tcp: fix NULL pointer dereference when restarting a Fast Open listener

When a valid RST is received for a Fast Open child accepted before the
three-way handshake completes, reqsk_fastopen_remove() adds the
request_sock to the listener's RST list if the listener is in TCP_LISTEN.

An application can call shutdown(SHUT_RD) on a listener socket.
inet_csk_listen_stop() checks rskq_rst_head without holding
fastopenq.lock. If rskq_rst_head is NULL, it does not acquire the lock
and therefore does not synchronize with an insertion by
reqsk_fastopen_remove() after that function has passed the TCP_LISTEN
check.

The more likely outcome of this race, even without a subsequent
listen(), is a leak. If reqsk_fastopen_remove() inserts a request_sock
after inet_csk_listen_stop() observes an empty list and returns, the
request_sock is stranded in rskq_rst_head and keeps a reference on the
listener. The socket is already in TCP_CLOSE, so a subsequent close()
does not call inet_csk_listen_stop() again.

If listen() is called again on the same socket before the delayed list
insertion, the insertion can overlap the queue initialization in
reqsk_queue_alloc(). If it occurs after rskq_rst_head is set to NULL but
before rskq_rst_tail is set to NULL, rskq_rst_head points to the
request_sock while rskq_rst_tail is NULL.

When a valid RST is later received for another accepted Fast Open child,
rskq_rst_head is not NULL, so reqsk_fastopen_remove() attempts to write
the request_sock pointer to rskq_rst_tail->dl_next while rskq_rst_tail is
NULL. KASAN reports:

  BUG: KASAN: null-ptr-deref in reqsk_fastopen_remove+0x147/0x210
  Write of size 8 at addr 0000000000000088 by task swapper/3/0
  ...
  Call Trace:
   <IRQ>
   ...
   reqsk_fastopen_remove+0x147/0x210
   tcp_check_req+0xa70/0x1060
   ...
   tcp_rcv_state_process+0x1cb/0x2ae0
   ...
   tcp_v4_do_rcv+0x1fe/0x740
   tcp_v4_rcv+0x1744/0x2070
   ...
   ip_protocol_deliver_rcu+0x5c/0x280
   ip_local_deliver_finish+0x171/0x330
   ip_local_deliver+0x107/0x370
   ...
   </IRQ>

To widen the race window, the issue was reproduced after adding
mdelay(3000) immediately after reqsk_fastopen_remove() checks that the
listener is in TCP_LISTEN and immediately after reqsk_queue_alloc() sets
rskq_rst_head to NULL.

Make inet_csk_listen_stop() acquire fastopenq.lock regardless of the
value of rskq_rst_head and detach the list while holding the lock. With
this synchronization, reqsk_fastopen_remove() either inserts the
request_sock before inet_csk_listen_stop() acquires the lock, in which
case inet_csk_listen_stop() reclaims it, or sees that the listener is
not in TCP_LISTEN and does not insert anything.

Fixes: 0536fcc039a8 ("tcp: prepare fastopen code for upcoming listener changes")
Cc: stable@vger.kernel.org
Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>
Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>
Reviewed-by: Eric Dumazet <edumazet@kernel.org>
Link: https://patch.msgid.link/avMkfJrXhCFXn8M_@v4bel
Signed-off-by: Jakub Kicinski <kuba@kernel.org>