ebtables: check struct type before computing gap

Check struct type before dereferencing fields in ebt_entry.
Failure to check can cause oops.
Signed-off-by: Chuck Ebbert <76306.1226@compuserve.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Adrian Bunk <bunk@stusta.de>

net/bridge/netfilter/ebtables.c

@@ -596,7 +596,7 @@ ebt_check_entry(struct ebt_entry *e, struct ebt_table_info *newinfo,
 	struct ebt_entry_target *t;
 	struct ebt_target *target;
 	unsigned int i, j, hook = 0, hookmask = 0;
-	size_t gap = e->next_offset - e->target_offset;
+	size_t gap;
 	int ret;
 
 	/* don't mess with the struct ebt_entries */
@@ -646,6 +646,7 @@ ebt_check_entry(struct ebt_entry *e, struct ebt_table_info *newinfo,
 	if (ret != 0)
 		goto cleanup_watchers;
 	t = (struct ebt_entry_target *)(((char *)e) + e->target_offset);
+	gap = e->next_offset - e->target_offset;
 	target = find_target_lock(t->u.name, &ret, &ebt_mutex);
 	if (!target)
 		goto cleanup_watchers;