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

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Date:   Sat, 28 Sep 2019 19:37:27 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Andy Lutomirski <luto@...capital.net>,
        Andy Lutomirski <luto@...nel.org>,
        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>,
        "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 Wed, 28 Aug 2019 21:07:24 -0700
Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:
> > 
> > This won’t make me much more comfortable, since CAP_BPF lets it do an ever-growing set of nasty things. I’d much rather one or both of two things happen:
> > 
> > 1. Give it CAP_TRACING only. It can leak my data, but it’s rather hard for it to crash my laptop, lose data, or cause other shenanigans.
> > 
> > 2. Improve it a bit do all the privileged ops are wrapped by capset().
> > 
> > Does this make sense?  I’m a security person on occasion. I find
> > vulnerabilities and exploit them deliberately and I break things by
> > accident on a regular basis. In my considered opinion, CAP_TRACING
> > alone, even extended to cover part of BPF as I’ve described, is
> > decently safe. Getting root with just CAP_TRACING will be decently
> > challenging, especially if I don’t get to read things like sshd’s
> > memory, and improvements to mitigate even that could be added.  I
> > am quite confident that attacks starting with CAP_TRACING will have
> > clear audit signatures if auditing is on.  I am also confident that
> > CAP_BPF *will* allow DoS and likely privilege escalation, and this
> > will only get more likely as BPF gets more widely used. And, if
> > BPF-based auditing ever becomes a thing, writing to the audit
> > daemon’s maps will be a great way to cover one’s tracks.  
> 
> CAP_TRACING, as I'm proposing it, will allow full tracefs access.
> I think Steven and Massami prefer that as well.
> That includes kprobe with probe_kernel_read.
> That also means mini-DoS by installing kprobes everywhere or running
> too much ftrace.

I was talking with Kees at Plumbers about this, and we were talking
about just using simple file permissions. I started playing with some
patches to allow the tracefs be visible but by default it would only be
visible by root.

 rwx------

Then a start up script (or perhaps mount options) could change the
group owner, and change this to:

 rwxrwx---

Where anyone in the group assigned (say "tracing") gets full access to
the file system.

The more I was playing with this, the less I see the need for
CAP_TRACING for ftrace and reading the format files.

-- Steve