Re: [PATCH v20 18/22] KVM: arm64: CCA: Don't expose unsupported capabilities for realm guests

[sashiko-bot@kernel.org]

Thank you for your contribution! Sashiko AI review found 1 potential issue(s) to consider:
- [High] kvm: arm64: CCA: Realm VCPU features bypass capability allowlist
--

commit e303d487e75069060645d8d27a0d2607328fb9fc
Author: Suzuki K Poulose <suzuki.poulose@arm.com>

KVM: arm64: CCA: Don't expose unsupported capabilities for realm guests

Limit the capabilities that are allowed for Realm VMs. Similarly block
the vm_ioctls backed by the capabilities.

Repurpose the kvm_pkvm_ioctl_allowed() to support both pKVM and Realm
ioctls. Rename the helper to kvm_vm_ioctl_allowed() and move it
into arch/arm64/kvm/arm.c. Also add a generic kvm_vm_ext_allowed()
to handle pKVM and Realm capability filtering and route them accordingly.

> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> index 0e77f59ded6d9..106b52e556e20 100644
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
[ ... ]
> @@ -136,6 +136,49 @@ int kvm_arch_vcpu_should_kick(struct kvm_vcpu *vcpu)
>  	return kvm_vcpu_exiting_guest_mode(vcpu) == IN_GUEST_MODE;
>  }
>  
> +static inline bool kvm_vm_ext_allowed(struct kvm *kvm, long ext)
> +{
> +	/*
> +	 * We could be called with kvm as NULL, so can't use kvm_vm_* for pKVM
> +	 * flavors
> +	 */
> +	if (is_protected_kvm_enabled())
> +		return kvm_pkvm_ext_allowed(kvm, ext);
> +	else if (kvm && kvm_vm_is_realm(kvm))
> +		return kvm_realm_ext_allowed(ext);
> +	else
> +		return true;
> +}

[Severity: High]
While this patch correctly limits VM-level capabilities and ioctls for Realm
VMs, does it miss restricting VCPU-level features during initialization?

Looking at kvm_vcpu_init_check_features() in arch/arm64/kvm/arm.c:

static int kvm_vcpu_init_check_features(struct kvm_vcpu *vcpu,
					const struct kvm_vcpu_init *init)
{
	unsigned long features = init->features[0];
	...
	if (features & ~system_supported_vcpu_features())
		return -EINVAL;

Since system_supported_vcpu_features() only checks global host capabilities,
can a malicious userspace process managing the VM enable restricted VCPU
features (such as KVM_ARM_VCPU_PMU_V3 or KVM_ARM_VCPU_EL1_32BIT) on a Realm
VCPU?

If userspace creates a Realm VM and VCPU, then invokes the KVM_ARM_VCPU_INIT
ioctl with restricted features set in init.features[0], this could activate
host KVM emulation logic for a Realm. Would this break the intended isolation
boundary and lead to inconsistent state or host/RMM crashes when KVM attempts
to emulate hardware for a Realm managed by the RMM?

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260924160504.853911-1-suzuki.poulose@arm.com?part=18