Messages in this thread | | | From | Alexei Starovoitov <> | Date | Tue, 14 Jan 2020 10:06:40 -0800 | Subject | Re: [PATCH v4 2/9] perf/core: open access for CAP_SYS_PERFMON privileged process |
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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.
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