[NETFILTER]: ip_tables: fix compat related crash

check_compat_entry_size_and_hooks iterates over the matches and calls
compat_check_calc_match, which loads the match and calculates the
compat offsets, but unlike the non-compat version, doesn't call
->checkentry yet. On error however it calls cleanup_matches, which in
turn calls ->destroy, which can result in crashes if the destroy
function (validly) expects to only get called after the checkentry
function.

Add a compat_release_match function that only drops the module reference
on error and rename compat_check_calc_match to compat_find_calc_match to
reflect the fact that it doesn't call the checkentry function.

Reported by Jan Engelhardt <jengelh@linux01.gwdg.de>

Signed-off-by: Dmitry Mishin <dim@openvz.org>
Signed-off-by: Patrick McHardy <kaber@trash.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netfilter_ipv4/ip_tables.h

@@ -264,6 +264,26 @@ ipt_get_target(struct ipt_entry *e)
 	__ret;							\
 })
 
+/* fn returns 0 to continue iteration */
+#define IPT_ENTRY_ITERATE_CONTINUE(entries, size, n, fn, args...) \
+({								\
+	unsigned int __i, __n;					\
+	int __ret = 0;						\
+	struct ipt_entry *__entry;				\
+								\
+	for (__i = 0, __n = 0; __i < (size);			\
+	     __i += __entry->next_offset, __n++) { 		\
+		__entry = (void *)(entries) + __i;		\
+		if (__n < n)					\
+			continue;				\
+								\
+		__ret = fn(__entry , ## args);			\
+		if (__ret != 0)					\
+			break;					\
+	}							\
+	__ret;							\
+})
+
 /*
  *	Main firewall chains definitions and global var's definitions.
  */