Re: [PATCH bpf-next] bpf, capabilities: introduce CAP_BPF

From: Steven Rostedt <rostedt@goodmis.org>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Andy Lutomirski <luto@amacapital.net>,
	Andy Lutomirski <luto@kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Kees Cook <keescook@chromium.org>,
	LSM List <linux-security-module@vger.kernel.org>,
	James Morris <jmorris@namei.org>, Jann Horn <jannh@google.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Masami Hiramatsu <mhiramat@kernel.org>,
	"David S. Miller" <davem@davemloft.net>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Network Development <netdev@vger.kernel.org>,
	bpf <bpf@vger.kernel.org>, kernel-team <kernel-team@fb.com>,
	Linux API <linux-api@vger.kernel.org>
Subject: Re: [PATCH bpf-next] bpf, capabilities: introduce CAP_BPF
Date: Sat, 28 Sep 2019 19:37:27 -0400	[thread overview]
Message-ID: <20190928193727.1769e90c@oasis.local.home> (raw)
In-Reply-To: <20190829040721.ef6rumbaunkavyrr@ast-mbp.dhcp.thefacebook.com>

On Wed, 28 Aug 2019 21:07:24 -0700
Alexei Starovoitov <alexei.starovoitov@gmail.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