From: Giuseppe Scrivano <gscrivan@redhat.com>
To: Snaipe <snaipe@arista.com>
Cc: alexander@mihalicyn.com, christian.brauner@ubuntu.com,
containers@lists.linux-foundation.org, cyphar@cyphar.com,
ebiederm@xmission.com, geofft@ldpreload.com, jcsible@cert.org,
josh@joshtriplett.org, keescook@chromium.org,
linux-kernel@vger.kernel.org, luto@amacapital.net,
mic@digikod.net, mpatel@redhat.com, ptikhomirov@virtuozzo.com,
sargun@sargun.me, serge@hallyn.com, stgraber@ubuntu.com,
vgoyal@redhat.com, watl@google.com
Subject: Re: LPC 2020 Hackroom Session: summary and next steps for isolated user namespaces
Date: Thu, 22 Apr 2021 11:18:01 +0200 [thread overview]
Message-ID: <87h7jyvfsm.fsf@redhat.com> (raw)
In-Reply-To: <20210421172714.912119-1-snaipe@arista.com> (snaipe@arista.com's message of "Wed, 21 Apr 2021 19:27:14 +0200")
Snaipe <snaipe@arista.com> writes:
> "Giuseppe Scrivano" <gscrivan@redhat.com> 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
next prev parent reply other threads:[~2021-04-22 9:18 UTC|newest]
Thread overview: 28+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-08-30 14:39 LPC 2020 Hackroom Session: summary and next steps for isolated user namespaces Christian Brauner
2020-10-10 4:26 ` Serge E. Hallyn
2020-10-11 20:53 ` Josh Triplett
2020-10-12 0:38 ` Andy Lutomirski
2020-10-12 5:01 ` Eric W. Biederman
2020-10-12 15:00 ` Serge E. Hallyn
2020-10-14 19:46 ` Eric W. Biederman
2020-10-15 14:27 ` Serge E. Hallyn
2020-10-17 15:04 ` Eric W. Biederman
2020-10-12 17:05 ` Giuseppe Scrivano
2020-10-13 12:46 ` Serge E. Hallyn
2020-10-13 15:17 ` Giuseppe Scrivano
2020-10-15 14:32 ` Serge E. Hallyn
2020-10-19 12:12 ` Giuseppe Scrivano
2021-04-21 17:27 ` Snaipe
2021-04-22 9:18 ` Giuseppe Scrivano [this message]
2021-04-23 14:36 ` Franklin “Snaipe” Mathieu
2021-05-07 13:37 ` Serge E. Hallyn
2021-05-10 13:02 ` Giuseppe Scrivano
2021-05-10 13:57 ` Giuseppe Scrivano
2020-10-15 15:31 ` Enrico Weigelt, metux IT consult
2020-10-17 16:51 ` Eric W. Biederman
2020-10-18 10:20 ` Christian Brauner
2020-10-18 13:05 ` The problem of setgroups and containers Eric W. Biederman
2020-10-19 0:15 ` Eric W. Biederman
2020-10-19 20:07 ` [RFC][PATCH] userns: Limit process in a user namespace to what the creator is allowed Eric W. Biederman
2020-10-20 14:11 ` Christian Brauner
2020-10-29 13:42 ` LPC 2020 Hackroom Session: summary and next steps for isolated user namespaces Enrico Weigelt, metux IT consult
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