Re: [RFC] perf/x86: Only expose userspace rdpmc for events on current CPU

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Jan 07, 2021 at 05:01:36PM -0700, Rob Herring wrote:
> Userspace access using rdpmc only makes sense if the event is valid for
> the current CPU. However, cap_user_rdpmc is currently set no matter which
> CPU the event is associated with. The result is userspace reading another
> CPU's event thinks it can use rdpmc to read the counter. In doing so, the
> wrong counter will be read.

Don't do that then?

> diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
> index a88c94d65693..6e6d4c1d03ca 100644
> --- a/arch/x86/events/core.c
> +++ b/arch/x86/events/core.c
> @@ -2490,7 +2490,8 @@ void arch_perf_update_userpage(struct perf_event *event,
>  	userpg->cap_user_time = 0;
>  	userpg->cap_user_time_zero = 0;
>  	userpg->cap_user_rdpmc =
> -		!!(event->hw.flags & PERF_X86_EVENT_RDPMC_ALLOWED);
> +		!!(event->hw.flags & PERF_X86_EVENT_RDPMC_ALLOWED) &&
> +		(event->oncpu == smp_processor_id());
>  	userpg->pmc_width = x86_pmu.cntval_bits;
>  
>  	if (!using_native_sched_clock() || !sched_clock_stable())

Isn't that a nop? That is, from the few sites I checked, we're always
calling this on the event's CPU.