Re: [PATCH v1 2/2] perf/core: Fake regs for leaked kernel samples

Jin, Yao
2020-08-07T05:32:46Z

Hi Peter,

On 8/6/2020 5:24 PM, peterz@infradead.org wrote:
> On Thu, Aug 06, 2020 at 11:18:27AM +0200, peterz@infradead.org wrote:
>> On Thu, Aug 06, 2020 at 10:26:29AM +0800, Jin, Yao wrote:
>>
>>>> +static struct pt_regs *sanitize_sample_regs(struct perf_event *event, struct pt_regs *regs)
>>>> +{
>>>> + struct pt_regs *sample_regs = regs;
>>>> +
>>>> + /* user only */
>>>> + if (!event->attr.exclude_kernel || !event->attr.exclude_hv ||
>>>> + !event->attr.exclude_host || !event->attr.exclude_guest)
>>>> + return sample_regs;
>>>> +
>>>
>>> Is this condition correct?
>>>
>>> Say counting user event on host, exclude_kernel = 1 and exclude_host = 0. It
>>> will go "return sample_regs" path.
>>
>> I'm not sure, I'm terminally confused on virt stuff.
>
> [A]
>
>> Suppose we have nested virt:
>>
>> L0-hv
>> |
>> G0/L1-hv
>> |
>> G1
>>
>> And we're running in G0, then:
>>
>> - 'exclude_hv' would exclude L0 events
>> - 'exclude_host' would ... exclude L1-hv events?
>> - 'exclude_guest' would ... exclude G1 events?
>
> [B]
>
>> Then the next question is, if G0 is a host, does the L1-hv run in
>> G0 userspace or G0 kernel space?
>>
>> I was assuming G0 userspace would not include anything L1 (kvm is a
>> kernel module after all), but what do I know.
>>
>>>> @@ -11609,7 +11636,8 @@ SYSCALL_DEFINE5(perf_event_open,
>>>> if (err)
>>>> return err;
>>>> - if (!attr.exclude_kernel) {
>>>> + if (!attr.exclude_kernel || !attr.exclude_callchain_kernel ||
>>>> + !attr.exclude_hv || !attr.exclude_host || !attr.exclude_guest) {
>>>> err = perf_allow_kernel(&attr);
>>>> if (err)
>>>> return err;
>>>>
>>>
>>> I can understand the conditions "!attr.exclude_kernel || !attr.exclude_callchain_kernel".
>>>
>>> But I'm not very sure about the "!attr.exclude_hv || !attr.exclude_host || !attr.exclude_guest".
>>
>> Well, I'm very sure G0 userspace should never see L0 or G1 state, so
>> exclude_hv and exclude_guest had better be true.
>>
>>> On host, exclude_hv = 1, exclude_guest = 1 and exclude_host = 0, right?
>>
>> Same as above, is G0 host state G0 userspace?
>>
>>> So even exclude_kernel = 1 but exclude_host = 0, we will still go
>>> perf_allow_kernel path. Please correct me if my understanding is wrong.
>>
>> Yes, because with those permission checks in place it means you have
>> permission to see kernel bits.
>
> So if I understand 'exclude_host' wrong -- a distinct possibility -- can
> we then pretty please have the above [A-B] corrected and put in a
> comment near perf_event_attr and the exclude_* comments changed to refer
> to that?
>

In my previous mail, I explained what I understood for 'exclude_host', but not sure if it's correct.
Needs more review comments.

Thanks
Jin Yao