Re: [PATCH v2 5/6] KVM: arm64: Handle out-of-bound write to HDCR_EL2.HPMN

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

HDCR? I thought you gave up on 32-bit a loooong time ago ;-)

On Wed, Apr 09, 2025 at 05:01:05PM +0100, Marc Zyngier wrote:
> We don't really pay attention to what gets written to MDCR_EL2.HPMN,
> and funky guests could play ugly games on us.
> 
> Restrict what gets written there, and limit the number of counters
> to what the PMU is allowed to have.
> 
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
>  arch/arm64/kvm/sys_regs.c | 34 +++++++++++++++++++++++++---------
>  1 file changed, 25 insertions(+), 9 deletions(-)
> 
> diff --git a/arch/arm64/kvm/sys_regs.c b/arch/arm64/kvm/sys_regs.c
> index 00b5396492d51..e53b8f82ca7f8 100644
> --- a/arch/arm64/kvm/sys_regs.c
> +++ b/arch/arm64/kvm/sys_regs.c
> @@ -2571,17 +2571,33 @@ static bool access_mdcr(struct kvm_vcpu *vcpu,
>  			struct sys_reg_params *p,
>  			const struct sys_reg_desc *r)
>  {
> -	u64 old = __vcpu_sys_reg(vcpu, MDCR_EL2);
> +	if (!p->is_write) {
> +		p->regval = __vcpu_sys_reg(vcpu, MDCR_EL2);
> +	} else {

nit: you can do an early return for an emulated read and get rid of a
level of indentation for the write case.

> +		u64 hpmn = FIELD_GET(MDCR_EL2_HPMN, p->regval);
> +		u64 old = __vcpu_sys_reg(vcpu, MDCR_EL2);
> +		u64 val = p->regval;
>  
> -	if (!access_rw(vcpu, p, r))
> -		return false;
> +		/*
> +		 * If HPMN is out of bounds, limit it to what we actually
> +		 * support. This matches the UNKNOWN definition of the field
> +		 * in that case, and keeps the emulation simple. Sort of.
> +		 */
> +		if (hpmn > vcpu->kvm->arch.pmcr_n) {
> +			hpmn = vcpu->kvm->arch.pmcr_n;
> +			u64_replace_bits(val, hpmn, MDCR_EL2_HPMN);
> +		}
>  
> -	/*
> -	 * Request a reload of the PMU to enable/disable the counters affected
> -	 * by HPME.
> -	 */
> -	if ((old ^ __vcpu_sys_reg(vcpu, MDCR_EL2)) & MDCR_EL2_HPME)
> -		kvm_make_request(KVM_REQ_RELOAD_PMU, vcpu);
> +		vcpu_write_sys_reg(vcpu, val, r->reg);
> +
> +		/*
> +		 * Request a reload of the PMU to enable/disable the
> +		 * counters affected by HPME.
> +		 */
> +
> +		if ((old ^ __vcpu_sys_reg(vcpu, MDCR_EL2)) & MDCR_EL2_HPME)
> +			kvm_make_request(KVM_REQ_RELOAD_PMU, vcpu);
> +	}
>  
>  	return true;
>  }
> -- 
> 2.39.2
> 

Thanks,
Oliver