Re: [PATCH v2] x86/kvm/vmx: do not use vm-exit instruction length for fast MMIO when running nested

[Michael S. Tsirkin]

On Thu, Jan 25, 2018 at 04:37:07PM +0100, Vitaly Kuznetsov wrote:
> I was investigating an issue with seabios >= 1.10 which stopped working
> for nested KVM on Hyper-V. The problem appears to be in
> handle_ept_violation() function: when we do fast mmio we need to skip
> the instruction so we do kvm_skip_emulated_instruction(). This, however,
> depends on VM_EXIT_INSTRUCTION_LEN field being set correctly in VMCS.
> However, this is not the case.
> 
> Intel's manual doesn't mandate VM_EXIT_INSTRUCTION_LEN to be set when
> EPT MISCONFIG occurs. While on real hardware it was observed to be set,
> some hypervisors follow the spec and don't set it; we end up advancing
> IP with some random value.
> 
> I checked with Microsoft and they confirmed they don't fill
> VM_EXIT_INSTRUCTION_LEN on EPT MISCONFIG.
> 
> Fix the issue by doing instruction skip through emulator when running
> nested.
> 
> Fixes: 68c3b4d1676d870f0453c31d5a52e7e65c7448ae
> Suggested-by: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> Suggested-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>

I would maybe also disable this when this is a kvm host
running a nested *guest*, just in case.

Acked-by: Michael S. Tsirkin <mst@xxxxxxxxxx>

> ---
> v1 -> v2:
>    inlay X86_FEATURE_HYPERVISOR case with EMULTYPE_SKIP optimization
>    [Paolo Bonzini, Radim KrÄmÃÅ]
> ---
>  arch/x86/kvm/vmx.c | 16 +++++++++++++++-
>  arch/x86/kvm/x86.c |  3 ++-
>  2 files changed, 17 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index c829d89e2e63..e105b439c372 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -6563,7 +6563,21 @@ static int handle_ept_misconfig(struct kvm_vcpu *vcpu)
>  	if (!is_guest_mode(vcpu) &&
>  	    !kvm_io_bus_write(vcpu, KVM_FAST_MMIO_BUS, gpa, 0, NULL)) {
>  		trace_kvm_fast_mmio(gpa);
> -		return kvm_skip_emulated_instruction(vcpu);
> +		/*
> +		 * Doing kvm_skip_emulated_instruction() depends on undefined
> +		 * behavior: Intel's manual doesn't mandate
> +		 * VM_EXIT_INSTRUCTION_LEN to be set in VMCS when EPT MISCONFIG
> +		 * occurs and while on real hardware it was observed to be set,
> +		 * other hypervisors (namely Hyper-V) don't set it, we end up
> +		 * advancing IP with some random value. Disable fast mmio when
> +		 * running nested and keep it for real hardware in hope that
> +		 * VM_EXIT_INSTRUCTION_LEN will always be set correctly.
> +		 */
> +		if (!static_cpu_has(X86_FEATURE_HYPERVISOR))
> +			return kvm_skip_emulated_instruction(vcpu);
> +		else
> +			return x86_emulate_instruction(vcpu, gpa, EMULTYPE_SKIP,
> +						       NULL, 0) == EMULATE_DONE;
>  	}
>  
>  	ret = kvm_mmu_page_fault(vcpu, gpa, PFERR_RSVD_MASK, NULL, 0);
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 1cec2c62a0b0..930aba87a723 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -5703,7 +5703,8 @@ int x86_emulate_instruction(struct kvm_vcpu *vcpu,
>  		 * handle watchpoints yet, those would be handled in
>  		 * the emulate_ops.
>  		 */
> -		if (kvm_vcpu_check_breakpoint(vcpu, &r))
> +		if (!(emulation_type & EMULTYPE_SKIP) &&
> +		    kvm_vcpu_check_breakpoint(vcpu, &r))
>  			return r;
>  
>  		ctxt->interruptibility = 0;
> -- 
> 2.14.3