Subject: Re: [PATCH] powerpc/perf: Fix handling of privilege level checks in perf interrupt context

From: Michael Ellerman <[email-masked]>
Date: Tue, 23 Feb 2021 23:54:28 +1100

Peter Zijlstra <[email-masked]> writes:
> On Tue, Feb 23, 2021 at 01:31:49AM -0500, Athira Rajeev wrote:
>> Running "perf mem record" in powerpc platforms with selinux enabled
>> resulted in soft lockup's. Below call-trace was seen in the logs:
...
>> 
>> Since the purpose of this security hook is to control access to
>> perf_event_open, it is not right to call this in interrupt context.
>> But in case of powerpc PMU, we need the privilege checks for specific
>> samples from branch history ring buffer and sampling register values.
>
> I'm confused... why would you need those checks at event time? Either
> the event has perf_event_attr::exclude_kernel and it then isn't allowed
> to expose kernel addresses, or it doesn't and it is.

Well one of us is confused that's for sure ^_^

I missed/forgot that we had that logic in open.

I think the reason we got here is that in the past we didn't have the
event in the low-level routines where we want to check,
power_pmu_bhrb_read() and perf_get_data_addr(), so we hacked in a
perf_paranoid_kernel() check. Which was wrong.

Then Joel's patch plumbed the event through and switched those paranoid
checks to perf_allow_kernel().

Anyway, we'll just switch those to exclude_kernel checks.

> There should never be an event-time question of permission like this. If
> you allow creation of an event, you're allowing the data it generates.

Ack.

cheers