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Message-ID: <20230126173445.97212-1-kuniyu@amazon.com>
Date: Thu, 26 Jan 2023 09:34:45 -0800
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <v4bel@...ori.io>
CC: <davem@...emloft.net>, <edumazet@...gle.com>, <imv4bel@...il.com>,
<kuba@...nel.org>, <kuniyu@...zon.com>,
<linux-hams@...r.kernel.org>, <netdev@...r.kernel.org>,
<pabeni@...hat.com>, <ralf@...ux-mips.org>,
<syzbot+caa188bdfc1eeafeb418@...kaller.appspotmail.com>
Subject: Re: [PATCH v2] netrom: Fix use-after-free caused by accept on already connected socket
From: Hyunwoo Kim <v4bel@...ori.io>
Date: Wed, 25 Jan 2023 20:16:36 -0800
> On Wed, Jan 25, 2023 at 08:40:35AM -0800, Kuniyuki Iwashima wrote:
> > From: Hyunwoo Kim <v4bel@...ori.io>
> > Date: Wed, 25 Jan 2023 01:17:10 -0800
> > > On Tue, Jan 24, 2023 at 05:43:47PM -0800, Kuniyuki Iwashima wrote:
> > > > Hi,
> > > >
> > > > I think the diff looks good but changelog is not correct.
> > > >
> > > > From: Hyunwoo Kim <v4bel@...ori.io>
> > > > Date: Mon, 23 Jan 2023 09:22:33 -0800
> > > > > If listen() and accept() are called on an AF_NETROM socket that
> > > > > has already been connect()ed, accept() succeeds in connecting.
> > > > > This is because nr_accept() dequeues the skb queued in
> > > > > `sk->sk_receive_queue` in nr_connect().
> > > >
> > > > This sentence is misleading. The skb sent by nr_connect() is queued
> > > > up for the peer's sk_receive_queue (socket_2 below), and also the reply
> > > > is not queued for connect()er (socket_1) because it is NR_CONNACK and
> > > > just consumed by nr_state1_machine() to transition to NR_STATE_3 and
> > > > TCP_ESTABLISHED.
> > > >
> > > >
> > > > >
> > > > > This causes nr_accept() to allocate and return a sock with the
> > > > > sk of the parent AF_NETROM socket.
> > > >
> > > > This happens because the peer (socket_2) sends some data by nr_sendmsg(),
> > > > whose type is NR_INFO and the data is queued up for the connect()er's
> > > > (socket_1) sk_receive_queue by
> > > >
> > > > - nr_state3_machine
> > > > - nr_queue_rx_frame
> > > > - sock_queue_rcv_skb
> > >
> > > you're right.
> > > I misanalyzed nr_rx_frame(). sorry.
> > >
> > > >
> > > >
> > > > > And here's where use-after-free
> > > > > can happen through complex race conditions:
> > > > > ```
> > > > > cpu0 cpu1
> > > > > 1. socket_2 = socket(AF_NETROM)
> > > > > listen(socket_2)
> > > > > accepted_socket = accept(socket_2) // loopback connection with socket_1
> > > > > 2. socket_1 = socket(AF_NETROM)
> > > > > nr_create() // sk refcount : 1
> > > > > connect(socket_1) // loopback connection with socket_2
> > > > > nr_connect()
> > > > > nr_establish_data_link()
> > > > > nr_write_internal()
> > > >
> > > > This is NR_CONNREQ for socket_2.
> > > >
> > > >
> > > > > nr_transmit_buffer()
> > > > > nr_route_frame()
> > > > > nr_loopback_queue()
> > > > > nr_loopback_timer()
> > > > > nr_rx_frame()
> > > > > nr_process_rx_frame()
> > > >
> > > > So, nr_process_rx_frame() is not called from nr_rx_frame() because the
> > > > kernel finds socket_2 by nr_find_listener(), and creates accepted_socket,
> > > > and then responds with NR_CONNACK to socket_1.
> > > >
> > > > And accepted_socket sends some data with NR_INFO, then, nr_state3_machine()
> > > > is called for socket_1 and the skb with sk pointing socket_1 is queued for
> > > > socket_1's sk_receive_queue.
> > > >
> > > > I think this part is lacking in this UAF scenario.
> > > >
> > > > What do you think ?
