Re: [PATCH net v2] tcp: fix NULL pointer dereference when restarting a Fast Open listener

[Kuniyuki Iwashima <kuniyu@google.com>]

On Tue, 10 Nov 2026 23:04:44 +0900, Hyunwoo Kim wrote:
> 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
> Assisted-by: LLM
> Signed-off-by: Hyunwoo Kim <imv4bel@gmail.com>

Reviewed-by: Kuniyuki Iwashima <kuniyu@google.com>