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

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Date:   Tue, 12 Jan 2021 16:32:30 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Rob Herring <robh@...nel.org>
Cc:     Jiri Olsa <jolsa@...hat.com>, Will Deacon <will@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>
Subject: Re: [RFC] perf/x86: Only expose userspace rdpmc for events on
 current CPU

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.