Re: [PATCH v4 2/9] perf/core: open access for CAP_SYS_PERFMON privileged process

[Masami Hiramatsu]

On Mon, 13 Jan 2020 21:17:49 -0800
Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> wrote:

> On Mon, Jan 13, 2020 at 7:25 PM Masami Hiramatsu <mhiramat@xxxxxxxxxx> wrote:
> >
> > On Sat, 11 Jan 2020 12:57:18 +0300
> > Alexey Budankov <alexey.budankov@xxxxxxxxxxxxxxx> wrote:
> >
> > >
> > > On 11.01.2020 3:35, arnaldo.melo@xxxxxxxxx wrote:
> >
> > > > Message-ID: <A7F0BF73-9189-44BA-9264-C88F2F51CBF3@xxxxxxxxxx>
> > > >
> > > > On January 10, 2020 9:23:27 PM GMT-03:00, Song Liu <songliubraving@xxxxxx> wrote:
> > > >>
> > > >>
> > > >>> On Jan 10, 2020, at 3:47 PM, Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> > > >> wrote:
> > > >>>
> > > >>> On Fri, 10 Jan 2020 13:45:31 -0300
> > > >>> Arnaldo Carvalho de Melo <acme@xxxxxxxxxx> wrote:
> > > >>>
> > > >>>> Em Sat, Jan 11, 2020 at 12:52:13AM +0900, Masami Hiramatsu escreveu:
> > > >>>>> On Fri, 10 Jan 2020 15:02:34 +0100 Peter Zijlstra
> > > >> <peterz@xxxxxxxxxxxxx> wrote:
> > > >>>>>> Again, this only allows attaching to previously created kprobes,
> > > >> it does
> > > >>>>>> not allow creating kprobes, right?
> > > >>>>
> > > >>>>>> That is; I don't think CAP_SYS_PERFMON should be allowed to create
> > > >>>>>> kprobes.
> > > >>>>
> > > >>>>>> As might be clear; I don't actually know what the user-ABI is for
> > > >>>>>> creating kprobes.
> > > >>>>
> > > >>>>> There are 2 ABIs nowadays, ftrace and ebpf. perf-probe uses ftrace
> > > >> interface to
> > > >>>>> define new kprobe events, and those events are treated as
> > > >> completely same as
> > > >>>>> tracepoint events. On the other hand, ebpf tries to define new
> > > >> probe event
> > > >>>>> via perf_event interface. Above one is that interface. IOW, it
> > > >> creates new kprobe.
> > > >>>>
> > > >>>> Masami, any plans to make 'perf probe' use the perf_event_open()
> > > >>>> interface for creating kprobes/uprobes?
> > > >>>
> > > >>> Would you mean perf probe to switch to perf_event_open()?
> > > >>> No, perf probe is for setting up the ftrace probe events. I think we
> > > >> can add an
> > > >>> option to use perf_event_open(). But current kprobe creation from
> > > >> perf_event_open()
> > > >>> is separated from ftrace by design.
> > > >>
> > > >> I guess we can extend event parser to understand kprobe directly.
> > > >> Instead of
> > > >>
> > > >>    perf probe kernel_func
> > > >>    perf stat/record -e probe:kernel_func ...
> > > >>
> > > >> We can just do
> > > >>
> > > >>    perf stat/record -e kprobe:kernel_func ...
> > > >
> > > >
> > > > You took the words from my mouth, exactly, that is a perfect use case, an alternative to the 'perf probe' one of making a disabled event that then gets activated via record/stat/trace, in many cases it's better, removes the explicit probe setup case.
> > >
> > > Arnaldo, Masami, Song,
> > >
> > > What do you think about making this also open to CAP_SYS_PERFMON privileged processes?
> > > Could you please also review and comment on patch 5/9 for bpf_trace.c?
> >
> > As we talked at RFC series of CAP_SYS_TRACING last year, I just expected
> > to open it for enabling/disabling kprobes, not for creation.
> >
> > If we can accept user who has no admin priviledge but the CAP_SYS_PERFMON,
> > to shoot their foot by their own risk, I'm OK to allow it. (Even though,
> > it should check the max number of probes to be created by something like
> > ulimit)
> > I think nowadays we have fixed all such kernel crash problems on x86,
> > but not sure for other archs, especially on the devices I can not reach.
> > I need more help to stabilize it.
> 
> I don't see how enable/disable is any safer than creation.
> If there are kernel bugs in kprobes the kernel will crash anyway.

Why? admin can test the probes before using it via bpf.

My point was only admin can make a dicision to allow (or delegate) the
priviledge to a user, and if it is OK, I don't mind it.
(Maybe it is better to give a knob to allow this CAP only for admin.)

> I think such partial CAP_SYS_PERFMON would be very confusing to the users.
> CAP_* is about delegation of root privileges to non-root.
> Delegating some of it is ok, but disallowing creation makes it useless
> for bpf tracing, so we would need to add another CAP later.
> Hence I suggest to do it right away instead of breaking
> sys_perf_even_open() access into two CAPs.

I understand that the single strong CAP will useful anyway (even if
it is CAP_SYS_ADMIN). I just concern that causes any issue and when
someone wants to mitigate it, it is sad if there is only way to disable
all tracing facilities.

What about providing a sysctl to control the power of the CAP? maybe
it is also good from the viewpoint of system security.

Thank you,

-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>