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

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Date:   Sun, 20 Nov 2022 14:43:06 +0300
From:   Kirill Tkhai <tkhai@...ru>
To:     Kuniyuki Iwashima <kuniyu@...zon.com>
Cc:     davem@...emloft.net, edumazet@...gle.com, kuba@...nel.org,
        netdev@...r.kernel.org, pabeni@...hat.com
Subject: Re: [PATCH net] unix: Fix race in SOCK_SEQPACKET's
 unix_dgram_sendmsg()

On 20.11.2022 12:46, Kirill Tkhai wrote:
> On 20.11.2022 12:09, Kuniyuki Iwashima wrote:
>> From:   Kirill Tkhai <tkhai@...ru>
>> Date:   Sun, 20 Nov 2022 02:16:47 +0300
>>> 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.
>>
>> nit: Then, also recvmsg() fails with -ENOTCONN.  And after connect(), even
>> both of recvmsg() and sendmsg() does not fail.
> 
> Possible, I wrote not clear. I mean sendmsg fails after connect, while recvmsg does not fail :)
> Here is sendmsg abort path:
> 
> unix_dgram_sendmsg()
>   sock_alloc_send_pskb()
>     if (sk->sk_shutdown & SEND_SHUTDOWN)
>       FAIL
> 
>> static int unix_seqpacket_recvmsg(struct socket *sock, struct msghdr *msg,
>> 				  size_t size, int flags)
>> {
>> 	struct sock *sk = sock->sk;
>>
>> 	if (sk->sk_state != TCP_ESTABLISHED)
>> 		return -ENOTCONN;
>>
>> 	return unix_dgram_recvmsg(sock, msg, size, flags);
>> }
>>
>>
>>>
>>> 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.
>>>
>>> Signed-off-by: Kirill Tkhai <tkhai@...ru>
>>
>> Fixes tag is needed:
>>
>> Fixes: 83301b5367a9 ("af_unix: Set TCP_ESTABLISHED for datagram sockets too")
>>> Before this commit, there was no state change and SEQPACKET sk also went
>> through the same path.  The bug was introduced because the commit did not
>> consider SEAPACKET.
>>
>> So, I think the fix should be like below, then we can free the peer faster.
>> Note unix_dgram_peer_wake_disconnect_wakeup() is dgram-specific too.
>>
>> ---8<---
>> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
>> index b3545fc68097..be40023a61fb 100644
>> --- a/net/unix/af_unix.c
>> +++ b/net/unix/af_unix.c
>> @@ -2001,11 +2001,14 @@ static int unix_dgram_sendmsg(struct socket *sock, struct msghdr *msg,
>>  		err = 0;
>>  		if (unix_peer(sk) == other) {
>>  			unix_peer(sk) = NULL;
>> -			unix_dgram_peer_wake_disconnect_wakeup(sk, other);
>> +
>> +			if (sk->sk_type == SOCK_DGRAM) {
>> +				unix_dgram_peer_wake_disconnect_wakeup(sk, other);
>> +				sk->sk_state = TCP_CLOSE;
>> +			}
>>  
>>  			unix_state_unlock(sk);
>>  
>> -			sk->sk_state = TCP_CLOSE;
>>  			unix_dgram_disconnected(sk, other);
>>  			sock_put(other);
>>  			err = -ECONNREFUSED;
>> ---8<---
>>
>> Also, it's better to mention that moving TCP_CLOSE under the lock resolves
>> another rare race with unix_dgram_connect() for DGRAM sk:
>>
>>   unix_state_unlock(sk);
>>   <--------------------------> connect() could set TCP_ESTABLISHED here.
>>   sk->sk_state = TCP_CLOSE;
> 
> Sounds good, I'll test this variant. Thanks!

Thinking again, I'd argue about disconnecting from peer (unix_peer(sk) = NULL) here.
Normally, unix_dgram_sendmsg() never came here like I wrote:

 unix_dgram_sendmsg()
   sock_alloc_send_pskb()
     if (sk->sk_shutdown & SEND_SHUTDOWN)
       FAIL with EPIPE

So, this optimization will work very rare, it fact there will not real performance profit.
But this creates a cornet case (safe but corner), which seems not good. Corner cases are not
good in general.

I'd leave an original variant. What do you think about this?

Kirill

>>> ---
>>>  net/unix/af_unix.c |   11 +++++++++--
>>>  1 file changed, 9 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
>>> index b3545fc68097..6fd745cfc492 100644
>>> --- a/net/unix/af_unix.c
>>> +++ b/net/unix/af_unix.c
>>> @@ -1999,13 +1999,20 @@ static int unix_dgram_sendmsg(struct socket *sock, struct msghdr *msg,
>>>  			unix_state_lock(sk);
>>>  
>>>  		err = 0;
>>> -		if (unix_peer(sk) == other) {
>>> +		if (sk->sk_type == SOCK_SEQPACKET) {
>>> +			/* We are here only when racing with unix_release_sock()
>>> +			 * is clearing @other. Never change state to TCP_CLOSE
>>> +			 * unlike SOCK_DGRAM wants.
>>> +			 */
>>> +			unix_state_unlock(sk);
>>> +			err = -EPIPE;
>>> +		} else if (unix_peer(sk) == other) {
>>>  			unix_peer(sk) = NULL;
>>>  			unix_dgram_peer_wake_disconnect_wakeup(sk, other);
>>>  
>>> +			sk->sk_state = TCP_CLOSE;
>>>  			unix_state_unlock(sk);
>>>  
>>> -			sk->sk_state = TCP_CLOSE;
>>>  			unix_dgram_disconnected(sk, other);
>>>  			sock_put(other);
>>>  			err = -ECONNREFUSED;
>