linux-kernel - Re: [PATCH bpf-next v4 3/8] bpf: lsm: provide attachment points for BPF LSM programs

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Date:   Thu, 20 Feb 2020 18:25:38 -0800
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     KP Singh <kpsingh@...omium.org>
Cc:     linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        Daniel Borkmann <daniel@...earbox.net>,
        James Morris <jmorris@...ei.org>,
        Kees Cook <keescook@...omium.org>,
        Jann Horn <jannh@...gle.com>,
        "David S. Miller" <davem@...emloft.net>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [PATCH bpf-next v4 3/8] bpf: lsm: provide attachment points for
 BPF LSM programs

On Thu, Feb 20, 2020 at 06:52:45PM +0100, KP Singh wrote:
> From: KP Singh <kpsingh@...gle.com>
> 
> The BPF LSM programs are implemented as fexit trampolines to avoid the
> overhead of retpolines. These programs cannot be attached to security_*
> wrappers as there are quite a few security_* functions that do more than
> just calling the LSM callbacks.
> 
> This was discussed on the lists in:
> 
>   https://lore.kernel.org/bpf/20200123152440.28956-1-kpsingh@chromium.org/T/#m068becce588a0cdf01913f368a97aea4c62d8266
> 
> Adding a NOP callback after all the static LSM callbacks are called has
> the following benefits:
> 
> - The BPF programs run at the right stage of the security_* wrappers.
> - They run after all the static LSM hooks allowed the operation,
>   therefore cannot allow an action that was already denied.
> 
> There are some hooks which do not call call_int_hooks or
> call_void_hooks. It's not possible to call the bpf_lsm_* functions
> without checking if there is BPF LSM program attached to these hooks.
> This is added further in a subsequent patch. For now, these hooks are
> marked as NO_BPF (i.e. attachment of BPF programs is not possible).

the commit log doesn't match the code.

> +
> +/* 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 int bpf_lsm_##NAME(__VA_ARGS__)  \

Did you check generated asm?
I think I saw cases when gcc ignored 'noinline' when function is defined in the
same file and still performed inlining while keeping the function body.
To be safe I think __weak is necessary. That will guarantee noinline.

And please reduce your cc next time. It's way too long.