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

[Giuseppe Scrivano]

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

> On Mon, Oct 12, 2020 at 07:05:10PM +0200, Giuseppe Scrivano wrote:
>> Josh Triplett <josh@xxxxxxxxxxxxxxxx> 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