mptcp: fix UaF in listener shutdown

mptcp: fix UaF in listener shutdown

[ Upstream commit 0a3f4f1f9c27215e4ddcd312558342e57b93e518 ]

  Backports notes: one simple conflict in net/mptcp/protocol.c with:

    commit f8c9dfbd875b ("mptcp: add pm listener events")

  Where one commit removes code in __mptcp_close_ssk() while the other
  one adds one line at the same place. We can simply remove the whole
  condition because this extra instruction is not present in v6.1.

As reported by Christoph after having refactored the passive
socket initialization, the mptcp listener shutdown path is prone
to an UaF issue.

  BUG: KASAN: use-after-free in _raw_spin_lock_bh+0x73/0xe0
  Write of size 4 at addr ffff88810cb23098 by task syz-executor731/1266

  CPU: 1 PID: 1266 Comm: syz-executor731 Not tainted 6.2.0-rc59af4eaa31c1f6c00c8f1e448ed99a45c66340dd5 #6
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.13.0-0-gf21b5a4aeb02-prebuilt.qemu.org 04/01/2014
  Call Trace:
   <TASK>
   dump_stack_lvl+0x6e/0x91
   print_report+0x16a/0x46f
   kasan_report+0xad/0x130
   kasan_check_range+0x14a/0x1a0
   _raw_spin_lock_bh+0x73/0xe0
   subflow_error_report+0x6d/0x110
   sk_error_report+0x3b/0x190
   tcp_disconnect+0x138c/0x1aa0
   inet_child_forget+0x6f/0x2e0
   inet_csk_listen_stop+0x209/0x1060
   __mptcp_close_ssk+0x52d/0x610
   mptcp_destroy_common+0x165/0x640
   mptcp_destroy+0x13/0x80
   __mptcp_destroy_sock+0xe7/0x270
   __mptcp_close+0x70e/0x9b0
   mptcp_close+0x2b/0x150
   inet_release+0xe9/0x1f0
   __sock_release+0xd2/0x280
   sock_close+0x15/0x20
   __fput+0x252/0xa20
   task_work_run+0x169/0x250
   exit_to_user_mode_prepare+0x113/0x120
   syscall_exit_to_user_mode+0x1d/0x40
   do_syscall_64+0x48/0x90
   entry_SYSCALL_64_after_hwframe+0x72/0xdc

The msk grace period can legitly expire in between the last
reference count dropped in mptcp_subflow_queue_clean() and
the later eventual access in inet_csk_listen_stop()

After the previous patch we don't need anymore special-casing
msk listener socket cleanup: the mptcp worker will process each
of the unaccepted msk sockets.

Just drop the now unnecessary code.

Please note this commit depends on the two parent ones:

  mptcp: refactor passive socket initialization
  mptcp: use the workqueue to destroy unaccepted sockets

Fixes: 6aeed9045071 ("mptcp: fix race on unaccepted mptcp sockets")
Cc: stable@vger.kernel.org
Reported-and-tested-by: Christoph Paasch <cpaasch@apple.com>
Closes: https://github.com/multipath-tcp/mptcp_net-next/issues/346
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Reviewed-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Sasha Levin <sashal@kernel.org>