linux-kernel - Re: [PATCH v4 2/9] perf/core: open access for CAP_SYS

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Date:   Fri, 3 Apr 2020 16:56:44 +0300
From:   Alexey Budankov <alexey.budankov@...ux.intel.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        James Morris <jmorris@...ei.org>, Jiri Olsa <jolsa@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Namhyung Kim <namhyung@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 2/9] perf/core: open access for CAP_SYS_PERFMON
 privileged process


On 01.04.2020 23:50, Alexey Budankov wrote:
> Hi Alexei,
> 
> On 15.01.2020 4:52, Alexei Starovoitov wrote:
>> On Tue, Jan 14, 2020 at 10:50 AM Alexey Budankov
>> <alexey.budankov@...ux.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@...ux.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.
> 
> Could this have you explicit Reviewed-by or Acked-by tag so
> the changes could be driven into the kernel?
> Latest v7 is here: https://lore.kernel.org/lkml/c8de937a-0b3a-7147-f5ef-69f467e87a13@linux.intel.com/

Posted v8 with all acquired tags so far:
https://lore.kernel.org/lkml/f96f8f8a-e65c-3f36-dc85-fc3f5191e8c5@linux.intel.com/

Thanks,
Alexey