> > > >
> > > >
> > > > > nr_state3_machine()
> > > > > nr_queue_rx_frame()
> > > > > sock_queue_rcv_skb()
> > > > > sock_queue_rcv_skb_reason()
> > > > > __sock_queue_rcv_skb()
> > > > > __skb_queue_tail(list, skb); // list : sk->sk_receive_queue
> > > > >
> > > > > 3. listen(socket_1)
> > > > > nr_listen()
> > > > > uaf_socket = accept(socket_1)
> > > > > nr_accept()
> > > > > skb_dequeue(&sk->sk_receive_queue);
> > > > > 4. close(accepted_socket)
> > > >
> > > > And I guess this close() is not so important in this case.
> > > > Even if we don't close the socket cleanly, the UAF will happen, right ?
> > > > (it might be caused in a different place like timer though)
> > >
> > > IMHO, I think this `close(accepted_socket)` is a necessary procedure.
> > > This is because `case NR_STATE_3:` in nr_release() is executed here,
> > > and nr_write_internal(sk, NR_DISCREQ) ~ nr_disconnect() sets socket_1's sk to NR_STATE_0.
> > > If sk of socket_1 is not set to NR_STATE_0, `case NR_STATE_3:` is executed instead of
> > > `case NR_STATE_0:` in `close(socket_1)` and sk is not released.
> >
> > No, it's released by the heartbeat timer. The heartbeat timer finally
> > calls nr_destroy_socket() as SOCK_DESTROY is flagged to the sk.
> >
> >
> > > This means you will not be able to trigger UAF on close(uaf_socket).
> >
> > I didn't say close(uaf_socket) is not needed, I mean we can
> > trigger UAF without close(__accepted_socket__).
>
> ok. understand. The reason why I put close(accepted_socket)
> is because Freed by task in the KASAN log I attached to the
> patch description is as follows:
Ah, right.
Sorry, I was confused with 3 close()s :/
> ```
> Freed by task 5128:
> kasan_save_stack+0x22/0x40 mm/kasan/common.c:45
> kasan_set_track+0x25/0x30 mm/kasan/common.c:52
> kasan_save_free_info+0x2b/0x40 mm/kasan/generic.c:518
> ____kasan_slab_free mm/kasan/common.c:236 [inline]
> ____kasan_slab_free+0x13b/0x1a0 mm/kasan/common.c:200
> kasan_slab_free include/linux/kasan.h:177 [inline]
> __cache_free mm/slab.c:3394 [inline]
> __do_kmem_cache_free mm/slab.c:3580 [inline]
> __kmem_cache_free+0xcd/0x3b0 mm/slab.c:3587
> sk_prot_free net/core/sock.c:2074 [inline]
> __sk_destruct+0x5df/0x750 net/core/sock.c:2166
> sk_destruct net/core/sock.c:2181 [inline]
> __sk_free+0x175/0x460 net/core/sock.c:2192
> sk_free+0x7c/0xa0 net/core/sock.c:2203
> sock_put include/net/sock.h:1991 [inline]
> nr_release+0x39e/0x460 net/netrom/af_netrom.c:554
> __sock_release+0xcd/0x280 net/socket.c:650
> sock_close+0x1c/0x20 net/socket.c:1365
> __fput+0x27c/0xa90 fs/file_table.c:320
> task_work_run+0x16f/0x270 kernel/task_work.c:179
> exit_task_work include/linux/task_work.h:38 [inline]
> do_exit+0xaa8/0x2950 kernel/exit.c:867
> do_group_exit+0xd4/0x2a0 kernel/exit.c:1012
> get_signal+0x21c3/0x2450 kernel/signal.c:2859
> arch_do_signal_or_restart+0x79/0x5c0 arch/x86/kernel/signal.c:306
> exit_to_user_mode_loop kernel/entry/common.c:168 [inline]
> exit_to_user_mode_prepare+0x15f/0x250 kernel/entry/common.c:203
> __syscall_exit_to_user_mode_work kernel/entry/common.c:285 [inline]
> syscall_exit_to_user_mode+0x1d/0x50 kernel/entry/common.c:296
> do_syscall_64+0x46/0xb0 arch/x86/entry/common.c:86
> entry_SYSCALL_64_after_hwframe+0x63/0xcd
> ```
>
> If, as you say, heartbeat is running and UAF is triggered,
> the KASAN Freed log will look like this:
> ```
> Freed by task 14094:
> kasan_save_stack+0x22/0x40 mm/kasan/common.c:45
> kasan_set_track+0x25/0x30 mm/kasan/common.c:52
