[PATCH 00/34] fs: idmapped mounts

[Serge E. Hallyn]

On Thu, Oct 29, 2020 at 11:37:23AM -0500, Eric W. Biederman wrote:
> Aleksa Sarai <cyphar at cyphar.com> writes:
> 
> > On 2020-10-29, Eric W. Biederman <ebiederm at xmission.com> wrote:
> >> Christian Brauner <christian.brauner at ubuntu.com> writes:
> >> 
> >> > Hey everyone,
> >> >
> >> > I vanished for a little while to focus on this work here so sorry for
> >> > not being available by mail for a while.
> >> >
> >> > Since quite a long time we have issues with sharing mounts between
> >> > multiple unprivileged containers with different id mappings, sharing a
> >> > rootfs between multiple containers with different id mappings, and also
> >> > sharing regular directories and filesystems between users with different
> >> > uids and gids. The latter use-cases have become even more important with
> >> > the availability and adoption of systemd-homed (cf. [1]) to implement
> >> > portable home directories.
> >> 
> >> Can you walk us through the motivating use case?
> >> 
> >> As of this year's LPC I had the distinct impression that the primary use
> >> case for such a feature was due to the RLIMIT_NPROC problem where two
> >> containers with the same users still wanted different uid mappings to
> >> the disk because the users were conflicting with each other because of
> >> the per user rlimits.
> >> 
> >> Fixing rlimits is straight forward to implement, and easier to manage
> >> for implementations and administrators.
> >
> > This is separate to the question of "isolated user namespaces" and
> > managing different mappings between containers. This patchset is solving
> > the same problem that shiftfs solved -- sharing a single directory tree
> > between containers that have different ID mappings. rlimits (nor any of
> > the other proposals we discussed at LPC) will help with this problem.
> 
> First and foremost: A uid shift on write to a filesystem is a security
> bug waiting to happen.  This is especially in the context of facilities
> like iouring, that play very agressive games with how process context
> makes it to  system calls.
> 
> The only reason containers were not immediately exploitable when iouring
> was introduced is because the mechanisms are built so that even if
> something escapes containment the security properties still apply.
> Changes to the uid when writing to the filesystem does not have that
> property.  The tiniest slip in containment will be a security issue.
> 
> This is not even the least bit theoretical.  I have seem reports of how
> shitfs+overlayfs created a situation where anyone could read
> /etc/shadow.
> 
> If you are going to write using the same uid to disk from different
> containers the question becomes why can't those containers configure
> those users to use the same kuid?

Because if user 'myapp' in two otherwise isolated containers both have
the same kuid, so that they can write to a shared directory, then root
in container 1 has privilege over all files owned by 'myapp' in
container 2.

Whereas if they can each have distinct kuids, but when writing to the
shared fs have a shared uid not otherwise belonging to either container,
their rootfs's can remain completely off limits to each other.

> What fixing rlimits does is it fixes one of the reasons that different
> containers could not share the same kuid for users that want to write to
> disk with the same uid.
> 
> 
> I humbly suggest that it will be more secure, and easier to maintain for
> both developers and users if we fix the reasons people want different
> containers to have the same user running with different kuids.
> 
> If not what are the reasons we fundamentally need the same on-disk user
> using multiple kuids in the kernel?
> 
> Eric