On Thu, Oct 29, 2020 at 11:37:23AM -0500, Eric W. Biederman wrote: > Aleksa Sarai <cyphar@xxxxxxxxxx> writes: > > > On 2020-10-29, Eric W. Biederman <ebiederm@xxxxxxxxxxxx> wrote: > >> Christian Brauner <christian.brauner@xxxxxxxxxx> 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