> kasan_save_free_info+0x2e/0x40 mm/kasan/generic.c:518
> ____kasan_slab_free mm/kasan/common.c:236 [inline]
> ____kasan_slab_free+0x160/0x1c0 mm/kasan/common.c:200
> kasan_slab_free include/linux/kasan.h:177 [inline]
> slab_free_hook mm/slub.c:1781 [inline]
> slab_free_freelist_hook+0x8b/0x1c0 mm/slub.c:1807
> slab_free mm/slub.c:3787 [inline]
> __kmem_cache_free+0xaf/0x3b0 mm/slub.c:3800
> sk_prot_free net/core/sock.c:2074 [inline]
> __sk_destruct+0x5df/0x750 net/core/sock.c:2166
> sk_destruct net/core/sock.c:2181 [inline]
> __sk_free+0x175/0x460 net/core/sock.c:2192
> sk_free+0x7c/0xa0 net/core/sock.c:2203
> sock_put include/net/sock.h:1991 [inline]
> nr_heartbeat_expiry+0x1d7/0x460 net/netrom/nr_timer.c:148
> call_timer_fn+0x1da/0x7c0 kernel/time/timer.c:1700
> expire_timers+0x2c6/0x5c0 kernel/time/timer.c:1751
> __run_timers kernel/time/timer.c:2022 [inline]
> __run_timers kernel/time/timer.c:1995 [inline]
> run_timer_softirq+0x326/0x910 kernel/time/timer.c:2035
> __do_softirq+0x1fb/0xadc kernel/softirq.c:571
> ```
>
> Anyway, I'm trying to keep close(accepted_socket) in the flow chart
> because the KASAN log I added to the first patch description is the first one.
>
> >
> >
> > >
> > > What do you think?
> > >
> > > So the modified flow chart looks like this:
> > > ```
> > > cpu0 cpu1
> > > 1. socket_2 = socket(AF_NETROM)
> > > .
> > > .
> > > listen(socket_2)
> > > accepted_socket = accept(socket_2)
> > > 2. socket_1 = socket(AF_NETROM)
> > > nr_create() // sk refcount : 1
> > > connect(socket_1)
> > > nr_connect()
> > > nr_establish_data_link()
> > > nr_write_internal()
> > > nr_transmit_buffer()
> > > nr_route_frame()
> > > nr_loopback_queue()
> > > nr_loopback_timer()
> > > nr_rx_frame()
> > > nr_process_rx_frame()
> > > nr_state1_machine()
> > > nr->state = NR_STATE_3;
> >
> > If you write the connect()'s stack trace, you should note the state
> > change at the socket_1 side by NR_CONNACK. The NR_STATE_3 here is
> > for socket_2 by NR_CONNREQ. We don't need the details of socket_2.
> >
> > Otherwise, you should just write connect(socket_1) only like the
> > step 1 (socket(), listen() etc).
>
> So, the modified flow chart is:
> ```
> cpu0 cpu1
> 1. socket_2 = socket(AF_NETROM)
> .
> .
> listen(socket_2)
> accepted_socket = accept(socket_2)
> 2. socket_1 = socket(AF_NETROM)
> nr_create() // sk refcount : 1
> connect(socket_1)
> 3. write(accepted_socket)
> nr_sendmsg()
> nr_output()
> nr_kick()
> nr_send_iframe()
> nr_transmit_buffer()
> nr_route_frame()
> nr_loopback_queue()
> nr_loopback_timer()
> nr_rx_frame()
> nr_process_rx_frame(sk, skb); // sk : socket_1's sk
> nr_state3_machine()
> nr_queue_rx_frame()
> sock_queue_rcv_skb()
> sock_queue_rcv_skb_reason()
> __sock_queue_rcv_skb()
> __skb_queue_tail(list, skb); // list : socket_1's sk->sk_receive_queue
> 4. listen(socket_1)
> nr_listen()
> uaf_socket = accept(socket_1)
> nr_accept()
> skb_dequeue(&sk->sk_receive_queue);
> 5. close(accepted_socket)
> nr_release()
> nr_write_internal(sk, NR_DISCREQ)
> nr_transmit_buffer() // NR_DISCREQ
> nr_route_frame()
> nr_loopback_queue()
> nr_loopback_timer()
> nr_rx_frame() // sk : socket_1's sk
> nr_process_rx_frame() // NR_STATE_3
> nr_state3_machine() // NR_DISCREQ
> nr_disconnect()
> nr_sk(sk)->state = NR_STATE_0;
> 6. close(socket_1) // sk refcount : 3
> nr_release() // NR_STATE_0
> sock_put(sk); // sk refcount : 0
> sk_free(sk);
> close(uaf_socket)
> nr_release()
> sock_hold(sk); // UAF
> ```
Ok, this looks good.
