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

[Josh Triplett]

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.