netdev - Re: [PATCH bpf-next v6 03/10] bpf: per-cgroup lsm flavor

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Message-ID: <CAKH8qBsR_kgQ3ETwm++AL7vZDcq1H-56eykqDdAcrveH5+ejzA@mail.gmail.com>
Date:   Tue, 10 May 2022 10:30:57 -0700
From:   Stanislav Fomichev <sdf@...gle.com>
To:     Martin KaFai Lau <kafai@...com>
Cc:     netdev@...r.kernel.org, bpf@...r.kernel.org, ast@...nel.org,
        daniel@...earbox.net, andrii@...nel.org
Subject: Re: [PATCH bpf-next v6 03/10] bpf: per-cgroup lsm flavor

On Tue, May 10, 2022 at 12:13 AM Martin KaFai Lau <kafai@...com> wrote:
>
> On Mon, May 09, 2022 at 04:38:36PM -0700, Stanislav Fomichev wrote:
> > > > +unsigned int __cgroup_bpf_run_lsm_current(const void *ctx,
> > > > +                                       const struct bpf_insn *insn)
> > > > +{
> > > > +     const struct bpf_prog *shim_prog;
> > > > +     struct cgroup *cgrp;
> > > > +     int ret = 0;
> > > From lsm_hook_defs.h, there are some default return values that are not 0.
> > > Is it ok to always return 0 in cases like the cgroup array is empty ?
> >
> > That's a good point, I haven't thought about it. You're right, it
> > seems like attaching to this hook for some LSMs will change the
> > default from some error to zero.
> > Let's start by prohibiting those hooks for now? I guess in theory,
> > when we generate a trampoline, we can put this default value as an
> > input arg to these new __cgroup_bpf_run_lsm_xxx helpers (in the
> > future)?
> After looking at arch_prepare_bpf_trampoline, return 0 here should be fine.
> If I read it correctly, when the shim_prog returns 0, the trampoline
> will call the original kernel function which is the bpf_lsm_##NAME()
> defined in bpf_lsm.c and it will then return the zero/-ve DEFAULT.

Not sure I read the same :-/ I'm assuming that for those cases we
actually end up generating fmod_ret trampoline which seems to be
unconditionally saving r0 into fp-8 ?

> > Another thing that seems to be related: there are a bunch of hooks
> > that return void, so returning EPERM from the cgroup programs won't
> > work as expected.
> > I can probably record, at verification time, whether lsm_cgroup
> > programs return any "non-success" return codes and prohibit attaching
> > these progs to the void hooks?
> hmm...yeah, BPF_LSM_CGROUP can be enforced to return either 0 or 1 as
> most other cgroup-progs do.
>
> Do you have a use case that needs to return something other than -EPERM ?

We do already enforce 0/1 for cgroup progs (and we have helpers to
expose custom errno). What I want to avoid is letting users attach
programs that try to return the error for the void hooks. And it seems
like we record that return range for a particular cgroup program and
verify it at attach time, WDYT?