Re: [PATCH v9 09/17] x86/split_lock: Handle #AC exception for split lock

[Sean Christopherson]

On Wed, Oct 16, 2019 at 11:29:00AM +0200, Thomas Gleixner wrote:
> >   - Modify the #AC handler to test/set the same atomic variable as the
> >     sysfs knob.  This is the "disabled by kernel" flow.
> 
> That's the #AC in kernel handler, right?

Yes.

> >   - Modify the debugfs/sysfs knob to only allow disabling split-lock
> >     detection.  This is the "disabled globally" path, i.e. sends IPIs to
> >     clear MSR_TEST_CTRL.split_lock on all online CPUs.
> 
> Why only disable? What's wrong with reenabling it? The shiny new driver you
> are working on is triggering #AC. So in order to test the fix, you need to
> reboot the machine instead of just unloading the module, reenabling #AC and
> then loading the fixed one?

A re-enabling path adds complexity (though not much) and is undesirable
for a production environment as a split-lock issue in the kernel isn't
going to magically disappear.  And I thought that disable-only was also
your preferred implementation based on a previous comment[*], but that
comment may have been purely in the scope of userspace applications.

Anyways, my personal preference would be to keep things simple and not
support a re-enabling path.  But then again, I do 99.9% of my development
in VMs so my vote probably shouldn't count regarding the module issue.

[*] https://lkml.kernel.org/r/alpine.DEB.2.21.1904180832290.3174@xxxxxxxxxxxxxxxxxxxxxxx

> >   - Modify the resume/init flow to clear MSR_TEST_CTRL.split_lock if it's
> >     been disabled on *any* CPU via #AC or via the knob.
> 
> Fine.
> 
> >   - Remove KVM loading of MSR_TEST_CTRL, i.e. KVM *never* writes the CPU's
> >     actual MSR_TEST_CTRL.  KVM still emulates MSR_TEST_CTRL so that the
> >     guest can do WRMSR and handle its own #AC faults, but KVM doesn't
> >     change the value in hardware.
> > 
> >       * Allowing guest to enable split-lock detection can induce #AC on
> >         the host after it has been explicitly turned off, e.g. the sibling
> >         hyperthread hits an #AC in the host kernel, or worse, causes a
> >         different process in the host to SIGBUS.
> >
> >       * Allowing guest to disable split-lock detection opens up the host
> >         to DoS attacks.
> 
> Wasn't this discussed before and agreed on that if the host has AC enabled
> that the guest should not be able to force disable it? I surely lost track
> of this completely so my memory might trick me.

Yes, I was restating that point, or at least attempting to.
 
> The real question is what you do when the host has #AC enabled and the
> guest 'disabled' it and triggers #AC. Is that going to be silently ignored
> or is the intention to kill the guest in the same way as we kill userspace?
> 
> The latter would be the right thing, but given the fact that the current
> kernels easily trigger #AC today, that would cause a major wreckage in
> hosting scenarios. So I fear we need to bite the bullet and have a knob
> which defaults to 'handle silently' and allows to enable the kill mechanics
> on purpose. 'Handle silently' needs some logging of course, at least a per
> guest counter which can be queried and a tracepoint.