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

[Jann Horn]

On Tue, Oct 18, 2016 at 10:35:23AM -0500, Eric W. Biederman wrote:
> Jann Horn <jann@xxxxxxxxx> writes:
> 
> > On Tue, Oct 18, 2016 at 09:56:53AM -0500, 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.
> >> 
> >> 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.
> 
> The better security when entering containers came from fixing the the
> check for unreadable files.  Because that is fundamentally what
> the mm dumpable settings are for.

Oh, interesting.


> > 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.)
> 
> That seems to be a part of this puzzle I have incompletely addressed,
> thank you.
> 
> It looks like I need to change either the owning user namespace or
> fail the exec.  Malicious dynamic linkers are doubly interesting.
> 
> As mount name spaces are also owned if I have privileges I can address
> the possibility of a malicious dynamic linker that way.  AKA who cares
> about the link if the owner of the mount namespace has permissions to
> read the file.

If you just check the owner of the mount namespace, someone could still
use a user namespace to chroot() the process. That should also be
sufficient to get the evil linker in. I think it really needs to be the
user namespace of the executing process that's checked, not the user
namespace associated with some mount namespace.


> I am going to look at failing the exec if the owning user namespace
> of the mm would not have permissions to read the file.  That should just
> be a couple of lines of code and easy to maintain.  Plus it does not
> appear that non-readable executables are particularly common.

Hm. Yeah, I guess mode 04111 probably isn't sooo common.
>From a security perspective, I think that should work.