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

[Paolo Bonzini]

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