netfilter: nft_exthdr: fix endianness of tcp option cast

I got a problem on MIPS with Big-Endian is turned on: every time when
NF trying to change TCP MSS it returns because of new.v16 was greater
than old.v16. But real MSS was 1460 and my rule was like this:

	add rule table chain tcp option maxseg size set 1400

And 1400 is lesser that 1460, not greater.

Later I founded that main causer is cast from u32 to __be16.

Debugging:

In example MSS = 1400(HEX: 0x578). Here is representation of each byte
like it is in memory by addresses from left to right(e.g. [0x0 0x1 0x2
0x3]). LE — Little-Endian system, BE — Big-Endian, left column is type.

	     LE               BE
	u32: [78 05 00 00]    [00 00 05 78]

As you can see, u32 representation will be casted to u16 from different
half of 4-byte address range. But actually nf_tables uses registers and
store data of various size. Actually TCP MSS stored in 2 bytes. But
registers are still u32 in definition:

	struct nft_regs {
		union {
			u32			data[20];
			struct nft_verdict	verdict;
		};
	};

So, access like regs->data[priv->sreg] exactly u32. So, according to
table presents above, per-byte representation of stored TCP MSS in
register will be:

	                     LE               BE
	(u32)regs->data[]:   [78 05 00 00]    [05 78 00 00]
	                                       ^^ ^^

We see that register uses just half of u32 and other 2 bytes may be
used for some another data. But in nft_exthdr_tcp_set_eval() it casted
just like u32 -> __be16:

	new.v16 = src

But u32 overfill __be16, so it get 2 low bytes. For clarity draw
one more table(<xx xx> means that bytes will be used for cast).

	                     LE                 BE
	u32:                 [<78 05> 00 00]    [00 00 <05 78>]
	(u32)regs->data[]:   [<78 05> 00 00]    [05 78 <00 00>]

As you can see, for Little-Endian nothing changes, but for Big-endian we
take the wrong half. In my case there is some other data instead of
zeros, so new MSS was wrongly greater.

For shooting this bug I used solution for ports ranges. Applying of this
patch does not affect Little-Endian systems.
Signed-off-by: Sergey Marinkevich <sergey.marinkevich@eltex-co.ru>
Acked-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

net/netfilter/nft_exthdr.c

@@ -228,7 +228,6 @@ static void nft_exthdr_tcp_set_eval(const struct nft_expr *expr,
 	unsigned int i, optl, tcphdr_len, offset;
 	struct tcphdr *tcph;
 	u8 *opt;
-	u32 src;
 
 	tcph = nft_tcp_header_pointer(pkt, sizeof(buff), buff, &tcphdr_len);
 	if (!tcph)
@@ -237,7 +236,6 @@ static void nft_exthdr_tcp_set_eval(const struct nft_expr *expr,
 	opt = (u8 *)tcph;
 	for (i = sizeof(*tcph); i < tcphdr_len - 1; i += optl) {
 		union {
-			u8 octet;
 			__be16 v16;
 			__be32 v32;
 		} old, new;
@@ -259,13 +257,13 @@ static void nft_exthdr_tcp_set_eval(const struct nft_expr *expr,
 		if (!tcph)
 			return;
 
-		src = regs->data[priv->sreg];
 		offset = i + priv->offset;
 
 		switch (priv->len) {
 		case 2:
 			old.v16 = get_unaligned((u16 *)(opt + offset));
-			new.v16 = src;
+			new.v16 = (__force __be16)nft_reg_load16(
+				&regs->data[priv->sreg]);
 
 			switch (priv->type) {
 			case TCPOPT_MSS:
@@ -283,7 +281,7 @@ static void nft_exthdr_tcp_set_eval(const struct nft_expr *expr,
 						 old.v16, new.v16, false);
 			break;
 		case 4:
-			new.v32 = src;
+			new.v32 = regs->data[priv->sreg];
 			old.v32 = get_unaligned((u32 *)(opt + offset));
 
 			if (old.v32 == new.v32)