vhost: return EINVAL if iovecs size does not match the message size

We've failed to copy and process vhost_iotlb_msg so let userspace at
least know about it. For instance before these patch the code below runs
without any error:

int main()
{
  struct vhost_msg msg;
  struct iovec iov;
  int fd;

  fd = open("/dev/vhost-net", O_RDWR);
  if (fd == -1) {
    perror("open");
    return 1;
  }

  iov.iov_base = &msg;
  iov.iov_len = sizeof(msg)-4;

  if (writev(fd, &iov,1) == -1) {
    perror("writev");
    return 1;
  }

  return 0;
}
Signed-off-by: NPavel Tikhomirov <ptikhomirov@virtuozzo.com>
Signed-off-by: NMichael S. Tsirkin <mst@redhat.com>

drivers/vhost/vhost.c

@@ -1034,8 +1034,10 @@ ssize_t vhost_chr_write_iter(struct vhost_dev *dev,
 	int type, ret;
 
 	ret = copy_from_iter(&type, sizeof(type), from);
-	if (ret != sizeof(type))
+	if (ret != sizeof(type)) {
+		ret = -EINVAL;
 		goto done;
+	}
 
 	switch (type) {
 	case VHOST_IOTLB_MSG:
@@ -1054,8 +1056,10 @@ ssize_t vhost_chr_write_iter(struct vhost_dev *dev,
 
 	iov_iter_advance(from, offset);
 	ret = copy_from_iter(&msg, sizeof(msg), from);
-	if (ret != sizeof(msg))
+	if (ret != sizeof(msg)) {
+		ret = -EINVAL;
 		goto done;
+	}
 	if (vhost_process_iotlb_msg(dev, &msg)) {
 		ret = -EFAULT;
 		goto done;