Re: [PATCH bpf-next v6 3/8] bpf: lsm: provide attachment points for BPF LSM programs

[Kees Cook]

On Wed, Mar 25, 2020 at 04:26:24PM +0100, KP Singh wrote:
> From: KP Singh <kpsingh@xxxxxxxxxx>
> 
> When CONFIG_BPF_LSM is enabled, nop functions, bpf_lsm_<hook_name>, are
> generated for each LSM hook. These functions are initialized as LSM
> hooks in a subsequent patch.
> 
> Signed-off-by: KP Singh <kpsingh@xxxxxxxxxx>
> Reviewed-by: Brendan Jackman <jackmanb@xxxxxxxxxx>
> Reviewed-by: Florent Revest <revest@xxxxxxxxxx>
> Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
> Acked-by: Yonghong Song <yhs@xxxxxx>
> ---
>  include/linux/bpf_lsm.h | 22 ++++++++++++++++++++++
>  kernel/bpf/bpf_lsm.c    | 14 ++++++++++++++
>  2 files changed, 36 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..83b96895829f
> --- /dev/null
> +++ b/include/linux/bpf_lsm.h
> @@ -0,0 +1,22 @@
> +/* 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, DEFAULT, NAME, ...) \
> +	RET bpf_lsm_##NAME(__VA_ARGS__);
> +#include <linux/lsm_hook_defs.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..1210a819ca52 100644
> --- a/kernel/bpf/bpf_lsm.c
> +++ b/kernel/bpf/bpf_lsm.c
> @@ -7,6 +7,20 @@
>  #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.
> + */
> +#define LSM_HOOK(RET, DEFAULT, NAME, ...) 	\
> +noinline __weak RET bpf_lsm_##NAME(__VA_ARGS__)	\

I don't think the __weak is needed any more here?

> +{						\
> +	return DEFAULT;				\

I'm impressed that LSM_RET_VOID actually works. :)

-Kees

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

-- 
Kees Cook