Re: [PATCH 2/2] KVM: arm64: PSCI: Forbid 64bit functions for 32bit guests

[Christoffer Dall <christoffer.dall@arm.com>]

On Wed, Apr 01, 2020 at 05:58:16PM +0100, Marc Zyngier wrote:
> Implementing (and even advertising) 64bit PSCI functions to 32bit
> guests is at least a bit odd, if not altogether violating the
> spec which says ("5.2.1 Register usage in arguments and return values"):
> 
> "Adherence to the SMC Calling Conventions implies that any AArch32
> caller of an SMC64 function will get a return code of 0xFFFFFFFF(int32).
> This matches the NOT_SUPPORTED error code used in PSCI"
> 
> Tighten the implementation by pretending these functions are not
> there for 32bit guests.
> 
> Signed-off-by: Marc Zyngier <maz@kernel.org>
> ---
>  virt/kvm/arm/psci.c | 24 ++++++++++++++++++++++++
>  1 file changed, 24 insertions(+)
> 
> diff --git a/virt/kvm/arm/psci.c b/virt/kvm/arm/psci.c
> index 69ff4a51ceb5..122795cdd984 100644
> --- a/virt/kvm/arm/psci.c
> +++ b/virt/kvm/arm/psci.c
> @@ -199,6 +199,21 @@ static void kvm_psci_narrow_to_32bit(struct kvm_vcpu *vcpu)
>  		vcpu_set_reg(vcpu, i, (u32)vcpu_get_reg(vcpu, i));
>  }
>  
> +static unsigned long kvm_psci_check_allowed_function(struct kvm_vcpu *vcpu, u32 fn)
> +{
> +	switch(fn) {
> +	case PSCI_0_2_FN64_CPU_SUSPEND:
> +	case PSCI_0_2_FN64_CPU_ON:
> +	case PSCI_0_2_FN64_AFFINITY_INFO:
> +		/* Disallow these functions for 32bit guests */
> +		if (vcpu_mode_is_32bit(vcpu))
> +			return PSCI_RET_NOT_SUPPORTED;
> +		break;
> +	}
> +
> +	return 0;
> +}
> +
>  static int kvm_psci_0_2_call(struct kvm_vcpu *vcpu)
>  {
>  	struct kvm *kvm = vcpu->kvm;
> @@ -206,6 +221,10 @@ static int kvm_psci_0_2_call(struct kvm_vcpu *vcpu)
>  	unsigned long val;
>  	int ret = 1;
>  
> +	val = kvm_psci_check_allowed_function(vcpu, psci_fn);
> +	if (val)
> +		goto out;
> +
>  	switch (psci_fn) {
>  	case PSCI_0_2_FN_PSCI_VERSION:
>  		/*
> @@ -273,6 +292,7 @@ static int kvm_psci_0_2_call(struct kvm_vcpu *vcpu)
>  		break;
>  	}
>  
> +out:
>  	smccc_set_retval(vcpu, val, 0, 0, 0);
>  	return ret;
>  }
> @@ -290,6 +310,10 @@ static int kvm_psci_1_0_call(struct kvm_vcpu *vcpu)
>  		break;
>  	case PSCI_1_0_FN_PSCI_FEATURES:
>  		feature = smccc_get_arg1(vcpu);
> +		val = kvm_psci_check_allowed_function(vcpu, feature);
> +		if (val)
> +			break;
> +
>  		switch(feature) {
>  		case PSCI_0_2_FN_PSCI_VERSION:
>  		case PSCI_0_2_FN_CPU_SUSPEND:
> -- 
> 2.25.0
> 

Reviewed-by: Christoffer Dall <christoffer.dall@arm.com>
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