[PATCH bpf-next v4 4/7] bpf: Attachment verification for BPF_MODIFY_RETURN

[Stephen Smalley]

On Wed, Mar 4, 2020 at 2:20 PM KP Singh <kpsingh at chromium.org> wrote:
>
> From: KP Singh <kpsingh at google.com>
>
> - Allow BPF_MODIFY_RETURN attachment only to functions that are:
>
>     * Whitelisted for error injection by checking
>       within_error_injection_list. Similar discussions happened for the
>       bpf_override_return helper.
>
>     * security hooks, this is expected to be cleaned up with the LSM
>       changes after the KRSI patches introduce the LSM_HOOK macro:
>
>         https://lore.kernel.org/bpf/20200220175250.10795-1-kpsingh@chromium.org/
>
> - The attachment is currently limited to functions that return an int.
>   This can be extended later other types (e.g. PTR).
>
> Signed-off-by: KP Singh <kpsingh at google.com>
> Acked-by: Andrii Nakryiko <andriin at fb.com>
> ---
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 2460c8e6b5be..ae32517d4ccd 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -9800,6 +9801,33 @@ static int check_struct_ops_btf_id(struct bpf_verifier_env *env)
>
>         return 0;
>  }
> +#define SECURITY_PREFIX "security_"
> +
> +static int check_attach_modify_return(struct bpf_verifier_env *env)
> +{
> +       struct bpf_prog *prog = env->prog;
> +       unsigned long addr = (unsigned long) prog->aux->trampoline->func.addr;
> +
> +       if (within_error_injection_list(addr))
> +               return 0;
> +
> +       /* This is expected to be cleaned up in the future with the KRSI effort
> +        * introducing the LSM_HOOK macro for cleaning up lsm_hooks.h.
> +        */
> +       if (!strncmp(SECURITY_PREFIX, prog->aux->attach_func_name,
> +                    sizeof(SECURITY_PREFIX) - 1)) {
> +
> +               if (!capable(CAP_MAC_ADMIN))
> +                       return -EPERM;

CAP_MAC_ADMIN was originally introduced for Smack and is not
all-powerful wrt SELinux, so this is not a sufficient check for
SELinux.
We would want an actual security hook called here so we can implement
a specific check over userspace
being able to attach BPF progs to LSM hooks.  CAP_MAC_ADMIN has other
connotations to SELinux (presently the
ability to set/get file security labels that are not known to the
currently loaded policy).