tools/ocaml/xenstored: only Dom0 can change node owner

Otherwise we can give quota away to another domain, either causing it to run
out of quota, or in case of Dom0 use unbounded amounts of memory and bypass
the quota system entirely.

This was fixed in the C version of xenstored in 2006 (c/s db34d2aaa5,
predating the XSA process by 5 years).

It was also fixed in the mirage version of xenstore in 2012, with a unit test
demonstrating the vulnerability:

  https://github.com/mirage/ocaml-xenstore/commit/6b91f3ac46b885d0530a51d57a9b3a57d64923a7
  https://github.com/mirage/ocaml-xenstore/commit/22ee5417c90b8fda905c38de0d534506152eace6

but possibly without realising that the vulnerability still affected the
in-tree oxenstored (added c/s f44af66041 in 2010).

This is XSA-352.

Signed-off-by: Edwin Török <edvin.torok@citrix.com>
Acked-by: Christian Lindig <christian.lindig@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>

tools/ocaml/xenstored/store.ml

@@ -437,7 +437,8 @@ let setperms store perm path nperms =
 	| Some node ->
 		let old_owner = Node.get_owner node in
 		let new_owner = Perms.Node.get_owner nperms in
-		if not ((old_owner = new_owner) || (Perms.Connection.is_dom0 perm)) then Quota.check store.quota new_owner 0;
+		if not ((old_owner = new_owner) || (Perms.Connection.is_dom0 perm)) then
+			raise Define.Permission_denied;
 		store.root <- path_setperms store perm path nperms;
 		Quota.del_entry store.quota old_owner;
 		Quota.add_entry store.quota new_owner