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From:   Stephen Smalley <stephen.smalley.work@gmail.com>
Date:   Tue, 24 Mar 2020 10:35:16 -0400
Message-ID: <CAEjxPJ4MukexdmAD=py0r7vkE6vnn6T1LVcybP_GSJYsAdRuxA@mail.gmail.com>
Subject: Re: [PATCH bpf-next v5 4/7] bpf: lsm: Implement attach, detach and execution
To:     KP Singh <kpsingh@chromium.org>
Cc:     linux-kernel@vger.kernel.org, bpf@vger.kernel.org,
        LSM List <linux-security-module@vger.kernel.org>,
        Brendan Jackman <jackmanb@google.com>,
        Florent Revest <revest@google.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        James Morris <jmorris@namei.org>,
        Kees Cook <keescook@chromium.org>,
        Paul Turner <pjt@google.com>, Jann Horn <jannh@google.com>,
        Florent Revest <revest@chromium.org>,
        Brendan Jackman <jackmanb@chromium.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Paul Moore <paul@paul-moore.com>
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Sender: linux-kernel-owner@vger.kernel.org
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On Mon, Mar 23, 2020 at 12:46 PM KP Singh <kpsingh@chromium.org> wrote:
>
> From: KP Singh <kpsingh@google.com>
>
> JITed BPF programs are dynamically attached to the LSM hooks
> using BPF trampolines. The trampoline prologue generates code to handle
> conversion of the signature of the hook to the appropriate BPF context.
>
> The allocated trampoline programs are attached to the nop functions
> initialized as LSM hooks.
>
> BPF_PROG_TYPE_LSM programs must have a GPL compatible license and
> and need CAP_SYS_ADMIN (required for loading eBPF programs).
>
> Upon attachment:
>
> * A BPF fexit trampoline is used for LSM hooks with a void return type.
> * A BPF fmod_ret trampoline is used for LSM hooks which return an
>   int. The attached programs can override the return value of the
>   bpf LSM hook to indicate a MAC Policy decision.
>
> Signed-off-by: KP Singh <kpsingh@google.com>
> Reviewed-by: Brendan Jackman <jackmanb@google.com>
> Reviewed-by: Florent Revest <revest@google.com>
> ---

> diff --git a/kernel/bpf/bpf_lsm.c b/kernel/bpf/bpf_lsm.c
> index 530d137f7a84..2a8131b640b8 100644
> --- a/kernel/bpf/bpf_lsm.c
> +++ b/kernel/bpf/bpf_lsm.c
> @@ -9,6 +9,9 @@
>  #include <linux/btf.h>
>  #include <linux/lsm_hooks.h>
>  #include <linux/bpf_lsm.h>
> +#include <linux/jump_label.h>
> +#include <linux/kallsyms.h>
> +#include <linux/bpf_verifier.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.
> @@ -27,6 +30,32 @@ noinline __weak void bpf_lsm_##NAME(__VA_ARGS__) {}
>  #include <linux/lsm_hook_names.h>
>  #undef LSM_HOOK
>
> +#define BPF_LSM_SYM_PREFX  "bpf_lsm_"
> +
> +int bpf_lsm_verify_prog(struct bpf_verifier_log *vlog,
> +                       const struct bpf_prog *prog)
> +{
> +       /* Only CAP_MAC_ADMIN users are allowed to make changes to LSM hooks
> +        */
> +       if (!capable(CAP_MAC_ADMIN))
> +               return -EPERM;

I had asked before, and will ask again: please provide an explicit LSM
hook for mediating whether one can make changes to the LSM hooks.
Neither CAP_MAC_ADMIN nor CAP_SYS_ADMIN suffices to check this for SELinux.