Re: [RFC PATCH v1 1/1] tracing/kprobe: Add multi-probe support for 'perf_kprobe' PMU

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Sun, 20 Aug 2023 23:09:22 -0700
Song Liu <song@kernel.org> wrote:

> On Sun, Aug 20, 2023 at 6:16 AM Masami Hiramatsu <mhiramat@kernel.org> wrote:
> >
> > On Sun, 20 Aug 2023 03:02:18 -0700
> > Song Liu <song@kernel.org> wrote:
> >
> > > On Sun, Aug 20, 2023 at 2:32 AM Masami Hiramatsu <mhiramat@kernel.org> wrote:
> > > >
> > > [...]
> > > > > >
> > > > > > perf_event_attr::kprobe_func = "_text";
> > > > > > perf_event_attr::probe_offset = OFFSET;
> > > > > >
> > > > > > Then, it should be able to specify the correct one. Of course you can use
> > > > > > other unique symbols around the target symbol.
> > > > >
> > > > > Trying to catch up with the thread.
> > > >
> > > > Thanks for your reply :)
> > > >
> > > > >
> > > > > Besides the CAP_* issue, we can do this with
> > > > >
> > > > > perf_event_attr::kprobe_func = NULL;
> > > > > perf_event_attr::kprobe_addr = address;
> > > >
> > > > As I pointed, you don't need actual address, instead, you can specify the
> > > > probe point via "unique symbol" + offset.
> > >
> > > Technically, this works. But it is weird to me.
> >
> > It's not so weired because it is a relative address, e.g. from _text,
> > this means "the address in the text section". And perf probe already
> > uses it a while.
> >
> > > > >
> > > > > Then for the CAP_*, I think we should give CAP_PERFMON access to
> > > > > /proc/kallsyms. Would this work?
> > > >
> > > > For the "unique symbol" + offset, you don't need the kallsyms, but need to
> > > > access the System.map or vmlinux image. In this case, we don't need to expand
> > > > the CAP_PERFMON capabilities.
> > >
> > > I agree this is not needed in this case. But I wonder whether it makes sense
> > > to give CAP_PERFMON access to /proc/kallsyms. Will this change make
> > > CAP_PERFMON less secure?
> >
> > Yes, because /proc/kallsyms will expose the real address of the all
> > symbols, which makes KASLR useless. But on the other hand, it maybe
> > already useless because BPF program can read any real address, right?
> > Hmm, from this point of view, is the CAP_PERFMON meaningful?
> > (maybe it can avoid loading modules etc.)
> 
> 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.

Thank you,

> 
> Thanks,
> Song
> 
> >
> > Thank you,
> >
> > >
> > > Thanks,
> > > Song
> >
> >
> > --
> > Masami Hiramatsu (Google) <mhiramat@kernel.org>


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>