netdev - Re: [PATCH bpf-next] bpf, capabilities: introduce CAP

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Message-ID: <CALCETrVVQs1s27y8fB17JtQi-VzTq1YZPTPy3k=fKhQB1X-KKA@mail.gmail.com>
Date:   Tue, 27 Aug 2019 19:00:40 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        James Morris <jmorris@...ei.org>, Jann Horn <jannh@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        "David S. Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>, kernel-team <kernel-team@...com>,
        Linux API <linux-api@...r.kernel.org>
Subject: Re: [PATCH bpf-next] bpf, capabilities: introduce CAP_BPF

> On Aug 27, 2019, at 5:55 PM, Andy Lutomirski <luto@...nel.org> wrote:
>
> On Tue, Aug 27, 2019 at 5:34 PM Alexei Starovoitov
> <alexei.starovoitov@...il.com> wrote:
>>

> From the previous discussion, you want to make progress toward solving
> a lot of problems with CAP_BPF.  One of them was making BPF
> firewalling more generally useful. By making CAP_BPF grant the ability
> to read kernel memory, you will make administrators much more nervous
> to grant CAP_BPF.  Similarly, and correct me if I'm wrong, most of
> these capabilities are primarily or only useful for tracing, so I
> don't see why users without CAP_TRACING should get them.
> bpf_trace_printk(), in particular, even has "trace" in its name :)
>
> Also, if a task has CAP_TRACING, it's expected to be able to trace the
> system -- that's the whole point.  Why shouldn't it be able to use BPF
> to trace the system better?

Let me put this a bit differently. Part of the point is that
CAP_TRACING should allow a user or program to trace without being able
to corrupt the system. CAP_BPF as you’ve proposed it *can* likely
crash the system.  For example, CAP_BPF allows bpf_map_get_fd_by_id()
in your patch.  If the system uses a BPF firewall that stores some of
its rules in maps, then bpf_map_get_fd_by_id() can be used to get a
writable fd to the map, which can be used to change the map, thus
preventing network access.  This means that no combination of
CAP_TRACING and CAP_BPF ends up allowing tracing without granting the
ability to reconfigure or otherwise corrupt the system.