From: Giuseppe Scrivano <gscrivan@redhat.com>
To: "Serge E. Hallyn" <serge@hallyn.com>
Cc: "Josh Triplett" <josh@joshtriplett.org>,
"Christian Brauner" <christian.brauner@ubuntu.com>,
containers@lists.linux-foundation.org,
"Alexander Mihalicyn" <alexander@mihalicyn.com>,
"Mrunal Patel" <mpatel@redhat.com>, "Wat Lim" <watl@google.com>,
"Aleksa Sarai" <cyphar@cyphar.com>,
"Pavel Tikhomirov" <ptikhomirov@virtuozzo.com>,
"Geoffrey Thomas" <geofft@ldpreload.com>,
"Eric W. Biederman" <ebiederm@xmission.com>,
"Joseph Christopher Sible" <jcsible@cert.org>,
"Mickaël Salaün" <mic@digikod.net>,
"Vivek Goyal" <vgoyal@redhat.com>,
"Andy Lutomirski" <luto@amacapital.net>,
"Stephane Graber" <stgraber@ubuntu.com>,
"Kees Cook" <keescook@chromium.org>,
"Sargun Dhillon" <sargun@sargun.me>,
linux-kernel@vger.kernel.org
Subject: Re: LPC 2020 Hackroom Session: summary and next steps for isolated user namespaces
Date: Tue, 13 Oct 2020 17:17:36 +0200 [thread overview]
Message-ID: <87o8l6qhnz.fsf@redhat.com> (raw)
In-Reply-To: <20201013124650.GA19668@mail.hallyn.com> (Serge E. Hallyn's message of "Tue, 13 Oct 2020 07:46:50 -0500")
"Serge E. Hallyn" <serge@hallyn.com> writes:
> On Mon, Oct 12, 2020 at 07:05:10PM +0200, Giuseppe Scrivano wrote:
>> 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
>
> Hi, you linked the setgroups patch, but do you also have a link to the
> attempt which you deemed was not worth it?
it was just part of a private discussion; but was 4 years ago so we can
probably revisit and accept the additional complexity since setgroups()
is still an issue with user namespaces.
>> 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.
> 2. whereas in my lockgroups proposal, lock_groups would only be taken into account
> for permission denial, this proposal would count for permission grants too. This
> means that if I have a group which is permitted to read /foo/topsecret, and I
> start a program in a new user namespace expecting it to drop that permission,
> I can't have that, right? The new program, will always have that permission?
right. The new mode I was working on cannot be used to drop granted permissions.
Giuseppe
next prev parent reply other threads:[~2020-10-13 15:17 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 [this message]
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
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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