netdev - Re: [PATCH v6 bpf-next 0/3] Introduce CAP

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Message-ID: <CAJPywT+c8uvi2zgUD_jObmi9T6j50THzjQHg-mudNrEC2HuJvg@mail.gmail.com>
Date:   Wed, 13 May 2020 11:50:42 +0100
From:   Marek Majkowski <marek@...udflare.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     David Miller <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        network dev <netdev@...r.kernel.org>, bpf@...r.kernel.org,
        kernel-team@...com, linux-security-module@...r.kernel.org,
        acme@...hat.com, jamorris@...ux.microsoft.com,
        Jann Horn <jannh@...gle.com>, kpsingh@...gle.com,
        kernel-team <kernel-team@...udflare.com>
Subject: Re: [PATCH v6 bpf-next 0/3] Introduce CAP_BPF

On Wed, May 13, 2020 at 4:19 AM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> CAP_BPF solves three main goals:
> 1. provides isolation to user space processes that drop CAP_SYS_ADMIN and switch to CAP_BPF.
>    More on this below. This is the major difference vs v4 set back from Sep 2019.
> 2. makes networking BPF progs more secure, since CAP_BPF + CAP_NET_ADMIN
>    prevents pointer leaks and arbitrary kernel memory access.
> 3. enables fuzzers to exercise all of the verifier logic. Eventually finding bugs
>    and making BPF infra more secure. Currently fuzzers run in unpriv.
>    They will be able to run with CAP_BPF.
>

Alexei, looking at this from a user point of view, this looks fine.

I'm slightly worried about REUSEPORT_EBPF. Currently without your
patch, as far as I understand it:

- You can load SOCKET_FILTER and SO_ATTACH_REUSEPORT_EBPF without any
permissions

- For loading BPF_PROG_TYPE_SK_REUSEPORT program and for SOCKARRAY map
creation CAP_SYS_ADMIN is needed. But again, no permissions check for
SO_ATTACH_REUSEPORT_EBPF later.

If I read the patchset correctly, the former SOCKET_FILTER case
remains as it is and is not affected in any way by presence or absence
of CAP_BPF.

The latter case is different. Presence of CAP_BPF is sufficient for
map creation, but not sufficient for loading SK_REUSEPORT program. It
still requires CAP_SYS_ADMIN. I think it's a good opportunity to relax
this CAP_SYS_ADMIN requirement. I think the presence of CAP_BPF should
be sufficient for loading BPF_PROG_TYPE_SK_REUSEPORT.

Our specific use case is simple - we want an application program -
like nginx - to control REUSEPORT programs. We will grant it CAP_BPF,
but we don't want to grant it CAP_SYS_ADMIN.

Marek