Messages in this thread | | | Subject | Re: [PATCH v4 2/9] perf/core: open access for CAP_SYS_PERFMON privileged process | From | Alexey Budankov <> | Date | Wed, 15 Jan 2020 08:15:10 +0300 |
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On 15.01.2020 4:52, Alexei Starovoitov wrote: > 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.
Thanks. I appreciate your cooperation.
> I would only change patch 1 to what Andy was proposing earlier:
Could you please share the link to the proposal to get more details? In this patch set discussion there was only this [1] on more generic naming of PERFMON cap from Andi Kleen.
> > 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); > }
Yes, this makes sense and adds up.
> 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.
Agreed, CAP_PERFMON sounds more generic, as it actually is.
Gratefully, Alexey
[1] https://lore.kernel.org/lkml/20191211203648.GA862919@tassilo.jf.intel.com/
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