Please send v4 without my Reviewed-by tag.
Thank you.
>
> And sorry for sending the v3 netrom patch first.
> I received an email that the v2 x25 patch with the wrong patch
> description was applied to the tree,
> so I made a v3 x25 patch in a hurry and sent the netrom as well.
>
> >
> >
> > > 3. write(accepted_socket)
> > > nr_sendmsg()
> > > nr_output()
> > > nr_kick()
> > > nr_send_iframe()
> > > nr_transmit_buffer()
> > > nr_route_frame()
> > > nr_loopback_queue()
> > > nr_loopback_timer()
> > > nr_process_rx_frame(sk, skb); // sk : socket_1's sk
> > > nr_state3_machine()
> > > nr_queue_rx_frame()
> > > sock_queue_rcv_skb()
> > > sock_queue_rcv_skb_reason()
> > > __sock_queue_rcv_skb()
> > > __skb_queue_tail(list, skb); // list : socket_1's sk->sk_receive_queue
> > > 4. listen(socket_1)
> > > nr_listen()
> > > uaf_socket = accept(socket_1)
> > > nr_accept()
> > > skb_dequeue(&sk->sk_receive_queue);
> > > 5. close(accepted_socket)
> > > nr_release()
> > > nr_write_internal(sk, NR_DISCREQ)
> > > nr_transmit_buffer() // NR_DISCREQ
> > > nr_route_frame()
> > > nr_loopback_queue()
> > > nr_loopback_timer()
> > > nr_rx_frame() // sk : socket_1's sk
> > > nr_process_rx_frame() // NR_STATE_3
> > > nr_state3_machine() // NR_DISCREQ
> > > nr_disconnect()
> > > nr_sk(sk)->state = NR_STATE_0;
> > > 6. close(socket_1) // sk refcount : 3
> > > nr_release() // NR_STATE_0
> > > sock_put(sk); // sk refcount : 0
> > > sk_free(sk);
> > > close(uaf_socket)
> > > nr_release()
> > > sock_hold(sk); // UAF
> > > ```
> > >
> > > Anything wrong?
> > >
> > >
> > > >
> > > > > nr_release()
> > > > > nr_write_internal(sk, NR_DISCREQ)
> > > > > nr_transmit_buffer() // NR_DISCREQ
> > > > > nr_route_frame()
> > > > > nr_loopback_queue()
> > > > > nr_loopback_timer()
> > > > > nr_rx_frame() // sk : socket_1's sk
> > > > > nr_process_rx_frame() // NR_STATE_3
> > > > > nr_state3_machine() // NR_DISCREQ
> > > > > nr_disconnect()
> > > > > nr_sk(sk)->state = NR_STATE_0;
> > > > > 5. close(socket_1) // sk refcount : 3
> > > > > nr_release() // NR_STATE_0
> > > > > sock_put(sk); // sk refcount : 0
> > > > > sk_free(sk);
> > > > > close(uaf_socket)
> > > > > nr_release()
> > > > > sock_hold(sk); // UAF
> > > > > ```
> > > > >
> > > > > KASAN report by syzbot:
> > > > > ```
> > > > > BUG: KASAN: use-after-free in nr_release+0x66/0x460 net/netrom/af_netrom.c:520
> > > > > Write of size 4 at addr ffff8880235d8080 by task syz-executor564/5128
> > > > >
> > > > > Call Trace:
> > > > > <TASK>
> > > > > __dump_stack lib/dump_stack.c:88 [inline]
> > > > > dump_stack_lvl+0xd1/0x138 lib/dump_stack.c:106
> > > > > print_address_description mm/kasan/report.c:306 [inline]
> > > > > print_report+0x15e/0x461 mm/kasan/report.c:417
> > > > > kasan_report+0xbf/0x1f0 mm/kasan/report.c:517
> > > > > check_region_inline mm/kasan/generic.c:183 [inline]
> > > > > kasan_check_range+0x141/0x190 mm/kasan/generic.c:189
> > > > > instrument_atomic_read_write include/linux/instrumented.h:102 [inline]
> > > > > atomic_fetch_add_relaxed include/linux/atomic/atomic-instrumented.h:116 [inline]
