Messages in this thread | | | Subject | Re: [PATCH v1 2/2] perf/core: Fake regs for leaked kernel samples | From | "Jin, Yao" <> | Date | Fri, 7 Aug 2020 13:32:39 +0800 |
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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
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