Re: [PATCH 01/13] KVM: arm64: Hide CNTHV_*_EL2 from userspace for nVHE guests

[Oliver Upton <oliver.upton@linux.dev>]

Hey,

On Mon, Sep 29, 2025 at 05:04:45PM +0100, Marc Zyngier wrote:
> Although we correctly UNDEF any CNTHV_*_EL2 access from the guest
> when E2H==0, we still expose these registers to userspace, which
> is a bad idea.
> 
> Drop the ad-hoc UNDEF injection and switch to a .visibility()
> callback which will also hide the register from userspace.
> 
> Fixes: 0e45981028550 ("KVM: arm64: timer: Don't adjust the EL2 virtual timer offset")
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
>  arch/arm64/kvm/sys_regs.c | 26 +++++++++++++-------------
>  1 file changed, 13 insertions(+), 13 deletions(-)
> 
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index ee8a7033c85bf..9f2f4e0b042e8 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -1594,16 +1594,6 @@ static bool access_arch_timer(struct kvm_vcpu *vcpu,
>  	return true;
>  }
>  
> -static bool access_hv_timer(struct kvm_vcpu *vcpu,
> -			    struct sys_reg_params *p,
> -			    const struct sys_reg_desc *r)
> -{
> -	if (!vcpu_el2_e2h_is_set(vcpu))
> -		return undef_access(vcpu, p, r);
> -
> -	return access_arch_timer(vcpu, p, r);
> -}
> -
>  static s64 kvm_arm64_ftr_safe_value(u32 id, const struct arm64_ftr_bits *ftrp,
>  				    s64 new, s64 cur)
>  {
> @@ -2831,6 +2821,16 @@ static unsigned int s1pie_el2_visibility(const struct kvm_vcpu *vcpu,
>  	return __el2_visibility(vcpu, rd, s1pie_visibility);
>  }
>  
> +static unsigned int cnthv_visibility(const struct kvm_vcpu *vcpu,
> +				     const struct sys_reg_desc *rd)
> +{
> +	if (vcpu_has_nv(vcpu) &&
> +	    !vcpu_has_feature(vcpu, KVM_ARM_VCPU_HAS_EL2_E2H0))
> +		return 0;
> +
> +	return REG_HIDDEN;
> +}

Hmm. We've already exposed these to userspace at this point, we just
conveniently last the get-reg-list test to assert the accessibility of
these (broken) exposures.

Given the amount of UAPI mishaps we've had with registers in the past I
don't have much appetite for taking away something we already
advertised.

What about making these RAZ/WI from userspace?

Thanks,
Oliver