> > > > > __refcount_add include/linux/refcount.h:193 [inline]
> > > > > __refcount_inc include/linux/refcount.h:250 [inline]
> > > > > refcount_inc include/linux/refcount.h:267 [inline]
> > > > > sock_hold include/net/sock.h:775 [inline]
> > > > > nr_release+0x66/0x460 net/netrom/af_netrom.c:520
> > > > > __sock_release+0xcd/0x280 net/socket.c:650
> > > > > sock_close+0x1c/0x20 net/socket.c:1365
> > > > > __fput+0x27c/0xa90 fs/file_table.c:320
> > > > > task_work_run+0x16f/0x270 kernel/task_work.c:179
> > > > > exit_task_work include/linux/task_work.h:38 [inline]
> > > > > do_exit+0xaa8/0x2950 kernel/exit.c:867
> > > > > do_group_exit+0xd4/0x2a0 kernel/exit.c:1012
> > > > > get_signal+0x21c3/0x2450 kernel/signal.c:2859
> > > > > arch_do_signal_or_restart+0x79/0x5c0 arch/x86/kernel/signal.c:306
> > > > > exit_to_user_mode_loop kernel/entry/common.c:168 [inline]
> > > > > exit_to_user_mode_prepare+0x15f/0x250 kernel/entry/common.c:203
> > > > > __syscall_exit_to_user_mode_work kernel/entry/common.c:285 [inline]
> > > > > syscall_exit_to_user_mode+0x1d/0x50 kernel/entry/common.c:296
> > > > > do_syscall_64+0x46/0xb0 arch/x86/entry/common.c:86
> > > > > entry_SYSCALL_64_after_hwframe+0x63/0xcd
> > > > > RIP: 0033:0x7f6c19e3c9b9
> > > > > Code: Unable to access opcode bytes at 0x7f6c19e3c98f.
> > > > > RSP: 002b:00007fffd4ba2ce8 EFLAGS: 00000246 ORIG_RAX: 0000000000000133
> > > > > RAX: 0000000000000116 RBX: 0000000000000003 RCX: 00007f6c19e3c9b9
> > > > > RDX: 0000000000000318 RSI: 00000000200bd000 RDI: 0000000000000006
> > > > > RBP: 0000000000000003 R08: 000000000000000d R09: 000000000000000d
> > > > > R10: 0000000000000000 R11: 0000000000000246 R12: 000055555566a2c0
> > > > > R13: 0000000000000011 R14: 0000000000000000 R15: 0000000000000000
> > > > > </TASK>
> > > > >
> > > > > Allocated by task 5128:
> > > > > kasan_save_stack+0x22/0x40 mm/kasan/common.c:45
> > > > > kasan_set_track+0x25/0x30 mm/kasan/common.c:52
> > > > > ____kasan_kmalloc mm/kasan/common.c:371 [inline]
> > > > > ____kasan_kmalloc mm/kasan/common.c:330 [inline]
> > > > > __kasan_kmalloc+0xa3/0xb0 mm/kasan/common.c:380
> > > > > kasan_kmalloc include/linux/kasan.h:211 [inline]
> > > > > __do_kmalloc_node mm/slab_common.c:968 [inline]
> > > > > __kmalloc+0x5a/0xd0 mm/slab_common.c:981
> > > > > kmalloc include/linux/slab.h:584 [inline]
> > > > > sk_prot_alloc+0x140/0x290 net/core/sock.c:2038
> > > > > sk_alloc+0x3a/0x7a0 net/core/sock.c:2091
> > > > > nr_create+0xb6/0x5f0 net/netrom/af_netrom.c:433
> > > > > __sock_create+0x359/0x790 net/socket.c:1515
> > > > > sock_create net/socket.c:1566 [inline]
> > > > > __sys_socket_create net/socket.c:1603 [inline]
> > > > > __sys_socket_create net/socket.c:1588 [inline]
> > > > > __sys_socket+0x133/0x250 net/socket.c:1636
> > > > > __do_sys_socket net/socket.c:1649 [inline]
> > > > > __se_sys_socket net/socket.c:1647 [inline]
> > > > > __x64_sys_socket+0x73/0xb0 net/socket.c:1647
> > > > > do_syscall_x64 arch/x86/entry/common.c:50 [inline]
> > > > > do_syscall_64+0x39/0xb0 arch/x86/entry/common.c:80
