vhost-scsi: protect vq->log_used with vq->mutex

The vhost-scsi completion path may access vq->log_base when vq->log_used is
already set to false.

    vhost-thread                       QEMU-thread

vhost_scsi_complete_cmd_work()
-> vhost_add_used()
   -> vhost_add_used_n()
      if (unlikely(vq->log_used))
                                      QEMU disables vq->log_used
                                      via VHOST_SET_VRING_ADDR.
                                      mutex_lock(&vq->mutex);
                                      vq->log_used = false now!
                                      mutex_unlock(&vq->mutex);

				      QEMU gfree(vq->log_base)
        log_used()
        -> log_write(vq->log_base)

Assuming the VMM is QEMU. The vq->log_base is from QEMU userpace and can be
reclaimed via gfree(). As a result, this causes invalid memory writes to
QEMU userspace.

The control queue path has the same issue.

Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
Acked-by: Jason Wang <jasowang@redhat.com>
Reviewed-by: Mike Christie <michael.christie@oracle.com>
Message-Id: <20250403063028.16045-2-dongli.zhang@oracle.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

drivers/vhost/scsi.c

@@ -627,6 +627,9 @@ static void vhost_scsi_complete_cmd_work(struct vhost_work *work)
 	int ret;
 
 	llnode = llist_del_all(&svq->completion_list);
+
+	mutex_lock(&svq->vq.mutex);
+
 	llist_for_each_entry_safe(cmd, t, llnode, tvc_completion_list) {
 		se_cmd = &cmd->tvc_se_cmd;
 
@@ -660,6 +663,8 @@ static void vhost_scsi_complete_cmd_work(struct vhost_work *work)
 		vhost_scsi_release_cmd_res(se_cmd);
 	}
 
+	mutex_unlock(&svq->vq.mutex);
+
 	if (signal)
 		vhost_signal(&svq->vs->dev, &svq->vq);
 }
@@ -1432,8 +1437,11 @@ static void vhost_scsi_tmf_resp_work(struct vhost_work *work)
 	else
 		resp_code = VIRTIO_SCSI_S_FUNCTION_REJECTED;
 
+	mutex_lock(&tmf->svq->vq.mutex);
 	vhost_scsi_send_tmf_resp(tmf->vhost, &tmf->svq->vq, tmf->in_iovs,
 				 tmf->vq_desc, &tmf->resp_iov, resp_code);
+	mutex_unlock(&tmf->svq->vq.mutex);
+
 	vhost_scsi_release_tmf_res(tmf);
 }