linux-kernel - Re: [RFC PATCH v1 1/1] tracing/kprobe: Add multi-probe support for 'perf

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Date:   Mon, 21 Aug 2023 11:07:58 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     "Masami Hiramatsu (Google)" <mhiramat@...nel.org>,
        Song Liu <song@...nel.org>,
        Francis Laniel <flaniel@...ux.microsoft.com>,
        linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [RFC PATCH v1 1/1] tracing/kprobe: Add multi-probe support for
 'perf_kprobe' PMU

On Mon, Aug 21, 2023 at 10:45:50AM -0400, Steven Rostedt wrote:
> On Mon, 21 Aug 2023 19:01:52 +0900
> Masami Hiramatsu (Google) <mhiramat@...nel.org> wrote:
> 
> > > kprobe BPF program has access to pt_regs, so it can read ip of the
> > > attached function. Can we do the same with regular kprobe (no bpf)?  
> > 
> > Yes, it can. So I think it is OK to expand CAP_PERFMON to access kallsyms.
> > But this means CAP_PERMON itself is not safe in some case.
> 
> What are the privileges that CAP_PERFMON gives. I can see why Kees told me
> to avoid capabilities when looking at what has access to tracefs. Because
> it becomes very difficult to know what the privileges you are giving when
> you give out a capability. I just stick to normal ACL (file permissions)
> and everything is much easier and simpler to know what has access to what.

At the very least, having a fd-based "handle" for access work. But yeah,
capabilities get ugly quickly.

Anyway... what does CAP_PERFMON have access to right now? If it is
allowed to read arbitrary kernel memory, then resolving symbols is fine.
If it doesn't, then no, it shouldn't: it becomes a oracle for probing
symbol locations.

-- 
Kees Cook