syzbot reported that subflow_check_req() was using uninitialized data in
subflow_check_req() [1]

This is because mp_opt.token is only set when OPTION_MPTCP_MPJ_SYN is also set.

While we are are it, fix mptcp_subflow_init_cookie_req()
to test for OPTION_MPTCP_MPJ_ACK.

[1]

BUG: KMSAN: uninit-value in subflow_token_join_request net/mptcp/subflow.c:91 [inline]
 BUG: KMSAN: uninit-value in subflow_check_req+0x1028/0x15d0 net/mptcp/subflow.c:209
  subflow_token_join_request net/mptcp/subflow.c:91 [inline]
  subflow_check_req+0x1028/0x15d0 net/mptcp/subflow.c:209
  subflow_v6_route_req+0x269/0x410 net/mptcp/subflow.c:367
  tcp_conn_request+0x153a/0x4240 net/ipv4/tcp_input.c:7164
 subflow_v6_conn_request+0x3ee/0x510
  tcp_rcv_state_process+0x2e1/0x4ac0 net/ipv4/tcp_input.c:6659
  tcp_v6_do_rcv+0x11bf/0x1fe0 net/ipv6/tcp_ipv6.c:1669
  tcp_v6_rcv+0x480b/0x4fb0 net/ipv6/tcp_ipv6.c:1900
  ip6_protocol_deliver_rcu+0xda6/0x2a60 net/ipv6/ip6_input.c:438
  ip6_input_finish net/ipv6/ip6_input.c:483 [inline]
  NF_HOOK include/linux/netfilter.h:314 [inline]
  ip6_input+0x15d/0x430 net/ipv6/ip6_input.c:492
  dst_input include/net/dst.h:461 [inline]
  ip6_rcv_finish+0x5db/0x870 net/ipv6/ip6_input.c:79
  NF_HOOK include/linux/netfilter.h:314 [inline]
  ipv6_rcv+0xda/0x390 net/ipv6/ip6_input.c:310
  __netif_receive_skb_one_core net/core/dev.c:5532 [inline]
  __netif_receive_skb+0x1a6/0x5a0 net/core/dev.c:5646
  netif_receive_skb_internal net/core/dev.c:5732 [inline]
  netif_receive_skb+0x58/0x660 net/core/dev.c:5791
  tun_rx_batched+0x3ee/0x980 drivers/net/tun.c:1555
  tun_get_user+0x53af/0x66d0 drivers/net/tun.c:2002
  tun_chr_write_iter+0x3af/0x5d0 drivers/net/tun.c:2048
  call_write_iter include/linux/fs.h:2020 [inline]
  new_sync_write fs/read_write.c:491 [inline]
  vfs_write+0x8ef/0x1490 fs/read_write.c:584
  ksys_write+0x20f/0x4c0 fs/read_write.c:637
  __do_sys_write fs/read_write.c:649 [inline]
  __se_sys_write fs/read_write.c:646 [inline]
  __x64_sys_write+0x93/0xd0 fs/read_write.c:646
  do_syscall_x64 arch/x86/entry/common.c:52 [inline]
  do_syscall_64+0x44/0x110 arch/x86/entry/common.c:83
 entry_SYSCALL_64_after_hwframe+0x63/0x6b

Local variable mp_opt created at:
  subflow_check_req+0x6d/0x15d0 net/mptcp/subflow.c:145
  subflow_v6_route_req+0x269/0x410 net/mptcp/subflow.c:367

CPU: 1 PID: 5924 Comm: syz-executor.3 Not tainted 6.7.0-rc8-syzkaller-00055-g5eff55d7 #0
Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 11/17/2023

Fixes: f296234c ("mptcp: Add handling of incoming MP_JOIN requests")
Reported-by: syzbot <syzkaller@googlegroups.com>
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Florian Westphal <fw@strlen.de>
Cc: Peter Krystad <peter.krystad@linux.intel.com>
Cc: Matthieu Baerts <matttbe@kernel.org>
Cc: Mat Martineau <martineau@kernel.org>
Cc: Geliang Tang <geliang.tang@linux.dev>
Reviewed-by: Simon Horman <horms@kernel.org>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20240111194917.4044654-5-edumazet@google.comSigned-off-by: Jakub Kicinski <kuba@kernel.org>