linux-kernel - Re: [PATCH bpf-next v5 3/7] bpf: lsm: provide attachment points for BPF LSM programs

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Date:   Tue, 24 Mar 2020 17:12:11 +0100
From:   KP Singh <kpsingh@...omium.org>
To:     Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc:     open list <linux-kernel@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, linux-security-module@...r.kernel.org,
        Brendan Jackman <jackmanb@...gle.com>,
        Florent Revest <revest@...gle.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        James Morris <jmorris@...ei.org>,
        Kees Cook <keescook@...omium.org>,
        Paul Turner <pjt@...gle.com>, Jann Horn <jannh@...gle.com>,
        Florent Revest <revest@...omium.org>,
        Brendan Jackman <jackmanb@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH bpf-next v5 3/7] bpf: lsm: provide attachment points for
 BPF LSM programs

On 24-Mär 11:39, KP Singh wrote:
> On 23-Mär 12:59, Andrii Nakryiko wrote:
> > On Mon, Mar 23, 2020 at 9:45 AM KP Singh <kpsingh@...omium.org> wrote:
> > >
> > > From: KP Singh <kpsingh@...gle.com>
> > >
> > > When CONFIG_BPF_LSM is enabled, nops functions, bpf_lsm_<hook_name>, are
> > > generated for each LSM hook. These nops are initialized as LSM hooks in
> > > a subsequent patch.
> > >
> > > Signed-off-by: KP Singh <kpsingh@...gle.com>
> > > Reviewed-by: Brendan Jackman <jackmanb@...gle.com>
> > > Reviewed-by: Florent Revest <revest@...gle.com>
> > > ---
> > >  include/linux/bpf_lsm.h | 21 +++++++++++++++++++++
> > >  kernel/bpf/bpf_lsm.c    | 19 +++++++++++++++++++
> > >  2 files changed, 40 insertions(+)
> > >  create mode 100644 include/linux/bpf_lsm.h
> > >
> > > diff --git a/include/linux/bpf_lsm.h b/include/linux/bpf_lsm.h
> > > new file mode 100644
> > > index 000000000000..c6423a140220
> > > --- /dev/null
> > > +++ b/include/linux/bpf_lsm.h
> > > @@ -0,0 +1,21 @@
> > > +/* SPDX-License-Identifier: GPL-2.0 */
> > > +
> > > +/*
> > > + * Copyright (C) 2020 Google LLC.
> > > + */
> > > +
> > > +#ifndef _LINUX_BPF_LSM_H
> > > +#define _LINUX_BPF_LSM_H
> > > +
> > > +#include <linux/bpf.h>
> > > +#include <linux/lsm_hooks.h>
> > > +
> > > +#ifdef CONFIG_BPF_LSM
> > > +
> > > +#define LSM_HOOK(RET, NAME, ...) RET bpf_lsm_##NAME(__VA_ARGS__);
> > > +#include <linux/lsm_hook_names.h>
> > > +#undef LSM_HOOK
> > > +
> > > +#endif /* CONFIG_BPF_LSM */
> > > +
> > > +#endif /* _LINUX_BPF_LSM_H */
> > > diff --git a/kernel/bpf/bpf_lsm.c b/kernel/bpf/bpf_lsm.c
> > > index 82875039ca90..530d137f7a84 100644
> > > --- a/kernel/bpf/bpf_lsm.c
> > > +++ b/kernel/bpf/bpf_lsm.c
> > > @@ -7,6 +7,25 @@
> > >  #include <linux/filter.h>
> > >  #include <linux/bpf.h>
> > >  #include <linux/btf.h>
> > > +#include <linux/lsm_hooks.h>
> > > +#include <linux/bpf_lsm.h>
> > > +
> > > +/* For every LSM hook  that allows attachment of BPF programs, declare a NOP
> > > + * function where a BPF program can be attached as an fexit trampoline.
> > > + */
> > > +#define LSM_HOOK(RET, NAME, ...) LSM_HOOK_##RET(NAME, __VA_ARGS__)
> > > +
> > > +#define LSM_HOOK_int(NAME, ...)                        \
> > > +noinline __weak int bpf_lsm_##NAME(__VA_ARGS__)        \
> > > +{                                              \
> > > +       return 0;                               \
> > > +}
> > > +
> > > +#define LSM_HOOK_void(NAME, ...) \
> > > +noinline __weak void bpf_lsm_##NAME(__VA_ARGS__) {}
> > > +
> > 
> > Could unify with:
> > 
> > #define LSM_HOOK(RET, NAME, ...) noinline __weak RET bpf_lsm_##NAME(__VA_ARGS__)
> > {
> >     return (RET)0;
> > }
> > 
> > then you don't need LSM_HOOK_int and LSM_HOOK_void.
> 
> Nice.
> 
> But, given that we are adding default values and that
> they are only needed for int hooks, we will need to keep the macros
> separate for int and void. Or, Am I missing a trick here?
> 
> - KP

Actually, was able to get it work. not setting a default for void
hooks makes the macros messier. So i just set it void. For example:

  LSM_HOOK(void, void, bprm_committing_creds, struct linux_binprm *bprm)

This also allows me to use the cleanup you suggested and not having
to split every usage into int and void.

- KP

> 
> > 
> > > +#include <linux/lsm_hook_names.h>
> > > +#undef LSM_HOOK
> > >
> > >  const struct bpf_prog_ops lsm_prog_ops = {
> > >  };
> > > --
> > > 2.20.1
> > >