libxl: Make copy of every xs backend in /libxl in _generic_add

We want to stop libxl trustingly reading information from the backend
directory (since this is, of course, writeable by the backend, which
might be a semi-trusted driver domain).

In principle it is wrong in current libxl for anything to try to
divine virtual device configuration from xenstore: the JSON domain
config ought to supply that, and xenstore should only tell us which
devices actually exist.

However:

Firstly, there are several existing places where configuration
information is retrieved from xenstore rather than JSON.  We do not
want to reen gineer this in a security patch.

Secondly, we want to make a security patch which can be backported to
versions of libxl without the JSON configuration machinery.

So we take the expedient approach of keeping a copy of the
configuration somewhere we trust, namely /libxl.  This is obviously
fairly low-risk, although it does write significantly more keys in
xenstore.

In this patch we make this change in libxl__device_generic_add.  This
is responsible for actually writing the vast majority of device
information to xenstore.  There are a few loose ends which will be
dealt with in a moment.

Likewise, changes to readers to use the new location will appear in
further patches.

This is part of XSA-178.

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

docs/misc/xenstore-paths.markdown

@@ -396,6 +396,10 @@ Path in xenstore to the frontend, normally
 Path in xenstore to the backend, normally
 /local/domain/$BACKEND_DOMID/backend/$KIND/$DOMID/$DEVID
 
+#### /libxl/$DOMID/device/$KIND/$DEVID/$NODE
+
+Trustworthy copy of /local/domain/$DOMID/backend/$KIND/$DEVID/$NODE.
+
 #### /libxl/$DOMID/dm-version ("qemu\_xen"|"qemu\_xen\_traditional") = [n,INTERNAL]
 
 The device model version for a domain.

tools/libxl/libxl_device.c

@@ -167,6 +167,29 @@ retry_transaction:
         xs_write(ctx->xsh, t, GCSPRINTF("%s/frontend", backend_path),
                  frontend_path, strlen(frontend_path));
         libxl__xs_writev(gc, t, backend_path, bents);
+
+        /*
+         * We make a copy of everything for the backend in the libxl
+         * path as well.  This means we don't need to trust the
+         * backend.  Ideally this information would not be used and we
+         * would use the information from the json configuration
+         * instead.  But there are still places in libxl that try to
+         * reconstruct a config from xenstore.
+         *
+         * This duplication will typically produces duplicate keys
+         * which will go out of date, but that's OK because nothing
+         * reads those.  For example, there is usually
+         *   /libxl/$guest/device/$kind/$devid/state
+         * which starts out containing XenbusStateInitialising ("1")
+         * just like the copy in
+         *  /local/domain/$driverdom/backend/$guest/$kind/$devid/state
+         * but which won't ever be updated.
+         *
+         * This duplication is superfluous and messy but as discussed
+         * the proper fix is more intrusive than we want to do now.
+         */
+        rc = libxl__xs_writev(gc, t, libxl_path, bents);
+        if (rc) goto out;
     }
 
     if (!create_transaction)