netdev - Re: [PATCH net v2] unix: Fix race in SOCK_SEQPACKET's unix_dgram

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Date:   Sat, 3 Dec 2022 01:43:53 +0300
From:   Kirill Tkhai <tkhai@...ru>
To:     Paolo Abeni <pabeni@...hat.com>, davem@...emloft.net,
        edumazet@...gle.com, kuba@...nel.org, netdev@...r.kernel.org,
        Kuniyuki Iwashima <kuniyu@...zon.com>
Subject: Re: [PATCH net v2] unix: Fix race in SOCK_SEQPACKET's
 unix_dgram_sendmsg()

On 01.12.2022 12:30, Paolo Abeni wrote:
> On Sun, 2022-11-27 at 01:46 +0300, Kirill Tkhai wrote:
>> There is a race resulting in alive SOCK_SEQPACKET socket
>> may change its state from TCP_ESTABLISHED to TCP_CLOSE:
>>
>> unix_release_sock(peer)                  unix_dgram_sendmsg(sk)
>>   sock_orphan(peer)
>>     sock_set_flag(peer, SOCK_DEAD)
>>                                            sock_alloc_send_pskb()
>>                                              if !(sk->sk_shutdown & SEND_SHUTDOWN)
>>                                                OK
>>                                            if sock_flag(peer, SOCK_DEAD)
>>                                              sk->sk_state = TCP_CLOSE
>>   sk->sk_shutdown = SHUTDOWN_MASK
>>
>>
>> After that socket sk remains almost normal: it is able to connect, listen, accept
>> and recvmsg, while it can't sendmsg.
>>
>> Since this is the only possibility for alive SOCK_SEQPACKET to change
>> the state in such way, we should better fix this strange and potentially
>> danger corner case.
>>
>> Also, move TCP_CLOSE assignment for SOCK_DGRAM sockets under state lock
>> to fix race with unix_dgram_connect():
>>
>> unix_dgram_connect(other)            unix_dgram_sendmsg(sk)
>>                                        unix_peer(sk) = NULL
>>                                        unix_state_unlock(sk)
>>   unix_state_double_lock(sk, other)
>>   sk->sk_state  = TCP_ESTABLISHED
>>   unix_peer(sk) = other
>>   unix_state_double_unlock(sk, other)
>>                                        sk->sk_state  = TCP_CLOSED
>>
>> This patch fixes both of these races.
>>
>> Fixes: 83301b5367a9 ("af_unix: Set TCP_ESTABLISHED for datagram sockets too")
> 
> I don't think this commmit introduces the issues, both behavior
> described above appear to be present even before?

1)Hm, I pointed to the commit suggested by Kuniyuki without checking it.

Possible, the real problem commit is dc56ad7028c5 "af_unix: fix potential NULL deref in unix_dgram_connect()",
since it added TCP_CLOSED assignment to unix_dgram_sendmsg().

2)What do you think about initial version of fix?

https://patchwork.kernel.org/project/netdevbpf/patch/38a920a7-cfba-7929-886d-c3c6effc0c43@ya.ru/

Despite there are some arguments, I'm not still sure that v2 is better.

Thanks,
Kirill