> > > > > entry_SYSCALL_64_after_hwframe+0x63/0xcd
> > > > >
> > > > > Freed by task 5128:
> > > > > kasan_save_stack+0x22/0x40 mm/kasan/common.c:45
> > > > > kasan_set_track+0x25/0x30 mm/kasan/common.c:52
> > > > > kasan_save_free_info+0x2b/0x40 mm/kasan/generic.c:518
> > > > > ____kasan_slab_free mm/kasan/common.c:236 [inline]
> > > > > ____kasan_slab_free+0x13b/0x1a0 mm/kasan/common.c:200
> > > > > kasan_slab_free include/linux/kasan.h:177 [inline]
> > > > > __cache_free mm/slab.c:3394 [inline]
> > > > > __do_kmem_cache_free mm/slab.c:3580 [inline]
> > > > > __kmem_cache_free+0xcd/0x3b0 mm/slab.c:3587
> > > > > sk_prot_free net/core/sock.c:2074 [inline]
> > > > > __sk_destruct+0x5df/0x750 net/core/sock.c:2166
> > > > > sk_destruct net/core/sock.c:2181 [inline]
> > > > > __sk_free+0x175/0x460 net/core/sock.c:2192
> > > > > sk_free+0x7c/0xa0 net/core/sock.c:2203
> > > > > sock_put include/net/sock.h:1991 [inline]
> > > > > nr_release+0x39e/0x460 net/netrom/af_netrom.c:554
> > > > > __sock_release+0xcd/0x280 net/socket.c:650
> > > > > sock_close+0x1c/0x20 net/socket.c:1365
> > > > > __fput+0x27c/0xa90 fs/file_table.c:320
> > > > > task_work_run+0x16f/0x270 kernel/task_work.c:179
> > > > > exit_task_work include/linux/task_work.h:38 [inline]
> > > > > do_exit+0xaa8/0x2950 kernel/exit.c:867
> > > > > do_group_exit+0xd4/0x2a0 kernel/exit.c:1012
> > > > > get_signal+0x21c3/0x2450 kernel/signal.c:2859
> > > > > arch_do_signal_or_restart+0x79/0x5c0 arch/x86/kernel/signal.c:306
> > > > > exit_to_user_mode_loop kernel/entry/common.c:168 [inline]
> > > > > exit_to_user_mode_prepare+0x15f/0x250 kernel/entry/common.c:203
> > > > > __syscall_exit_to_user_mode_work kernel/entry/common.c:285 [inline]
> > > > > syscall_exit_to_user_mode+0x1d/0x50 kernel/entry/common.c:296
> > > > > do_syscall_64+0x46/0xb0 arch/x86/entry/common.c:86
> > > > > entry_SYSCALL_64_after_hwframe+0x63/0xcd
> > > > > ```
> > > > >
> > > > > To fix this problem, nr_listen() returns -EINVAL for sockets that successfully nr_connect().
> > > > >
> > > > > Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> > > > > Reported-by: syzbot+caa188bdfc1eeafeb418@...kaller.appspotmail.com
> > > > > Signed-off-by: Hyunwoo Kim <v4bel@...ori.io>
> > > > > ---
> > > > > net/netrom/af_netrom.c | 5 +++++
> > > > > 1 file changed, 5 insertions(+)
> > > > >
> > > > > diff --git a/net/netrom/af_netrom.c b/net/netrom/af_netrom.c
> > > > > index 6f7f4392cffb..5a4cb796150f 100644
> > > > > --- a/net/netrom/af_netrom.c
> > > > > +++ b/net/netrom/af_netrom.c
> > > > > @@ -400,6 +400,11 @@ static int nr_listen(struct socket *sock, int backlog)
> > > > > struct sock *sk = sock->sk;
> > > > >
> > > > > lock_sock(sk);
> > > > > + if (sock->state != SS_UNCONNECTED) {
> > > > > + release_sock(sk);
> > > > > + return -EINVAL;
> > > > > + }
> > > > > +
> > > > > if (sk->sk_state != TCP_LISTEN) {
> > > > > memset(&nr_sk(sk)->user_addr, 0, AX25_ADDR_LEN);
> > > > > sk->sk_max_ack_backlog = backlog;
> > > > > --
> > > > > 2.25.1
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