Re: [PATCH v16 15/45] KVM: arm64: CCA: Don't expose unsupported capabilities for realm guests

[Gavin Shan <gshan@redhat.com>]

Hi Steve,

On 8/3/26 11:43 PM, Steven Price wrote:
> From: Suzuki K Poulose <suzuki.poulose@arm.com>
> 
> RMM v2.0 provides no mechanism for the host to perform debug operations
> on the guest. So limit the extensions that are visible to an allowlist
> so that only those capabilities we can support are advertised.
> 
> Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
> Signed-off-by: Steven Price <steven.price@arm.com>
> ---
> Changes since v13:
>   * Add missing check in kvm_vm_ioctl_enable_cap().
> Changes since v10:
>   * Add a kvm_realm_ext_allowed() function which limits which extensions
>     are exposed to an allowlist. This removes the need for special casing
>     various extensions.
> Changes since v7:
>   * Remove the helper functions and inline the kvm_is_realm() check with
>     a ternary operator.
>   * Rewrite the commit message to explain this patch.
> ---
>   arch/arm64/kvm/arm.c | 26 ++++++++++++++++++++++++++
>   1 file changed, 26 insertions(+)
> 
> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index c4d906861736..6f39831dcf5d 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
> @@ -135,6 +135,26 @@ int kvm_arch_vcpu_should_kick(struct kvm_vcpu *vcpu)
>   	return kvm_vcpu_exiting_guest_mode(vcpu) == IN_GUEST_MODE;
>   }
>   
> +static bool kvm_realm_ext_allowed(long ext)
> +{
> +	switch (ext) {
> +	case KVM_CAP_IRQCHIP:
> +	case KVM_CAP_ARM_PSCI:
> +	case KVM_CAP_ARM_PSCI_0_2:
> +	case KVM_CAP_NR_VCPUS:
> +	case KVM_CAP_MAX_VCPUS:
> +	case KVM_CAP_MAX_VCPU_ID:
> +	case KVM_CAP_MSI_DEVID:
> +	case KVM_CAP_ARM_VM_IPA_SIZE:
> +	case KVM_CAP_ARM_PTRAUTH_ADDRESS:
> +	case KVM_CAP_ARM_PTRAUTH_GENERIC:
> +	case KVM_CAP_ARM_RMI:
> +	case KVM_CAP_SYNC_MMU:
> +		return true;
> +	}
> +	return false;
> +}
> +

Nitpick: The code can be simplified a bit if @kvm is passed to kvm_realm_ext_allowed().
In this way, kvm_realm_ext_allowed() looks similar to kvm_pkvm_ext_allowed().

static bool kvm_realm_ext_allowed(struct kvm *kvm, long ext)
{
	if (!kvm || !kvm_is_realm(kvm))
		return true;

	switch (ext) {
	case KVM_CAP_IRQCHIP:
		:
	case KVM_CAP_SYNC_MMU:
		return true;
	}

	return false;
}

>   int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
>   			    struct kvm_enable_cap *cap)
>   {
> @@ -146,6 +166,9 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
>   	if (is_protected_kvm_enabled() && !kvm_pkvm_ext_allowed(kvm, cap->cap))
>   		return -EINVAL;
>   
> +	if (kvm && kvm_is_realm(kvm) && !kvm_realm_ext_allowed(cap->cap))
> +		return -EINVAL;
> +

	if (!kvm_realm_ext_allowed(kvm, cap->cap))
		return -EINVAL;

>   	switch (cap->cap) {
>   	case KVM_CAP_ARM_NISV_TO_USER:
>   		r = 0;
> @@ -380,6 +403,9 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>   	if (is_protected_kvm_enabled() && !kvm_pkvm_ext_allowed(kvm, ext))
>   		return 0;
>   
> +	if (kvm && kvm_is_realm(kvm) && !kvm_realm_ext_allowed(ext))
> +		return 0;
> +

	if (!kvm_realm_ext_allowed(kvm, ext))
		return 0;

>   	switch (ext) {
>   	case KVM_CAP_IRQCHIP:
>   		r = vgic_present;

Thanks,
Gavin