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

Seth Forshee
2016-03-07T13:32:51Z

On Sun, Mar 06, 2016 at 04:07:49PM -0600, Eric W. Biederman wrote:
> Seth Forshee <seth.forshee@canonical.com> writes:
>
> > 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.
>
> If I was not clear I was suggesting that we allow a sufficiently
> privileged user in the filesysteme's s_user_ns to allow chowning files
> with INVALID_UID and INVALID_GID.

Right, I got that.

> The global root user would always be able to do that because unless
> capabilities are dropped it is sufficiently privileged in ever user
> namespace.

Sure. I was just commenting on one result - that ns-root has to chown
the file before being privileged wrt that file but global root does not,
on account of the fact that the invalid ids are mapped in init_user_ns.

Seth