Messages in this thread | | | Date | Thu, 6 Aug 2020 11:24:54 +0200 | From | peterz@infradea ... | Subject | Re: [PATCH v1 2/2] perf/core: Fake regs for leaked kernel samples |
| |
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?
| |