vhost: scsi: add weight support

This patch will check the weight and exit the loop if we exceeds the
weight. This is useful for preventing scsi kthread from hogging cpu
which is guest triggerable.

This addresses CVE-2019-3900.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Stefan Hajnoczi <stefanha@redhat.com>
Fixes: 057cbf49 ("tcm_vhost: Initial merge for vhost level target fabric driver")
Signed-off-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

CVE-2019-3900

(backported from commit c1ea02f1)
[tyhicks: Backport to Xenial:
 - Minor context adjustment in local variables
 - Adjust context around the loop in vhost_scsi_handle_vq()
 - No need to modify vhost_scsi_ctl_handle_vq() since it was added later
   in commit 0d02dbd6 ("vhost/scsi: Respond to control queue
   operations")]
Signed-off-by: Tyler Hicks <tyhicks@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Connor Kuehl <connor.kuehl@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

drivers/vhost/scsi.c

@@ -861,7 +861,7 @@ vhost_scsi_handle_vq(struct vhost_scsi *vs, struct vhost_virtqueue *vq)
 	u64 tag;
 	u32 exp_data_len, data_direction;
 	unsigned out, in;
-	int head, ret, prot_bytes;
+	int head, ret, prot_bytes, c = 0;
 	size_t req_size, rsp_size = sizeof(struct virtio_scsi_cmd_resp);
 	size_t out_size, in_size;
 	u16 lun;
@@ -880,7 +880,7 @@ vhost_scsi_handle_vq(struct vhost_scsi *vs, struct vhost_virtqueue *vq)
 
 	vhost_disable_notify(&vs->dev, vq);
 
-	for (;;) {
+	do {
 		head = vhost_get_vq_desc(vq, vq->iov,
 					 ARRAY_SIZE(vq->iov), &out, &in,
 					 NULL, NULL);
@@ -1096,7 +1096,7 @@ vhost_scsi_handle_vq(struct vhost_scsi *vs, struct vhost_virtqueue *vq)
 		 */
 		INIT_WORK(&cmd->work, vhost_scsi_submission_work);
 		queue_work(vhost_scsi_workqueue, &cmd->work);
-	}
+	} while (likely(!vhost_exceeds_weight(vq, ++c, 0)));
 out:
 	mutex_unlock(&vq->mutex);
 }