scsi-cd: Fix inserting read-only media in empty drive

scsi-disks decides whether it has a read-only device by looking at
whether the BlockBackend specified as drive=... is read-only. In the
case of an anonymous BlockBackend (with a node name specified in
drive=...), this is the read-only flag of the attached node. In the case
of an empty anonymous BlockBackend, it's always read-write because
nothing prevented it from being read-write.

This is a problem because scsi-cd would take write permissions on the
anonymous BlockBackend of an empty drive created without a drive=...
option. Using blockdev-insert-medium with a read-only node fails then
with the error message "Block node is read-only".

Fix scsi_realize() so that scsi-cd devices always take read-only
permissions on their BlockBackend instead.

Fixes: https://bugzilla.redhat.com/show_bug.cgi?id=1733920
Signed-off-by: Kevin Wolf <[email protected]>
Reviewed-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Reviewed-by: Markus Armbruster <[email protected]>

hw/scsi/scsi-disk.c

@@ -2318,6 +2318,7 @@ static void scsi_disk_unit_attention_reported(SCSIDevice *dev)
 static void scsi_realize(SCSIDevice *dev, Error **errp)
 {
     SCSIDiskState *s = DO_UPCAST(SCSIDiskState, qdev, dev);
+    bool read_only;
 
     if (!s->qdev.conf.blk) {
         error_setg(errp, "drive property not set");
@@ -2351,8 +2352,13 @@ static void scsi_realize(SCSIDevice *dev, Error **errp)
             return;
         }
     }
-    if (!blkconf_apply_backend_options(&dev->conf,
-                                       blk_is_read_only(s->qdev.conf.blk),
+
+    read_only = blk_is_read_only(s->qdev.conf.blk);
+    if (dev->type == TYPE_ROM) {
+        read_only = true;
+    }
+
+    if (!blkconf_apply_backend_options(&dev->conf, read_only,
                                        dev->type == TYPE_DISK, errp)) {
         return;
     }