Subject: Re: [PATCH] netrom: Fix use-after-free caused by accept on already connected socket

From: Hyunwoo Kim <[email-masked]>
Date: Sat, 21 Jan 2023 07:12:27 -0800

On Sat, Jan 21, 2023 at 07:08:59AM -0800, Hyunwoo Kim wrote:
> 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 causes nr_accept() to allocate and return a sock with the
> sk of the parent AF_NETROM socket. 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()
>             nr_transmit_buffer()
>             nr_route_frame()
>             nr_loopback_queue()
>             nr_loopback_timer()
>             nr_rx_frame()
>             nr_process_rx_frame()
>             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)
>                                                                     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
> 
> CPU: 0 PID: 5128 Comm: syz-executor564 Not tainted 6.2.0-rc1-syzkaller #0
> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 10/26/2022
> 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().
> 
> Reported-by: [email-masked]
> Signed-off-by: Hyunwoo Kim <[email-masked]>
> ---
>  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..dcfa606684d7 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_CONNECTED) {
> +		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
> 

Dear,

Looks like a patch was recently submitted for another use-after-free bug in netrom:
https://lore.kernel.org/all/[email-masked]/

However, the connect -> accept bug I reported is unaffected by the above patch and still occurs.

Regards,
Hyunwoo Kim