Re: [PATCH v3 0/4] Introduce security_create_user_ns()

[KP Singh <kpsingh@kernel.org>]

On Mon, Aug 1, 2022 at 6:35 PM Paul Moore <paul@paul-moore.com> wrote:
>
> On Mon, Aug 1, 2022 at 11:25 AM Casey Schaufler <casey@schaufler-ca.com> wrote:
> > On 8/1/2022 6:13 AM, Frederick Lawler wrote:
> > > On 7/22/22 7:20 AM, Paul Moore wrote:
> > >> On July 22, 2022 2:12:03 AM Martin KaFai Lau <kafai@fb.com> wrote:
> > >>
> > >>> On Thu, Jul 21, 2022 at 12:28:04PM -0500, Frederick Lawler wrote:
> > >>>> While creating a LSM BPF MAC policy to block user namespace
> > >>>> creation, we
> > >>>> used the LSM cred_prepare hook because that is the closest hook to
> > >>>> prevent
> > >>>> a call to create_user_ns().
> > >>>>
> > >>>> The calls look something like this:
> > >>>>
> > >>>> cred = prepare_creds()
> > >>>> security_prepare_creds()
> > >>>> call_int_hook(cred_prepare, ...
> > >>>> if (cred)
> > >>>> create_user_ns(cred)
> > >>>>
> > >>>> We noticed that error codes were not propagated from this hook and
> > >>>> introduced a patch [1] to propagate those errors.
> > >>>>
> > >>>> The discussion notes that security_prepare_creds()
> > >>>> is not appropriate for MAC policies, and instead the hook is
> > >>>> meant for LSM authors to prepare credentials for mutation. [2]
> > >>>>
> > >>>> Ultimately, we concluded that a better course of action is to
> > >>>> introduce
> > >>>> a new security hook for LSM authors. [3]
> > >>>>
> > >>>> This patch set first introduces a new security_create_user_ns()
> > >>>> function
> > >>>> and userns_create LSM hook, then marks the hook as sleepable in BPF.
> > >>> Patch 1 and 4 still need review from the lsm/security side.
> > >>
> > >>
> > >> This patchset is in my review queue and assuming everything checks
> > >> out, I expect to merge it after the upcoming merge window closes.
> > >>
> > >> I would also need an ACK from the BPF LSM folks, but they're CC'd on
> > >> this patchset.
> > >
> > > Based on last weeks comments, should I go ahead and put up v4 for
> > > 5.20-rc1 when that drops, or do I need to wait for more feedback?
> >
> > As the primary consumer of this hook is BPF I would really expect their
> > reviewed-by before accepting this.
>
> We love all our in-tree LSMs equally.  As long as there is at least
> one LSM which provides an implementation and has ACK'd the hook, and
> no other LSMs have NACK'd the hook, then I have no problem merging it.
> I doubt it will be necessary in this case, but if we need to tweak the
> hook in the future we can definitely do that; we've done this in the
> past when it has made sense.
>
> As a reminder, the LSM hooks are *not* part of the "don't break
> userspace" promise.  I know it gets a little muddy with the way the

That's correct. Also, with BPF LSM, we encourage users to
build the application / bpf program logic to be resilient to changes
in the LSM hooks.

I am happy to share how we've done it, if folks are interested.

- KP

> BPF LSM works, but just as we don't want to allow one LSM to impact
> the runtime controls on another, we don't want to allow one LSM to
> freeze the hooks for everyone.
>
> --
> paul-moore.com