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

[Serge E. Hallyn]

On Tue, Oct 13, 2020 at 05:17:36PM +0200, Giuseppe Scrivano wrote:
> "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.

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?

> > 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