Re: [PATCH net v3] Fix race for duplicate reqsk on identical SYN

luoxuanqiang
2024-06-21T01:36:30Z

在 2024/6/21 03:14, Kuniyuki Iwashima 写道:
> From: luoxuanqiang <luoxuanqiang@kylinos.cn>
> Date: Thu, 20 Jun 2024 10:38:36 +0800
>> 在 2024/6/20 03:53, Kuniyuki Iwashima 写道:
>>> From: luoxuanqiang <luoxuanqiang@kylinos.cn>
>>> Date: Wed, 19 Jun 2024 14:54:15 +0800
>>>> 在 2024/6/18 01:59, Kuniyuki Iwashima 写道:
>>>>> From: luoxuanqiang <luoxuanqiang@kylinos.cn>
>>>>> Date: Mon, 17 Jun 2024 15:56:40 +0800
>>>>>> When bonding is configured in BOND_MODE_BROADCAST mode, if two identical
>>>>>> SYN packets are received at the same time and processed on different CPUs,
>>>>>> it can potentially create the same sk (sock) but two different reqsk
>>>>>> (request_sock) in tcp_conn_request().
>>>>>>
>>>>>> These two different reqsk will respond with two SYNACK packets, and since
>>>>>> the generation of the seq (ISN) incorporates a timestamp, the final two
>>>>>> SYNACK packets will have different seq values.
>>>>>>
>>>>>> The consequence is that when the Client receives and replies with an ACK
>>>>>> to the earlier SYNACK packet, we will reset(RST) it.
>>>>>>
>>>>>> ========================================================================
>>>>>>
>>>>>> This behavior is consistently reproducible in my local setup,
>>>>>> which comprises:
>>>>>>
>>>>>> | NETA1 ------ NETB1 |
>>>>>> PC_A --- bond --- | | --- bond --- PC_B
>>>>>> | NETA2 ------ NETB2 |
>>>>>>
>>>>>> - PC_A is the Server and has two network cards, NETA1 and NETA2. I have
>>>>>> bonded these two cards using BOND_MODE_BROADCAST mode and configured
>>>>>> them to be handled by different CPU.
>>>>>>
>>>>>> - PC_B is the Client, also equipped with two network cards, NETB1 and
>>>>>> NETB2, which are also bonded and configured in BOND_MODE_BROADCAST mode.
>>>>>>
>>>>>> If the client attempts a TCP connection to the server, it might encounter
>>>>>> a failure. Capturing packets from the server side reveals:
>>>>>>
>>>>>> 10.10.10.10.45182 > localhost: Flags [S], seq 320236027,
>>>>>> 10.10.10.10.45182 > localhost: Flags [S], seq 320236027,
>>>>>> localhost > 10.10.10.10.45182: Flags [S.], seq 2967855116,
>>>>>> localhost > 10.10.10.10.45182: Flags [S.], seq 2967855123, <==
>>>>>> 10.10.10.10.45182 > localhost: Flags [.], ack 4294967290,
>>>>>> 10.10.10.10.45182 > localhost: Flags [.], ack 4294967290,
>>>>>> localhost > 10.10.10.10.45182: Flags [R], seq 2967855117, <==
>>>>>> localhost > 10.10.10.10.45182: Flags [R], seq 2967855117,
>>>>>>
>>>>>> Two SYNACKs with different seq numbers are sent by localhost,
>>>>>> resulting in an anomaly.
>>>>>>
>>>>>> ========================================================================
>>>>>>
>>>>>> The attempted solution is as follows:
>>>>>> In the tcp_conn_request(), while inserting reqsk into the ehash table,
>>>>>> it also checks if an entry already exists. If found, it avoids
>>>>>> reinsertion and releases it.
>>>>>>
>>>>>> Simultaneously, In the reqsk_queue_hash_req(), the start of the
>>>>>> req->rsk_timer is adjusted to be after successful insertion.
