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

From: Hyunwoo Kim <[email-masked]>
Date: Mon, 23 Jan 2023 08:57:06 -0800

Hi,

Thank you for your review.

On Mon, Jan 23, 2023 at 08:22:00AM -0800, Kuniyuki Iwashima wrote:
> Hi,
> 
> Thanks for the patch!
> 
> From:   Hyunwoo Kim <[email-masked]>
> Date:   Sat, 21 Jan 2023 07:08:59 -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 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);
> 
> Sorry, I didn't understand how this is populated by close(accepted_socket),
> especially how skb->sk is set as socket_1's sk.

When calling close(accepted_socket), accepted_socket is currently in NR_STATE_3 state, 
so nr_release() calls `nr_write_internal(sk, NR_DISCREQ)`.
In a later flow, nr_rx_frame() is called, where nr_find_socket() is used to get socket_1's sk 
from the global list `nr_list` (Because `circuit_index` and `circuit_id` used for search were index/id of socket_1's sk):
```
        sk = NULL;

        if (circuit_index == 0 && circuit_id == 0) {
                if (frametype == NR_CONNACK && flags == NR_CHOKE_FLAG)
                        sk = nr_find_peer(peer_circuit_index, peer_circuit_id, src);
        } else {
                if (frametype == NR_CONNREQ)
                        sk = nr_find_peer(circuit_index, circuit_id, src);
                else
                        sk = nr_find_socket(circuit_index, circuit_id);    // here
        }
```

And nr_process_rx_frame(), nr_state3_machine(), nr_disconnect() are executed sequentially, 
and `nr_sk(sk)->state = NR_STATE_0;` is executed, so the state of socket_1 sk becomes NR_STATE_0.

As a result, `5. In close(socket_1)`, the code of `case NR_STATE_0:` of nr_release() is executed 
to free the sk, and finally, by calling `close(uaf_socket)`, UAF occurs by referring to the freed sk.

> 
> 
> >                                                                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().
> >
> 
> I'd add
> 
>   Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> 
> > 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) {
> 
> I guess the same issue happens for SS_CONNECTING (non-blocking connect()),
> so this should be
> 
>         if (sock->state != SS_UNCONNECTED) {
> 
> ?
> 
> Same for the rose and x25 patches.
> https://lore.kernel.org/netdev/20230122173957.GA99728@ubuntu/
> https://lore.kernel.org/netdev/20230122170925.GA98061@ubuntu/

you're right.
I will submit the fixed v2 patches.

Regards,
Hyunwoo Kim

> 
> 
> Thanks,
> Kuniyuki
> 
> > +		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