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

From: Giuseppe Scrivano <gscrivan@redhat.com>
To: Josh Triplett <josh@joshtriplett.org>
Cc: "Alexander Mihalicyn" <alexander@mihalicyn.com>,
	"Joseph Christopher Sible" <jcsible@cert.org>,
	"Kees Cook" <keescook@chromium.org>,
	containers@lists.linux-foundation.org,
	linux-kernel@vger.kernel.org, "Vivek Goyal" <vgoyal@redhat.com>,
	"Geoffrey Thomas" <geofft@ldpreload.com>,
	"Andy Lutomirski" <luto@amacapital.net>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	"Mickaël Salaün" <mic@digikod.net>, "Wat Lim" <watl@google.com>,
	"Mrunal Patel" <mpatel@redhat.com>,
	"Pavel Tikhomirov" <ptikhomirov@virtuozzo.com>
Subject: Re: LPC 2020 Hackroom Session: summary and next steps for isolated user namespaces
Date: Mon, 12 Oct 2020 19:05:10 +0200	[thread overview]
Message-ID: <87tuuzv0hl.fsf@redhat.com> (raw)
In-Reply-To: <20201011205306.GC17441@localhost> (Josh Triplett's message of "Sun, 11 Oct 2020 13:53:06 -0700")

Josh Triplett <josh@joshtriplett.org> writes:

> On Fri, Oct 09, 2020 at 11:26:06PM -0500, Serge E. Hallyn wrote:
>> > 3. Find a way to allow setgroups() in a user namespace while keeping
>> >    in mind the case of groups used for negative access control.
>> >    This was suggested by Josh Triplett and Geoffrey Thomas. Their idea was to
>> >    investigate adding a prctl() to allow setgroups() to be called in a user
>> >    namespace at the cost of restricting paths to the most restrictive
>> >    permission. So if something is 0707 it needs to be treated as if it's 0000
>> >    even though the caller is not in its owning group which is used for negative
>> >    access control (how these new semantics will interact with ACLs will also
>> >    need to be looked into).
>> 
>> I should probably think this through more, but for this problem, would it
>> not suffice to add a new prevgroups grouplist to the struct cred, maybe
>> struct group_info *locked_groups, and every time an unprivileged task creates
>> a new user namespace, add all its current groups to this list?
>
> So, effectively, you would be allowed to drop permissions, but
> locked_groups would still be checked for restrictions?
>
> That seems like it'd introduce a new level of complexity (a new facet of
> permission) to manage. Not opposed, but it does seem more complex than
> just opting out of using groups for negative permissions.

I have played with something similar in the past.  At that time I've
discussed it only privately with Eric and we agreed it wasn't worth the
extra complexity:

https://github.com/giuseppe/linux/commit/7e0701b389c497472d11fab8570c153a414050af

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()?

Giuseppe

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