Re: [PATCH 1/8] perf/x86/intel: Fix memory corruption

[Peter Zijlstra]

On Wed, Mar 20, 2019 at 11:22:20PM +0100, Peter Zijlstra wrote:
> On Wed, Mar 20, 2019 at 01:47:28PM -0700, Stephane Eranian wrote:
> 
> > Right now, if I do:
> > 
> > echo 0 > /sys/bus/event_source/devices/cpu/allow_tsx_force_abort
> > 
> > Then I don't have the guarantee on when there will be no abort when I
> > return from the echo.  the MSR is accessed only on PMU scheduling. I
> > would expect a sysadmin to want some guarantee if this is to be
> > switched on/off at runtime. If not, then having a boot time option is
> > better in my opinion.
> 
> Something like cycling the nmi watchdog or:
> 
>   perf stat -a -e cycles sleep 1
> 
> should be enough to force reschedule the events on every CPU.
> 
> Again, I'm not adverse to 'fixing' this if it can be done with limited
> LoC. But I don't really see this as critical.
> 
> > This other bit I noticed is that cpuc->tfa_shadow is used to avoid the
> > wrmsr(), but I don't see the code that makes sure the init value (0)
> > matches the value of the MSR.  Is this MSR guarantee to be zero on
> > reset? 
> 
> That was my understanding.
> 
> > How about on kexec()?
> 
> Good point, we might want to fix that.

Something like the below perhaps?

---
Subject: perf/x86/intel: Initialize TFA MSR

Stephane reported that we don't initialize the TFA MSR, which could lead
to trouble if the RESET value is not 0 or on kexec.

Reported-by: Stephane Eranian <eranian@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
 arch/x86/events/intel/core.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
index 8baa441d8000..2d3caf2d1384 100644
--- a/arch/x86/events/intel/core.c
+++ b/arch/x86/events/intel/core.c
@@ -3575,6 +3575,12 @@ static void intel_pmu_cpu_starting(int cpu)
 
 	cpuc->lbr_sel = NULL;
 
+	if (x86_pmu.flags & PMU_FL_TFA) {
+		WARN_ON_ONCE(cpuc->tfa_shadow);
+		cpuc->tfa_shadow = ~0ULL;
+		intel_set_tfa(cpuc, false);
+	}
+
 	if (x86_pmu.version > 1)
 		flip_smm_bit(&x86_pmu.attr_freeze_on_smi);