KVM: x86: Limit check IDs for KVM_SET_BOOT_CPU_ID

Do not accept IDs which are definitely invalid by limit checking the
passed value against KVM_MAX_VCPU_IDS and 'max_vcpu_ids' if it was
already set.

This ensures invalid values, especially on 64-bit systems, don't go
unnoticed and lead to a valid id by chance when truncated by the final
assignment.

Fixes: 73880c8 ("KVM: Break dependency between vcpu index in vcpus array and vcpu_id.")
Signed-off-by: Mathias Krause <minipli@grsecurity.net>
Link: https://lore.kernel.org/r/20240614202859.3597745-3-minipli@grsecurity.net
Co-developed-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: minipli-oss

arch/x86/kvm/x86.c

@@ -7228,6 +7228,9 @@ int kvm_arch_vm_ioctl(struct file *filp, unsigned int ioctl, unsigned long arg)
 		mutex_lock(&kvm->lock);
 		if (kvm->created_vcpus)
 			r = -EBUSY;
+		else if (arg > KVM_MAX_VCPU_IDS ||
+			 (kvm->arch.max_vcpu_ids && arg > kvm->arch.max_vcpu_ids))
+			r = -EINVAL;
 		else
 			kvm->arch.bsp_vcpu_id = arg;
 		mutex_unlock(&kvm->lock);