netfilter: nfnetlink_osf: fix mss parsing on big-endian architectures

[ Upstream commit a625c94 ]

The MSS calculation in nf_osf_match_one() manually shifts bytes to
construct a 16-bit value before passing it to ntohs().

This works on little-endian hosts but it does not work on big-endian as
the bytes are being always shifted and set in the same way for all
architectures.

Use get_unaligned_be16() to fix this on big-endian systems. It also
simplifies the code.

Fixes: 11eeef4 ("netfilter: passive OS fingerprint xtables match")
Signed-off-by: Fernando Fernandez Mancera <fmancera@suse.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

Author: ffmancera

net/netfilter/nfnetlink_osf.c

@@ -95,11 +95,7 @@ static bool nf_osf_match_one(const struct sk_buff *skb,
 
 			switch (*optp) {
 			case OSFOPT_MSS:
-				mss = optp[3];
-				mss <<= 8;
-				mss |= optp[2];
-
-				mss = ntohs((__force __be16)mss);
+				mss = get_unaligned_be16(&optp[2]);
 				break;
 			case OSFOPT_TS:
 				break;