Re: [PATCH v2 02/21] KVM: VMX: Drop bits 31:16 when shoving exception error code into VMCS

[Maxim Levitsky <mlevitsk@redhat.com>]

On Tue, 2022-06-14 at 20:47 +0000, Sean Christopherson wrote:
> Deliberately truncate the exception error code when shoving it into the
> VMCS (VM-Entry field for vmcs01 and vmcs02, VM-Exit field for vmcs12).
> Intel CPUs are incapable of handling 32-bit error codes and will never
> generate an error code with bits 31:16, but userspace can provide an
> arbitrary error code via KVM_SET_VCPU_EVENTS.  Failure to drop the bits
> on exception injection results in failed VM-Entry, as VMX disallows
> setting bits 31:16.  Setting the bits on VM-Exit would at best confuse
> L1, and at worse induce a nested VM-Entry failure, e.g. if L1 decided to
> reinject the exception back into L2.

Wouldn't it be better to fail KVM_SET_VCPU_EVENTS instead if it tries
to set error code with uppper 16 bits set?

Or if that is considered ABI breakage, then KVM_SET_VCPU_EVENTS code
can truncate the user given value to 16 bit.

Best regards,
	Maxim Levitsky


> 
> Cc: stable@vger.kernel.org
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> ---
>  arch/x86/kvm/vmx/nested.c |  9 ++++++++-
>  arch/x86/kvm/vmx/vmx.c    | 11 ++++++++++-
>  2 files changed, 18 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index ee6f27dffdba..33ffc8bcf9cd 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -3833,7 +3833,14 @@ static void nested_vmx_inject_exception_vmexit(struct kvm_vcpu *vcpu,
>  	u32 intr_info = nr | INTR_INFO_VALID_MASK;
>  
>  	if (vcpu->arch.exception.has_error_code) {
> -		vmcs12->vm_exit_intr_error_code = vcpu->arch.exception.error_code;
> +		/*
> +		 * Intel CPUs will never generate an error code with bits 31:16
> +		 * set, and more importantly VMX disallows setting bits 31:16
> +		 * in the injected error code for VM-Entry.  Drop the bits to
> +		 * mimic hardware and avoid inducing failure on nested VM-Entry
> +		 * if L1 chooses to inject the exception back to L2.
> +		 */
> +		vmcs12->vm_exit_intr_error_code = (u16)vcpu->arch.exception.error_code;
>  		intr_info |= INTR_INFO_DELIVER_CODE_MASK;
>  	}
>  
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 5e14e4c40007..ec98992024e2 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1621,7 +1621,16 @@ static void vmx_queue_exception(struct kvm_vcpu *vcpu)
>  	kvm_deliver_exception_payload(vcpu);
>  
>  	if (has_error_code) {
> -		vmcs_write32(VM_ENTRY_EXCEPTION_ERROR_CODE, error_code);
> +		/*
> +		 * Despite the error code being architecturally defined as 32
> +		 * bits, and the VMCS field being 32 bits, Intel CPUs and thus
> +		 * VMX don't actually supporting setting bits 31:16.  Hardware
> +		 * will (should) never provide a bogus error code, but KVM's
> +		 * ABI lets userspace shove in arbitrary 32-bit values.  Drop
> +		 * the upper bits to avoid VM-Fail, losing information that
> +		 * does't really exist is preferable to killing the VM.
> +		 */
> +		vmcs_write32(VM_ENTRY_EXCEPTION_ERROR_CODE, (u16)error_code);
>  		intr_info |= INTR_INFO_DELIVER_CODE_MASK;
>  	}
>