From:   Hyunwoo Kim <redacted>
Date:   Mon, 23 Jan 2023 10:22:09 -0800

On Mon, Jan 23, 2023 at 09:24:57AM -0800, Kuniyuki Iwashima wrote:

quoted

From:   Hyunwoo Kim <redacted>
Date:   Mon, 23 Jan 2023 08:57:06 -0800

quoted

Hi,

Thank you for your review.


On Mon, Jan 23, 2023 at 08:22:00AM -0800, Kuniyuki Iwashima wrote:

quoted

Hi,

Thanks for the patch!

From:   Hyunwoo Kim <redacted>
Date:   Sat, 21 Jan 2023 07:08:59 -0800

quoted

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.

Right, but nr_process_rx_frame() just sends out NR_DISCACK for
accepted_socket, and nr_disconnect() does not queue skb for socket_1 ?

IIUC, uaf_socket's sk is the same one with socket_1, but I'm wondering
how the sk is queued up for socket_1's recv queue at 4.

As I understand it, you know that socket_1 and uaf_socket's sk are the same, 
but you're confused about how the accepted_socket's sk of number 4 is equal 
to socket_1's sk, am I correct?

No.  I'm not sure how close(accepeted_socket) populates socket_1's
sk_receive_queue by a skb->sk with socket_1's sk so that uaf_socket
has the same sk with socket_1.

The sk of socket_1 and the sk of accepted_socket are completely different.
In the flow chart I made, socket_1 and uaf_socket have the same sk,
socket_2 has distinct sk, and accepted_socket has distinct non-overlapping sk.
(There are a total of three sks in the flowchart.)

The reason accepted_socket can refer to the sk of socket_1 in #4 is that 
`->your_index` and `->your_id` of the sk of accepted_socket point to the sk of socket_1.
As explained before, this index/id is used to get the sk of socket_1. 
(In flowchart #4, nr_process_rx_frame() is called using the sk of socket_1 retrieved 
from nr_list, rather than using accepted_socket's own sk)

I understand this, but to make "uaf_socket = accept(socket_1)" happen,
we need to put a skb in socket_1's recv_q in 4. with the found sk.

As you said, 4. finds the sk by nr_find_socket() (as not listener) and ends
up calling nr_process_rx_frame(), which does not queue the skb with the sk
for socket_1's queue, right ?

In short, where is a skb created with socket_1's sk and queued for
socket_1's recv_q ?  We don't call sock_queue_rcv_skb() at 4.

Please let me know if I'm misunderstanding and giving a strange answer.

quoted

quoted

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.

quoted

quoted

                                                               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")

quoted

Reported-by: syzbot+caa188bdfc1eeafeb418@syzkaller.appspotmail.com
Signed-off-by: Hyunwoo Kim <redacted>
---
 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/ (local)
https://lore.kernel.org/netdev/20230122170925.GA98061@ubuntu/ (local)

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

Regards,
Hyunwoo Kim

quoted

Thanks,
Kuniyuki

quoted

+		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