linux-kernel - Re: [PATCH bpf-next v4 1/3] bpf: add destructive kfunc flag

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Message-ID: <CAP01T75Qern=-hvYONBMom7T3ycs-bpGxR1n5CdrehJBwrOTuQ@mail.gmail.com>
Date:   Tue, 9 Aug 2022 14:40:30 +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 v4 1/3] bpf: add destructive kfunc flag

On Tue, 9 Aug 2022 at 12:53, 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>
> ---
>  Documentation/bpf/kfuncs.rst | 9 +++++++++
>  include/linux/btf.h          | 1 +
>  kernel/bpf/verifier.c        | 5 +++++
>  3 files changed, 15 insertions(+)
>
> diff --git a/Documentation/bpf/kfuncs.rst b/Documentation/bpf/kfuncs.rst
> index c0b7dae6dbf5..2e97e08be7de 100644
> --- a/Documentation/bpf/kfuncs.rst
> +++ b/Documentation/bpf/kfuncs.rst
> @@ -146,6 +146,15 @@ that operate (change some property, perform some operation) on an object that
>  was obtained using an acquire kfunc. Such kfuncs need an unchanged pointer to
>  ensure the integrity of the operation being performed on the expected object.
>
> +2.4.5 KF_DESTRUCTIVE flag

This should be 2.4.6.

> +--------------------------
> +
> +The KF_DESTRUCTIVE flag is used to indicate functions calling which is
> +destructive to the system. For example such a call can result in system
> +rebooting or panicking. Due to this additional restrictions apply to these
> +calls. At the moment they only require CAP_SYS_BOOT capability, but more can be
> +added later.
> +
>  2.5 Registering the kfuncs
>  --------------------------
>
> 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 */
>
>  struct btf;
>  struct btf_member;
> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> index 843a966cd02b..163cc0a2dc5a 100644
> --- a/kernel/bpf/verifier.c
> +++ b/kernel/bpf/verifier.c
> @@ -7598,6 +7598,11 @@ 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
>