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From:   Alexei Starovoitov <alexei.starovoitov@gmail.com>
Date:   Tue, 14 Jan 2020 10:06:40 -0800
Message-ID: <CAADnVQKddDCRV9Zp7N_TR51wc5rtRwFN-pSZHLiXDXe23+B_5Q@mail.gmail.com>
Subject: Re: [PATCH v4 2/9] perf/core: open access for CAP_SYS_PERFMON
 privileged process
To:     Alexey Budankov <alexey.budankov@linux.intel.com>
Cc:     Masami Hiramatsu <mhiramat@kernel.org>,
        Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>,
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On Tue, Jan 14, 2020 at 1:47 AM Alexey Budankov
<alexey.budankov@linux.intel.com> wrote:
> >>
> >> 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.
> > 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.
> >
>
> Alexei, Masami,
>
> Thanks for your meaningful input.
> If we know in advance that it still can crash the system in some cases and on
> some archs, even though root fully controls delegation thru CAP_SYS_PERFMON,
> such delegation looks premature until the crashes are avoided. So it looks like
> access to eBPF for CAP_SYS_PERFMON privileged processes is the subject for
> a separate patch set.

perf_event_open is always dangerous. sw cannot guarantee non-bugginess of hw.
imo adding a cap just for pmc is pointless.
if you add a new cap it should cover all of sys_perf_event_open syscall.
subdividing it into sw vs hw counters, kprobe create vs enable, etc will
be the source of ongoing confusion. nack to such cap.