch and maintain the current MNT_NODEV behavior.
> >  4. Add a new s_iflags flag to indicate a super block is from an
> >     unprivileged mount, and use this in your patch instead of s_user_ns.
> >
> > Any preference, or any other ideas?
> 
> In general this is only an issue if uids and gids on the filesystem
> do not map into the user namespace.

Yes, both capable_wrt_inode_uidgid and inode_owner_or_capable will
return true for a privileged user in the current namespace if the ids
map into that namespace.

> Therefore the general fix is to limit the logic of checking for
> capabilities in s_user_ns if we are dealing with INVALID_UID and
> INVALID_GID.  For proc and kernfs that should never be the case
> so the problem becomes a non-issue.
> 
> Further I would look at limiting that relaxation to just
> inode_change_ok.  So that we can easily wrap that check per filesystem
> and deny the relaxation for proc and kernfs.  proc and kernfs already
> have wrappers for .setattr so denying changes when !uid_vaid and
> !gid_valid would be a trivial addition, and ensure calamity does
> not ensure.
> 
> Furthmore by limiting any additional to inode_change_ok we keep
> the work of the additional tests off of the fast paths.

So then the inode would need to be chowned before a privileged user in a
non-init namespace would be capable towards it. That seems workable. It
looks like INVALID_UID and INVALID_GID do map into init_user_ns (which
seems a bit odd) so real root remains capable towards those indoes.

That seems okay to me then.

Thanks,
Seth