mptcp: fix possible NULL pointer dereference on close

After the blamed commit below, the MPTCP release callback can
dereference the first subflow pointer via __mptcp_set_connected()
and send buffer auto-tuning. Such pointer is always expected to be
valid, except at socket destruction time, when the first subflow is
deleted and the pointer zeroed.

If the connect event is handled by the release callback while the
msk socket is finally released, MPTCP hits the following splat:

  general protection fault, probably for non-canonical address 0xdffffc00000000f2: 0000 [#1] PREEMPT SMP KASAN
  KASAN: null-ptr-deref in range [0x0000000000000790-0x0000000000000797]
  CPU: 1 PID: 26719 Comm: syz-executor.2 Not tainted 6.6.0-syzkaller-10102-gff269e2cd5ad #0
  Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/09/2023
  RIP: 0010:mptcp_subflow_ctx net/mptcp/protocol.h:542 [inline]
  RIP: 0010:__mptcp_propagate_sndbuf net/mptcp/protocol.h:813 [inline]
  RIP: 0010:__mptcp_set_connected+0x57/0x3e0 net/mptcp/subflow.c:424
  RAX: dffffc0000000000 RBX: 0000000000000000 RCX: ffffffff8a62323c
  RDX: 00000000000000f2 RSI: ffffffff8a630116 RDI: 0000000000000790
  RBP: ffff88803334b100 R08: 0000000000000001 R09: 0000000000000000
  R10: 0000000000000001 R11: 0000000000000034 R12: ffff88803334b198
  R13: ffff888054f0b018 R14: 0000000000000000 R15: ffff88803334b100
  FS:  0000000000000000(0000) GS:ffff8880b9900000(0000) knlGS:0000000000000000
  CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
  CR2: 00007fbcb4f75198 CR3: 000000006afb5000 CR4: 00000000003506f0
  DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000
  DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400
  Call Trace:
   <TASK>
   mptcp_release_cb+0xa2c/0xc40 net/mptcp/protocol.c:3405
   release_sock+0xba/0x1f0 net/core/sock.c:3537
   mptcp_close+0x32/0xf0 net/mptcp/protocol.c:3084
   inet_release+0x132/0x270 net/ipv4/af_inet.c:433
   inet6_release+0x4f/0x70 net/ipv6/af_inet6.c:485
   __sock_release+0xae/0x260 net/socket.c:659
   sock_close+0x1c/0x20 net/socket.c:1419
   __fput+0x270/0xbb0 fs/file_table.c:394
   task_work_run+0x14d/0x240 kernel/task_work.c:180
   exit_task_work include/linux/task_work.h:38 [inline]
   do_exit+0xa92/0x2a20 kernel/exit.c:876
   do_group_exit+0xd4/0x2a0 kernel/exit.c:1026
   get_signal+0x23ba/0x2790 kernel/signal.c:2900
   arch_do_signal_or_restart+0x90/0x7f0 arch/x86/kernel/signal.c:309
   exit_to_user_mode_loop kernel/entry/common.c:168 [inline]
   exit_to_user_mode_prepare+0x11f/0x240 kernel/entry/common.c:204
   __syscall_exit_to_user_mode_work kernel/entry/common.c:285 [inline]
   syscall_exit_to_user_mode+0x1d/0x60 kernel/entry/common.c:296
   do_syscall_64+0x4b/0x110 arch/x86/entry/common.c:88
   entry_SYSCALL_64_after_hwframe+0x63/0x6b
  RIP: 0033:0x7fb515e7cae9
  Code: Unable to access opcode bytes at 0x7fb515e7cabf.
  RSP: 002b:00007fb516c560c8 EFLAGS: 00000246 ORIG_RAX: 000000000000002e
  RAX: 000000000000003c RBX: 00007fb515f9c120 RCX: 00007fb515e7cae9
  RDX: 0000000000000000 RSI: 0000000020000140 RDI: 0000000000000006
  RBP: 00007fb515ec847a R08: 0000000000000000 R09: 0000000000000000
  R10: 0000000000000000 R11: 0000000000000246 R12: 0000000000000000
  R13: 000000000000006e R14: 00007fb515f9c120 R15: 00007ffc631eb968
   </TASK>

To avoid sparkling unneeded conditionals, address the issue explicitly
checking msk->first only in the critical place.

Fixes: 8005184f ("mptcp: refactor sndbuf auto-tuning")
Cc: stable@vger.kernel.org
Reported-by:  <syzbot+9dfbaedb6e6baca57a32@syzkaller.appspotmail.com>
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/454


Reported-by: Eric Dumazet <edumazet@google.com>
Closes: https://lore.kernel.org/netdev/CANn89iLZUA6S2a=K8GObnS62KK6Jt4B7PsAs7meMFooM8xaTgw@mail.gmail.com/


Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts <matttbe@kernel.org>
Link: https://lore.kernel.org/r/20231114-upstream-net-20231113-mptcp-misc-fixes-6-7-rc2-v1-2-7b9cd6a7b7f4@kernel.org


Signed-off-by: Jakub Kicinski <kuba@kernel.org>