Re: [PATCH v2 12/15] KVM: x86: Harden is_64_bit_hypercall() against bugs on 32-bit kernels

[Binbin Wu]

On 5/15/2026 5:53 AM, Sean Christopherson wrote:
> Unconditionally return %false for is_64_bit_hypercall() on 32-bit kernels
> to guard against incorrectly setting guest_state_protected, and because
> in a (very) hypothetical world where 32-bit KVM supports protected guests,
> assuming a hypercall was made in 64-bit mode is flat out wrong.
> 
> Reviewed-by: Kai Huang <kai.huang@xxxxxxxxx>
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>

Reviewed-by: Binbin Wu <binbin.wu@xxxxxxxxxxxxxxx>

> ---
>  arch/x86/kvm/regs.h | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/arch/x86/kvm/regs.h b/arch/x86/kvm/regs.h
> index 52bed14f43e3..d4d2a47a4968 100644
> --- a/arch/x86/kvm/regs.h
> +++ b/arch/x86/kvm/regs.h
> @@ -39,12 +39,16 @@ static inline bool is_64_bit_mode(struct kvm_vcpu *vcpu)
>  
>  static inline bool is_64_bit_hypercall(struct kvm_vcpu *vcpu)
>  {
> +#ifdef CONFIG_X86_64
>  	/*
>  	 * If running with protected guest state, the CS register is not
>  	 * accessible. The hypercall register values will have had to been
>  	 * provided in 64-bit mode, so assume the guest is in 64-bit.
>  	 */
>  	return vcpu->arch.guest_state_protected || is_64_bit_mode(vcpu);
> +#else
> +	return false;
> +#endif
>  }
>  
>  static __always_inline unsigned long kvm_reg_mode_mask(struct kvm_vcpu *vcpu)