Re: LPC 2020 Hackroom Session: summary and next steps for isolated user namespaces

[Giuseppe Scrivano]

Hi Serge,

"Serge E. Hallyn" <serge@xxxxxxxxxx> writes:

> On Thu, Apr 22, 2021 at 11:18:01AM +0200, Giuseppe Scrivano wrote:
>> 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?
>
> Can you send these to lkml?  I'll have to go back through our previous
> discussions, but offhand this looks good to me.

I've just sent them to lkml.

Regards,
Giuseppe