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

[Jann Horn <jann@thejh.net>]

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On Tue, Oct 18, 2016 at 09:56:53AM -0500, Eric W. Biederman wrote:
> Michal Hocko <mhocko@kernel.org> 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.
> 
> The core goal is to enforce the unreadability of an mm when an
> non-readable file is executed.  This is a time of mm creation property.
> The enforcement of which fits very well with the security/permission
> checking role of the user namespace.

How is that going to work? I thought the core goal was better security for
entering containers.

If I want to dump a non-readable file, afaik, I can just make a new user
namespace, then run the file in there and dump its memory.
I guess you could fix that by entirely prohibiting the execution of a
non-readable file whose owner UID is not mapped. (Adding more dumping
restrictions wouldn't help much because you could still e.g. supply a
malicious dynamic linker if you control the mount namespace.)

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