KVM: Disallow fork() and similar games when using a VM

We don't want the meaning of guest userspace changing under our feet.
Signed-off-by: Avi Kivity <avi@qumranet.com>

drivers/kvm/kvm.h

@@ -305,6 +305,7 @@ struct kvm_vm_stat {
 
 struct kvm {
 	struct mutex lock; /* protects everything except vcpus */
+	struct mm_struct *mm; /* userspace tied to this vm */
 	int naliases;
 	struct kvm_mem_alias aliases[KVM_ALIAS_SLOTS];
 	int nmemslots;

drivers/kvm/kvm_main.c

@@ -165,6 +165,8 @@ static struct kvm *kvm_create_vm(void)
 	if (IS_ERR(kvm))
 		goto out;
 
+	kvm->mm = current->mm;
+	atomic_inc(&kvm->mm->mm_count);
 	kvm_io_bus_init(&kvm->pio_bus);
 	mutex_init(&kvm->lock);
 	kvm_io_bus_init(&kvm->mmio_bus);
@@ -202,12 +204,15 @@ void kvm_free_physmem(struct kvm *kvm)
 
 static void kvm_destroy_vm(struct kvm *kvm)
 {
+	struct mm_struct *mm = kvm->mm;
+
 	spin_lock(&kvm_lock);
 	list_del(&kvm->vm_list);
 	spin_unlock(&kvm_lock);
 	kvm_io_bus_destroy(&kvm->pio_bus);
 	kvm_io_bus_destroy(&kvm->mmio_bus);
 	kvm_arch_destroy_vm(kvm);
+	mmdrop(mm);
 }
 
 static int kvm_vm_release(struct inode *inode, struct file *filp)
@@ -818,6 +823,8 @@ static long kvm_vcpu_ioctl(struct file *filp,
 	void __user *argp = (void __user *)arg;
 	int r;
 
+	if (vcpu->kvm->mm != current->mm)
+		return -EIO;
 	switch (ioctl) {
 	case KVM_RUN:
 		r = -EINVAL;
@@ -976,6 +983,8 @@ static long kvm_vm_ioctl(struct file *filp,
 	void __user *argp = (void __user *)arg;
 	int r;
 
+	if (kvm->mm != current->mm)
+		return -EIO;
 	switch (ioctl) {
 	case KVM_CREATE_VCPU:
 		r = kvm_vm_ioctl_create_vcpu(kvm, arg);

kernel/fork.c

@@ -393,6 +393,7 @@ void fastcall __mmdrop(struct mm_struct *mm)
 	destroy_context(mm);
 	free_mm(mm);
 }
+EXPORT_SYMBOL_GPL(__mmdrop);
 
 /*
  * Decrement the use count and release all resources for an mm.