libxl: Do not trust backend for disk eject vdev

For disk eject, use configured vdev from /libxl, not backend.

The backend directory is writeable by driver domains.  This means that
a malicious driver domain could cause libxl to see a wrong vdev,
confusing the user or the toolstack.

Use the vdev from the /libxl space, rather than the backend.

For convenience, we read the vdev from the /libxl space into the evg
during setup and copy it on each event, rather than reading it afresh
each time (which would in any case involve generating or saving a copy
of the relevant /libxl path).

This is part of XSA-178.

Signed-off-by: Ian Jackson <ian.jackson@eu.citrix.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>

tools/libxl/libxl.c

@@ -1357,8 +1357,7 @@ static void disk_eject_xswatch_callback(libxl__egc *egc, libxl__ev_xswatch *w,
     disk->pdev_path = strdup(""); /* xxx fixme malloc failure */
     disk->format = LIBXL_DISK_FORMAT_EMPTY;
     /* this value is returned to the user: do not free right away */
-    disk->vdev = xs_read(CTX->xsh, XBT_NULL,
-                         GCSPRINTF("%s/dev", backend), NULL);
+    disk->vdev = libxl__strdup(NOGC, evg->vdev);
     disk->removable = 1;
     disk->readwrite = 0;
     disk->is_cdrom = 1;
@@ -1381,9 +1380,6 @@ int libxl_evenable_disk_eject(libxl_ctx *ctx, uint32_t guest_domid,
     evg->domid = guest_domid;
     LIBXL_LIST_INSERT_HEAD(&CTX->disk_eject_evgens, evg, entry);
 
-    evg->vdev = strdup(vdev);
-    if (!evg->vdev) { rc = ERROR_NOMEM; goto out; }
-
     uint32_t domid = libxl_get_stubdom_id(ctx, guest_domid);
 
     if (!domid)
@@ -1401,6 +1397,13 @@ int libxl_evenable_disk_eject(libxl_ctx *ctx, uint32_t guest_domid,
                                  devid);
     evg->be_ptr_path = libxl__sprintf(NOGC, "%s/backend", libxl_path);
 
+    const char *configured_vdev;
+    rc = libxl__xs_read_checked(gc, XBT_NULL,
+            GCSPRINTF("%s/vdev", libxl_path), &configured_vdev);
+    if (rc) goto out;
+
+    evg->vdev = libxl__strdup(NOGC, configured_vdev);
+
     rc = libxl__ev_xswatch_register(gc, &evg->watch,
                                     disk_eject_xswatch_callback, path);
     if (rc) goto out;