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

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Message-ID: <20200324103910.GA7135@chromium.org>
Date:   Tue, 24 Mar 2020 11:39:10 +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 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

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