mptcp: fix length of MP_PRIO suboption

With version 0 of the protocol it was legal to encode the 'Subflow Id' in
the MP_PRIO suboption, to specify which subflow would change its 'Backup'
flag. This has been removed from v1 specification: thus, according to RFC
8684 §3.3.8, the resulting 'Length' for MP_PRIO changed from 4 to 3 byte.

Current Linux generates / parses MP_PRIO according to the old spec, using
'Length' equal to 4, and hardcoding 1 as 'Subflow Id'; RFC compliance can
improve if we change 'Length' in other to become 3, leaving a 'Nop' after
the MP_PRIO suboption. In this way the kernel will emit and accept *only*
MP_PRIO suboptions that are compliant to version 1 of the MPTCP protocol.

 unpatched 5.11-rc kernel:
 [root@bottarga ~]# tcpdump -tnnr unpatched.pcap | grep prio
 reading from file unpatched.pcap, link-type LINUX_SLL (Linux cooked v1)
 dropped privs to tcpdump
 IP 10.0.3.2.48433 > 10.0.1.1.10006: Flags [.], ack 1, win 502, options [nop,nop,TS val 4032325513 ecr 1876514270,mptcp prio non-backup id 1,mptcp dss ack 14084896651682217737], length 0

 patched 5.11-rc kernel:
 [root@bottarga ~]# tcpdump -tnnr patched.pcap | grep prio
 reading from file patched.pcap, link-type LINUX_SLL (Linux cooked v1)
 dropped privs to tcpdump
 IP 10.0.3.2.49735 > 10.0.1.1.10006: Flags [.], ack 1, win 502, options [nop,nop,TS val 1276737699 ecr 2686399734,mptcp prio non-backup,nop,mptcp dss ack 18433038869082491686], length 0

Changes since v2:
 - when accounting for option space, don't increment 'TCPOLEN_MPTCP_PRIO'
   and use 'TCPOLEN_MPTCP_PRIO_ALIGN' instead, thanks to Matthieu Baerts.
Changes since v1:
 - refactor patch to avoid using 'TCPOLEN_MPTCP_PRIO' with its old value,
   thanks to Geliang Tang.

Fixes: 067065422f ("mptcp: add the outgoing MP_PRIO support")
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Davide Caratti <dcaratti@redhat.com>
Reviewed-by: Matteo Croce <mcroce@linux.microsoft.com>
Link: https://lore.kernel.org/r/846cdd41e6ad6ec88ef23fee1552ab39c2f5a3d1.1612184361.git.dcaratti@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/mptcp/options.c

@@ -699,10 +699,11 @@ static bool mptcp_established_options_mp_prio(struct sock *sk,
 	if (!subflow->send_mp_prio)
 		return false;
 
-	if (remaining < TCPOLEN_MPTCP_PRIO)
+	/* account for the trailing 'nop' option */
+	if (remaining < TCPOLEN_MPTCP_PRIO_ALIGN)
 		return false;
 
-	*size = TCPOLEN_MPTCP_PRIO;
+	*size = TCPOLEN_MPTCP_PRIO_ALIGN;
 	opts->suboptions |= OPTION_MPTCP_PRIO;
 	opts->backup = subflow->request_bkup;
 

net/mptcp/protocol.h

@@ -60,7 +60,8 @@
 #define TCPOLEN_MPTCP_ADD_ADDR6_BASE_PORT	24
 #define TCPOLEN_MPTCP_PORT_LEN		4
 #define TCPOLEN_MPTCP_RM_ADDR_BASE	4
-#define TCPOLEN_MPTCP_PRIO		4
+#define TCPOLEN_MPTCP_PRIO		3
+#define TCPOLEN_MPTCP_PRIO_ALIGN	4
 #define TCPOLEN_MPTCP_FASTCLOSE		12
 
 /* MPTCP MP_JOIN flags */