KVM: kill file->f_count abuse in kvm

Use kvm own refcounting instead of playing with ->filp->f_count.
That will allow to get rid of a lot of crap in anon_inode_getfd() and
kill a race in kvm_dev_ioctl_create_vm() (file might have been closed
immediately by another thread, so ->filp might point to already freed
struct file when we get around to setting it).
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Avi Kivity <avi@qumranet.com>

include/linux/kvm_host.h

@@ -110,7 +110,6 @@ struct kvm {
 					KVM_PRIVATE_MEM_SLOTS];
 	struct kvm_vcpu *vcpus[KVM_MAX_VCPUS];
 	struct list_head vm_list;
-	struct file *filp;
 	struct kvm_io_bus mmio_bus;
 	struct kvm_io_bus pio_bus;
 	struct kvm_vm_stat stat;

virt/kvm/kvm_main.c

@@ -818,7 +818,7 @@ static int kvm_vcpu_release(struct inode *inode, struct file *filp)
 {
 	struct kvm_vcpu *vcpu = filp->private_data;
 
-	fput(vcpu->kvm->filp);
+	kvm_put_kvm(vcpu->kvm);
 	return 0;
 }
 
@@ -840,9 +840,10 @@ static int create_vcpu_fd(struct kvm_vcpu *vcpu)
 
 	r = anon_inode_getfd(&fd, &inode, &file,
 			     "kvm-vcpu", &kvm_vcpu_fops, vcpu);
-	if (r)
+	if (r) {
+		kvm_put_kvm(vcpu->kvm);
 		return r;
-	atomic_inc(&vcpu->kvm->filp->f_count);
+	}
 	return fd;
 }
 
@@ -877,6 +878,7 @@ static int kvm_vm_ioctl_create_vcpu(struct kvm *kvm, int n)
 	mutex_unlock(&kvm->lock);
 
 	/* Now it's all set up, let userspace reach it */
+	kvm_get_kvm(kvm);
 	r = create_vcpu_fd(vcpu);
 	if (r < 0)
 		goto unlink;
@@ -1176,12 +1178,10 @@ static int kvm_dev_ioctl_create_vm(void)
 		return PTR_ERR(kvm);
 	r = anon_inode_getfd(&fd, &inode, &file, "kvm-vm", &kvm_vm_fops, kvm);
 	if (r) {
-		kvm_destroy_vm(kvm);
+		kvm_put_kvm(kvm);
 		return r;
 	}
 
-	kvm->filp = file;
-
 	return fd;
 }