CVE-2023-32269: Use After Free (CWE-416)
value: An issue was discovered in the Linux kernel before 6.1.11. In net/netrom/af_netrom.c, there is a use-after-free because accept is also allowed for a successfully connected AF_NETROM socket. However, in order for an attacker to exploit this, the system must have netrom routing configured or the attacker must have the CAP_NET_ADMIN capability.
CWE description: The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory "belongs" to the code that operates on the new pointer.
CVSS v3 base score: 6.7
Published: 2023-05-05T17:15Z
Repository: https://github.com/torvalds/linux
Language: C
Files changed: net/netrom/af_netrom.c
Commit date: 2023-01-30 07:30:47 +0000
Commit message: netrom: Fix use-after-free caused by accept on already connected socket

If you call listen() and accept() on an already connect()ed
AF_NETROM socket, accept() can successfully connect.
This is because when the peer socket sends data to sendmsg,
the skb with its own sk stored in the connected socket's
sk->sk_receive_queue is connected, and nr_accept() dequeues
the skb waiting in the sk->sk_receive_queue.

As a result, nr_accept() allocates and returns a sock with
the sk of the parent AF_NETROM socket.

And here 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)
       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
```

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 issue, nr_listen() returns -EINVAL for sockets that
successfully nr_connect().

Reported-by: syzbot+caa188bdfc1eeafeb418@syzkaller.appspotmail.com
Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Signed-off-by: Hyunwoo Kim <v4bel@theori.io>
Reviewed-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

Diff: 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
@@ -398,10 +398,15 @@ static int nr_getsockopt(struct socket *sock, int level, int optname,
 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;
 		sk->sk_state           = TCP_LISTEN;
 		release_sock(sk);

Fixed code: 	if (sock->state != SS_UNCONNECTED) {
		release_sock(sk);
		return -EINVAL;
	}