Messages in this thread | | | From | Alexei Starovoitov <> | Date | Tue, 14 Jan 2020 17:52:10 -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 10:50 AM Alexey Budankov <alexey.budankov@linux.intel.com> wrote: > > > On 14.01.2020 21:06, Alexei Starovoitov wrote: > > 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. > > Sure, software cannot guarantee, but known software bugs could still be fixed, > that's what I meant. > > > 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. > > > > Well, as this patch set already covers complete perf_event_open functionality, > and also eBPF related parts too, could you please review and comment on it? > Does the patches 2/9 and 5/9 already bring all required extentions?
yes. the current patches 2 and 5 look good to me. I would only change patch 1 to what Andy was proposing earlier:
static inline bool perfmon_capable(void) { if (capable_noaudit(CAP_PERFMON)) return capable(CAP_PERFMON); if (capable_noaudit(CAP_SYS_ADMIN)) return capable(CAP_SYS_ADMIN);
return capable(CAP_PERFMON); } I think Andy was trying to preserve the order of audit events.
I'm also suggesting to drop SYS from the cap name. It doesn't add any value to the name.
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