linux-kernel - Re: [perf] perf_fuzzer causes unchecked MSR access error

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Date:   Thu, 4 Mar 2021 14:33:17 -0500
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Vince Weaver <vincent.weaver@...ne.edu>
Cc:     Stephane Eranian <eranian@...gle.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.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>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>
Subject: Re: [perf] perf_fuzzer causes unchecked MSR access error



On 3/3/2021 3:22 PM, Vince Weaver wrote:
> On Wed, 3 Mar 2021, Liang, Kan wrote:
> 
>> We never use bit 58. It should be a new issue.

Actually, KVM uses it. They create a fake event called VLBR_EVENT, which 
uses bit 58. It's introduced from the commit 097e4311cda9 ("perf/x86: 
Add constraint to create guest LBR event without hw counter").

Since it's a fake event, it doesn't support PEBS. Perf should reject it 
if it sets the precise_ip.

The below patch should fix the MSR access error.

diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
index 5bac48d..1ea3c67 100644
--- a/arch/x86/events/intel/core.c
+++ b/arch/x86/events/intel/core.c
@@ -3659,6 +3659,10 @@ static int intel_pmu_hw_config(struct perf_event 
*event)
  		return ret;

  	if (event->attr.precise_ip) {
+
+		if ((event->attr.config & INTEL_ARCH_EVENT_MASK) == 
INTEL_FIXED_VLBR_EVENT)
+			return -EINVAL;
+
  		if (!(event->attr.freq || (event->attr.wakeup_events && 
!event->attr.watermark))) {
  			event->hw.flags |= PERF_X86_EVENT_AUTO_RELOAD;
  			if (!(event->attr.sample_type &

Thanks,
Kan