CVE-2024-36953: Undefined Security Weakness

In the Linux kernel, the following vulnerability has been resolved:

KVM: arm64: vgic-v2: Check for non-NULL vCPU in vgic_v2_parse_attr()

vgic_v2_parse_attr() is responsible for finding the vCPU that matches the user-provided CPUID, which (of course) may not be valid. If the ID is invalid, kvm_get_vcpu_by_id() returns NULL, which isn't handled gracefully.

Similar to the GICv3 uaccess flow, check that kvm_get_vcpu_by_id() actually returns something and fail the ioctl if not.