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

Thomas Gleixner
2019-10-16T09:47:18Z

On Wed, 16 Oct 2019, Paolo Bonzini wrote:
> On 25/09/19 20:09, Sean Christopherson wrote:
> > - 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.
> >
> > - KVM advertises split-lock detection to guest/userspace if and only if
> > split_lock_detect_disabled is zero.
> >
> > - Add a pr_warn_once() in KVM that triggers if split locks are disabled
> > after support has been advertised to a guest.
> >
> > Does this sound sane?
>
> Not really, unfortunately. Just never advertise split-lock detection to
> guests. If the host has enabled split-lock detection, trap #AC and
> forward it to the host handler---which would disable split lock
> detection globally and reenter the guest.

Which completely defeats the purpose.

1) Sane guest

Guest kernel has #AC handler and you basically prevent it from detecting
malicious user space and killing it. You also prevent #AC detection in
the guest kernel which limits debugability.

2) Malicious guest

Trigger #AC to disable the host detection and then carry out the DoS
attack.

Try again.

Thanks,

tglx