On Mon, Aug 1, 2022 at 6:35 PM Paul Moore <paul@xxxxxxxxxxxxxx> wrote: > > On Mon, Aug 1, 2022 at 11:25 AM Casey Schaufler <casey@xxxxxxxxxxxxxxxx> 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@xxxxxx> 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
