Re: [PATCH v18 20/23] 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: host Denial of Service via WARN_ON_ONCE in kvm_arch_vm_ioctl_allowed()
--

commit f9fdb12a2363dfa41ace289596ec2d2bd95cde48
Author: Suzuki K Poulose <suzuki.poulose@arm.com>
KVM: arm64: CCA: Don't expose unsupported capabilities for realm guests

This commit limits the capabilities allowed for Realm VMs and blocks
vm_ioctls backed by unsupported capabilities. It introduces generic
capability and ioctl filtering helpers kvm_arch_vm_ext_allowed() and
kvm_arch_vm_ioctl_allowed() to support both pKVM and Realm guests.

> diff --git a/arch/arm64/kvm/arm.c b/arch/arm64/kvm/arm.c
> --- a/arch/arm64/kvm/arm.c
> +++ b/arch/arm64/kvm/arm.c
[ ... ]
> +static inline bool kvm_arch_vm_ioctl_allowed(struct kvm *kvm, unsigned int ioctl)
> +{
> +	long ext;
> +	int r;
> +
> +	r = kvm_get_cap_for_kvm_ioctl(ioctl, &ext);
> +	if (WARN_ON_ONCE(r < 0))
> +		return false;

[Severity: High]
Could this allow an unprivileged host userspace process to crash the host?

When kvm_arch_vm_ioctl() receives an unrecognized or garbage ioctl, 
kvm_get_cap_for_kvm_ioctl() returns a negative value. This evaluates to true
and triggers the WARN_ON_ONCE() here. On systems configured with panic_on_warn
(which is common in production and Android environments), this results in a
kernel panic and a complete host denial of service.

[ ... ]
> @@ -2150,7 +2202,7 @@ int kvm_arch_vm_ioctl(struct file *filp, unsigned int ioctl, unsigned long arg)
>  	void __user *argp = (void __user *)arg;
>  	struct kvm_device_attr attr;
>  
> -	if (is_protected_kvm_enabled() && !kvm_pkvm_ioctl_allowed(kvm, ioctl))
> +	if (!kvm_arch_vm_ioctl_allowed(kvm, ioctl))
>  		return -EINVAL;

[Severity: High]
Does removing the is_protected_kvm_enabled() check expose the WARN_ON_ONCE()
above to all ARM64 KVM deployments?

Previously, the warning in kvm_pkvm_ioctl_allowed() was strictly guarded by 
is_protected_kvm_enabled(). By unconditionally calling 
kvm_arch_vm_ioctl_allowed() for all VMs, any local user with access to 
/dev/kvm can open a VM file descriptor and issue an unrecognized ioctl to 
trigger the warning. This exposes the panic condition to all ARM64 KVM 
deployments rather than strictly limiting it to pKVM configurations.

-- 
Sashiko AI review · https://sashiko.dev/#/patchset/20260915160141.3543048-1-suzuki.poulose@arm.com?part=20