vhost: add helper to parse userspace vring state/file

The next patches add new vhost worker ioctls which will need to get a
vhost_virtqueue from a userspace struct which specifies the vq's index.
This moves the vhost_vring_ioctl code to do this to a helper so it can
be shared.
Signed-off-by: Mike Christie <michael.christie@oracle.com>
Message-Id: <20230626232307.97930-14-michael.christie@oracle.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/vhost/vhost.c

@@ -599,6 +599,27 @@ static struct vhost_worker *vhost_worker_create(struct vhost_dev *dev)
 	return NULL;
 }
 
+static int vhost_get_vq_from_user(struct vhost_dev *dev, void __user *argp,
+				  struct vhost_virtqueue **vq, u32 *id)
+{
+	u32 __user *idxp = argp;
+	u32 idx;
+	long r;
+
+	r = get_user(idx, idxp);
+	if (r < 0)
+		return r;
+
+	if (idx >= dev->nvqs)
+		return -ENOBUFS;
+
+	idx = array_index_nospec(idx, dev->nvqs);
+
+	*vq = dev->vqs[idx];
+	*id = idx;
+	return 0;
+}
+
 /* Caller should have device mutex */
 long vhost_dev_set_owner(struct vhost_dev *dev)
 {
@@ -1618,21 +1639,15 @@ long vhost_vring_ioctl(struct vhost_dev *d, unsigned int ioctl, void __user *arg
 	struct file *eventfp, *filep = NULL;
 	bool pollstart = false, pollstop = false;
 	struct eventfd_ctx *ctx = NULL;
-	u32 __user *idxp = argp;
 	struct vhost_virtqueue *vq;
 	struct vhost_vring_state s;
 	struct vhost_vring_file f;
 	u32 idx;
 	long r;
 
-	r = get_user(idx, idxp);
+	r = vhost_get_vq_from_user(d, argp, &vq, &idx);
 	if (r < 0)
 		return r;
-	if (idx >= d->nvqs)
-		return -ENOBUFS;
-
-	idx = array_index_nospec(idx, d->nvqs);
-	vq = d->vqs[idx];
 
 	if (ioctl == VHOST_SET_VRING_NUM ||
 	    ioctl == VHOST_SET_VRING_ADDR) {