Snaipe <snaipe@xxxxxxxxxx> writes: > "Giuseppe Scrivano" <gscrivan@xxxxxxxxxx> writes: >>>> >> instead of a prctl, I've added a new mode to /proc/PID/setgroups that >>>> >> allows setgroups in a userns locking the current gids. >>>> >> >>>> >> What do you think about using /proc/PID/setgroups instead of a new >>>> >> prctl()? >>>> > >>>> > It's better than not having it, but two concerns - >>>> > >>>> > 1. some userspace, especially testsuites, could become confused by the fact >>>> > that they can't drop groups no matter how hard they try, since these will all >>>> > still show up as regular groups. >>>> >>>> I forgot to send a link to a second patch :-) that completes the feature: >>>> https://github.com/giuseppe/linux/commit/1c5fe726346b216293a527719e64f34e6297f0c2 >>>> >>>> When the new mode is used, the gids that are not known in the userns do >>>> not show up in userspace. >>> >>> Ah, right - and of course those gids better not be mapped into the namespace :) >>> >>> But so, this is the patch you said you agreed was not worth the extra >>> complexity? >> >> yes, these two patches are what looked too complex at that time. The >> problem still exists though, we could perhaps reconsider if the >> extra-complexity is acceptable to address it. > > Hey Folks, sorry for necro-bumping, but I've found this discussion > while searching for this specific issue, and it seems like the most > recent relevant discussion on the matter. I'd like to chime in with > our personal experience. > > We have a tool[1] that allows unprivileged use of namespaces > (when using a userns, which is the default). > > The primary use-case of said tool is lightweight containerization, > but we're also using it for other mundane usages, like a better > substitute for fakeroot to build and package privileged software > (e.g. sudo or ping, which needs to be installed with special > capabilities) unprivileged, or to copy file trees that are owned by > the user or sub-ids. > > For the first use-case, it's always safe to drop unmapped groups, > because the target rootfs is always owned by the user or its sub-ids. > > For the other use-cases, this is more problematic, as you're all > well-aware of. Our position right now is that the tool will always > allow setgroups in user namespace, and that it's not safe to use on > systems that rely on negative access groups. > > I think that something that's not mentioned is that if a user setgroups > to a fixed list of subgids, dropping all unmapped gids, they don't just > gain the ability to access these negative-access files, they also lose > legitimate access to files that their unmapped groups allow them to > access. This is fine for our first use-case, but a bit surprising for > the second one -- and since setgroups never lets us keep unmapped gids, > we have no way to keep these desired groups. > > From a first glance, a sysctl that explicitly controls that would not > address the above problem, but keeping around the original group list > of the owner of the user ns would have the desired semantics. > > Giuseppe's patch seems to address this use case, which would personally > make me very happy. > > [1]: https://github.com/aristanetworks/bst thanks for the feedback. We are still facing the issue with rootless Podman, and these patches (listed here so you won't need to dig into archives): https://github.com/giuseppe/linux/commit/7e0701b389c497472d11fab8570c153a414050af https://github.com/giuseppe/linux/commit/1c5fe726346b216293a527719e64f34e6297f0c2 would solve the issue for us as well and we can use setgroups within a user namespace in a safe way. Any comments on this approach? Could we move forward with it? Regards, Giuseppe _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linuxfoundation.org/mailman/listinfo/containers
