linux-kernel - Re: [PATCH bpf-next v3 1/3] bpf: add destructive kfunc flag

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Date:   Mon, 8 Aug 2022 14:14:33 +0200
From:   Kumar Kartikeya Dwivedi <memxor@...il.com>
To:     Artem Savkov <asavkov@...hat.com>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>, bpf@...r.kernel.org,
        netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
        Andrea Arcangeli <aarcange@...hat.com>,
        Daniel Vacek <dvacek@...hat.com>,
        Jiri Olsa <olsajiri@...il.com>, Song Liu <song@...nel.org>,
        Daniel Xu <dxu@...uu.xyz>
Subject: Re: [PATCH bpf-next v3 1/3] bpf: add destructive kfunc flag

On Mon, 8 Aug 2022 at 11:48, Artem Savkov <asavkov@...hat.com> wrote:
>
> Add KF_DESTRUCTIVE flag for destructive functions. Functions with this
> flag set will require CAP_SYS_BOOT capabilities.
>
> Signed-off-by: Artem Savkov <asavkov@...hat.com>
> ---
>  include/linux/btf.h   | 1 +
>  kernel/bpf/verifier.c | 5 +++++
>  2 files changed, 6 insertions(+)
>
> diff --git a/include/linux/btf.h b/include/linux/btf.h
> index cdb376d53238..51a0961c84e3 100644
> --- a/include/linux/btf.h
> +++ b/include/linux/btf.h
> @@ -49,6 +49,7 @@
>   * for this case.
>   */
>  #define KF_TRUSTED_ARGS (1 << 4) /* kfunc only takes trusted pointer arguments */
> +#define KF_DESTRUCTIVE  (1 << 5) /* kfunc performs destructive actions */
>

Please also document this flag in Documentation/bpf/kfuncs.rst.

And maybe instead of KF_DESTRUCTIVE, it might be more apt to call this
KF_CAP_SYS_BOOT. While it is true you had a destructive flag for
programs being loaded earlier, so there was a mapping between the two
UAPI and kfunc flags, what it has boiled down to is that this flag
just requires CAP_SYS_BOOT (in addition to other capabilities) during
load. So that name might express the intent a bit better. We might
soon have similar flags encoding requirements of other capabilities on
load.

The flag rename is just a suggestion, up to you.

>  struct btf;
>  struct btf_member;
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 096fdac70165..e52ca1631d3f 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -7584,6 +7584,11 @@ static int check_kfunc_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
>                         func_name);
>                 return -EACCES;
>         }
> +       if (*kfunc_flags & KF_DESTRUCTIVE && !capable(CAP_SYS_BOOT)) {
> +               verbose(env, "destructive kfunc calls require CAP_SYS_BOOT capabilities\n");
> +               return -EACCES;
> +       }
> +
>         acq = *kfunc_flags & KF_ACQUIRE;
>
>         /* Check the arguments */
> --
> 2.37.1
>