Re: [PATCH 1/2] KVM: x86: Allow userspace to opt out of hypercall patching

[Sean Christopherson <seanjc@google.com>]

On Wed, Mar 16, 2022, Oliver Upton wrote:
> KVM handles the VMCALL/VMMCALL instructions very strangely. Even though
> both of these instructions really should #UD when executed on the wrong
> vendor's hardware (i.e. VMCALL on SVM, VMMCALL on VMX), KVM replaces the
> guest's instruction with the appropriate instruction for the vendor.
> Nonetheless, older guest kernels without commit c1118b3602c2 ("x86: kvm:
> use alternatives for VMCALL vs. VMMCALL if kernel text is read-only")
> do not patch in the appropriate instruction using alternatives, likely
> motivating KVM's intervention.
> 
> Add a quirk allowing userspace to opt out of hypercall patching.

A quirk may not be appropriate, per Paolo, the whole cross-vendor thing is
intentional.

https://lore.kernel.org/all/20211210222903.3417968-1-seanjc@google.com

> If the quirk is disabled, KVM synthesizes a #UD in the guest.

...

> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index d3a9ce07a565..685c4bc453b4 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -9291,6 +9291,17 @@ static int emulator_fix_hypercall(struct x86_emulate_ctxt *ctxt)
>  	char instruction[3];
>  	unsigned long rip = kvm_rip_read(vcpu);
>  
> +	/*
> +	 * If the quirk is disabled, synthesize a #UD and let the guest pick up
> +	 * the pieces.
> +	 */
> +	if (!kvm_check_has_quirk(vcpu->kvm, KVM_X86_QUIRK_FIX_HYPERCALL_INSN)) {
> +		ctxt->exception.error_code_valid = false;
> +		ctxt->exception.vector = UD_VECTOR;
> +		ctxt->have_exception = true;
> +		return X86EMUL_PROPAGATE_FAULT;

This should return X86EMUL_UNHANDLEABLE instead of manually injecting a #UD.  That
will also end up generating a #UD in most cases, but will play nice with
KVM_CAP_EXIT_ON_EMULATION_FAILURE.