Re: [REVIEW][PATCH] mm: Add a user_ns owner to mm_struct and fix ptrace_may_access

From: Michal Hocko
Date: Tue Oct 18 2016 - 14:06:38 EST

On Tue 18-10-16 09:56:53, Eric W. Biederman wrote:
> Michal Hocko <mhocko@xxxxxxxxxx> writes:
> > On Mon 17-10-16 11:39:49, Eric W. Biederman wrote:
> >>
> >> During exec dumpable is cleared if the file that is being executed is
> >> not readable by the user executing the file. A bug in
> >> ptrace_may_access allows reading the file if the executable happens to
> >> enter into a subordinate user namespace (aka clone(CLONE_NEWUSER),
> >> unshare(CLONE_NEWUSER), or setns(fd, CLONE_NEWUSER).
> >>
> >> This problem is fixed with only necessary userspace breakage by adding
> >> a user namespace owner to mm_struct, captured at the time of exec,
> >> so it is clear in which user namespace CAP_SYS_PTRACE must be present
> >> in to be able to safely give read permission to the executable.
> >>
> >> The function ptrace_may_access is modified to verify that the ptracer
> >> has CAP_SYS_ADMIN in task->mm->user_ns instead of task->cred->user_ns.
> >> This ensures that if the task changes it's cred into a subordinate
> >> user namespace it does not become ptraceable.
> >
> > I haven't studied your patch too deeply but one thing that immediately
> > raised a red flag was that mm might be shared between processes (aka
> > thread groups). What prevents those two to sit in different user
> > namespaces?
> >
> > I am primarily asking because this generated a lot of headache for the
> > memcg handling as those processes might sit in different cgroups while
> > there is only one correct memcg for them which can disagree with the
> > cgroup associated with one of the processes.
> That is a legitimate concern, but I do not see any of those kinds of
> issues here.
> Part of the memcg pain comes from the fact that control groups are
> process centric, and part of the pain comes from the fact that it is
> possible to change control groups. What I am doing is making the mm
> owned by a user namespace (at creation time), and I am not allowing
> changes to that ownership. The credentials of the tasks that use that mm
> may be in the same user namespace or descendent user namespaces.

OK, then my worries about this weird "threading" model is void.

Thanks for the clarification.
Michal Hocko