Re: [PATCH v5 2/4] fuse: Support fuse filesystems outside of init_user_ns

[Seth Forshee]

On Tue, Nov 18, 2014 at 09:09:34AM -0800, Andy Lutomirski wrote:
> On Tue, Nov 18, 2014 at 7:21 AM, Seth Forshee
> <seth.forshee@xxxxxxxxxxxxx> wrote:
> > On Wed, Nov 12, 2014 at 10:22:54AM -0600, Seth Forshee wrote:
> >> On Wed, Nov 12, 2014 at 02:09:15PM +0100, Miklos Szeredi wrote:
> >> > On Tue, Nov 11, 2014 at 09:37:10AM -0600, Eric W. Biederman wrote:
> >> >
> >> > > > Maybe I'm being dense, but can someone give a concrete example of such an
> >> > > > attack?
> >> > >
> >> > > There are two variants of things at play here.
> >> > >
> >> > > There is the classic if you don't freeze your context at open time when
> >> > > you pass that file descriptor to another process unexpected things can
> >> > > happen.
> >> > >
> >> > > An essentially harmless but extremely confusing example is what happens
> >> > > to a partial read when it stops halfway through a uid value and the next
> >> > > read on the same file descriptor is from a process in a different user
> >> > > namespace.  Which uid value should be returned to userspace.
> >> >
> >> > Fuse device doesn't currently do partial reads, so that's a non-issue.
> >> >
> >> > > Now if I am in a nefarious mood I can create a unprivileged user
> >> > > namespace, open /dev/fuse and mount a fuse filesystem.  Pass the file
> >> > > descriptor to /dev/fuse to a processes that is in the default user
> >> > > namespace (and thus can use any uid/gid).   With that file desctipor
> >> > > report that there is a setuid 0 exectuable on that file system.
> >> >
> >> > Yes, and this would also be prevented by MNT_NOSUID, which would be a good idea
> >> > anyway.  I just don't see the reason we'd want to allow clearing MNT_NOSUID in a
> >> > private namespace.
> >> >
> >> > So we don't currently see a use case for relaxing either the MNT_NOSUID
> >> > restriction or for relaxing the requirement on the user namespace the fuse
> >> > server is in.  Is that correct?
> >> >
> >> > If so, we should leave both restrictions in place since that allows the greatest
> >> > flexibility in the future, is either of those needs to be relaxed.
> >>
> >> I'm not aware of specific use cases for either at this point. However,
> >> Andy's patch [1] will limit suid to the set of namespaces where the user
> >> who mounted the filesystem already has privileges. Enforcing MNT_NOSUID
> >> will require enforcement in the vfs, and in that case we definitely need
> >> to decide whether the policy is to implicitly add the flag or fail the
> >> mount attempt if the flag is not present [2].
> >
> > I asked around a bit, and it turns out there are use cases for nested
> > containers (i.e. a container within a container) where the rootfs for
> > the outer container mounts a filesystem containing the rootfs for the
> > inner container. If that mount is nosuid then suid utilities like ping
> > aren't going to work in the inner container.
> >
> > So since there's a use case for suid in a userns mount and we have what
> > we belive are sufficient protections against using this as a vector to
> > get privileges outside the container, I'm planning to move ahead without
> > the MNT_NOSUID restriction. Any objections?
> 
> Are you talking about MNT_NOSUID the flag or my ns-dependent thing?

I'm talking about dropping the proposed requirement from Miklos that all
fuse userns mounts are required to have the MNT_NOSUID flag. I intend to
keep your ns-dependent thing.

Thanks,
Seth

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/