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

Paolo Bonzini
2019-10-16T09:40:47Z

On 25/09/19 20:09, Sean Christopherson wrote:
> We're trying to sort out the trainwreck, but there's an additional wrinkle
> that I'd like your input on.

That's not exactly a wrinkle...

> - 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.

Paolo