Messages in this thread Patch in this message | | | Date | Thu, 6 Aug 2020 11:18:27 +0200 | From | peterz@infradea ... | Subject | Re: [PATCH v1 2/2] perf/core: Fake regs for leaked kernel samples |
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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.
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?
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.
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