[PATCH 9/9] vhost: scsi: add weight support

[Tyler Hicks]

From: Jason Wang <jasowang at redhat.com>

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 at redhat.com>
Cc: Stefan Hajnoczi <stefanha at redhat.com>
Fixes: 057cbf49a1f0 ("tcm_vhost: Initial merge for vhost level target fabric driver")
Signed-off-by: Jason Wang <jasowang at redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha at redhat.com>
Signed-off-by: Michael S. Tsirkin <mst at redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha at redhat.com>

CVE-2019-3900

(backported from commit c1ea02f15ab5efb3e93fc3144d895410bf79fcf2)
[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 0d02dbd68c47 ("vhost/scsi: Respond to control queue
   operations")]
Signed-off-by: Tyler Hicks <tyhicks at canonical.com>
---
 drivers/vhost/scsi.c | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/drivers/vhost/scsi.c b/drivers/vhost/scsi.c
index 631fa4a768d7..3698dd66c720 100644
--- a/drivers/vhost/scsi.c
+++ b/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);
 }
-- 
2.7.4