>>>>>>
>>>>>> Signed-off-by: luoxuanqiang <luoxuanqiang@kylinos.cn>
>>>>>> ---
>>>>>> include/net/inet_connection_sock.h | 4 ++--
>>>>>> net/dccp/ipv4.c | 2 +-
>>>>>> net/dccp/ipv6.c | 2 +-
>>>>>> net/ipv4/inet_connection_sock.c | 19 +++++++++++++------
>>>>>> net/ipv4/tcp_input.c | 9 ++++++++-
>>>>>> 5 files changed, 25 insertions(+), 11 deletions(-)
>>>>>>
>>>>>> diff --git a/include/net/inet_connection_sock.h b/include/net/inet_connection_sock.h
>>>>>> index 7d6b1254c92d..8ebab6220dbc 100644
>>>>>> --- a/include/net/inet_connection_sock.h
>>>>>> +++ b/include/net/inet_connection_sock.h
>>>>>> @@ -263,8 +263,8 @@ struct dst_entry *inet_csk_route_child_sock(const struct sock *sk,
>>>>>> struct sock *inet_csk_reqsk_queue_add(struct sock *sk,
>>>>>> struct request_sock *req,
>>>>>> struct sock *child);
>>>>>> -void inet_csk_reqsk_queue_hash_add(struct sock *sk, struct request_sock *req,
>>>>>> - unsigned long timeout);
>>>>>> +bool inet_csk_reqsk_queue_hash_add(struct sock *sk, struct request_sock *req,
>>>>>> + unsigned long timeout, bool *found_dup_sk);
>>>>>> struct sock *inet_csk_complete_hashdance(struct sock *sk, struct sock *child,
>>>>>> struct request_sock *req,
>>>>>> bool own_req);
>>>>>> diff --git a/net/dccp/ipv4.c b/net/dccp/ipv4.c
>>>>>> index ff41bd6f99c3..13aafdeb9205 100644
>>>>>> --- a/net/dccp/ipv4.c
>>>>>> +++ b/net/dccp/ipv4.c
>>>>>> @@ -657,7 +657,7 @@ int dccp_v4_conn_request(struct sock *sk, struct sk_buff *skb)
>>>>>> if (dccp_v4_send_response(sk, req))
>>>>>> goto drop_and_free;
>>>>>>
>>>>>> - inet_csk_reqsk_queue_hash_add(sk, req, DCCP_TIMEOUT_INIT);
>>>>>> + inet_csk_reqsk_queue_hash_add(sk, req, DCCP_TIMEOUT_INIT, NULL);
>>>>>> reqsk_put(req);
>>>>>> return 0;
>>>>>>
>>>>>> diff --git a/net/dccp/ipv6.c b/net/dccp/ipv6.c
>>>>>> index 85f4b8fdbe5e..493cdb12ce2b 100644
>>>>>> --- a/net/dccp/ipv6.c
>>>>>> +++ b/net/dccp/ipv6.c
>>>>>> @@ -400,7 +400,7 @@ static int dccp_v6_conn_request(struct sock *sk, struct sk_buff *skb)
>>>>>> if (dccp_v6_send_response(sk, req))
>>>>>> goto drop_and_free;
>>>>>>
>>>>>> - inet_csk_reqsk_queue_hash_add(sk, req, DCCP_TIMEOUT_INIT);
>>>>>> + inet_csk_reqsk_queue_hash_add(sk, req, DCCP_TIMEOUT_INIT, NULL);
>>>>>> reqsk_put(req);
>>>>>> return 0;
>>>>>>
>>>>>> diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
>>>>>> index d81f74ce0f02..2fa9b33ae26a 100644
>>>>>> --- a/net/ipv4/inet_connection_sock.c
>>>>>> +++ b/net/ipv4/inet_connection_sock.c
>>>>>> @@ -1122,25 +1122,32 @@ static void reqsk_timer_handler(struct timer_list *t)
>>>>>> inet_csk_reqsk_queue_drop_and_put(oreq->rsk_listener, oreq);
>>>>>> }
>>>>>>
>>>>>> -static void reqsk_queue_hash_req(struct request_sock *req,
>>>>>> - unsigned long timeout)
>>>>>> +static bool reqsk_queue_hash_req(struct request_sock *req,
>>>>>> + unsigned long timeout, bool *found_dup_sk)
>>>>>> {
>>>>> Given any changes here in reqsk_queue_hash_req() conflicts with 4.19
>>>>> (oldest stable) and DCCP does not check found_dup_sk, you can define
>>>>> found_dup_sk here, then you need not touch DCCP at all.
>>>> Apologies for not fully understanding your advice. If we cannot modify
>>>> the content of reqsk_queue_hash_req() and should avoid touching the DCCP
>>>> part, it seems the issue requires reworking some interfaces. Specifically:
>>>>
>>>> The call flow to add reqsk to ehash is as follows:
>>>>
>>>> tcp_conn_request()
>>>>
>>>> dccp_v4(6)_conn_request()
>>>>
>>>> -> inet_csk_reqsk_queue_hash_add()
>>>>
>>>> -> reqsk_queue_hash_req()
>>>>
>>>> -> inet_ehash_insert()
>>>>
>>>> tcp_conn_request() needs to call the same interface inet_csk_reqsk_queue_hash_add()
>>>> as dccp_v4(6)_conn_request(), but the critical section for installation check and
>>>> insertion into ehash is within inet_ehash_insert().
>>>> If reqsk_queue_hash_req() should not be modified, then we need to rewrite
>>>> the interfaces to distinguish them. I don't see how redefining found_dup_sk
>>>> alone can resolve this conflict point.
>>> I just said we cannot avoid conflict so suggested avoiding found_dup_sk
>>> in inet_csk_reqsk_queue_hash_add().
>>>
>>> But I finally ended up modifying DCCP because we return before setting
>>> refcnt.
>>>
>>> ---8<---
>>> diff --git a/net/dccp/ipv4.c b/net/dccp/ipv4.c
>>> index ff41bd6f99c3..b2a8aed35eb0 100644
>>> --- a/net/dccp/ipv4.c
>>> +++ b/net/dccp/ipv4.c
>>> @@ -657,8 +657,11 @@ int dccp_v4_conn_request(struct sock *sk, struct sk_buff *skb)
>>> if (dccp_v4_send_response(sk, req))
>>> goto drop_and_free;
>>>
>>> - inet_csk_reqsk_queue_hash_add(sk, req, DCCP_TIMEOUT_INIT);
>>> - reqsk_put(req);
>>> + if (unlikely(inet_csk_reqsk_queue_hash_add(sk, req, DCCP_TIMEOUT_INIT)))
>>> + reqsk_free(req);
>>> + else
>>> + reqsk_put(req);
>>> +
>>> return 0;
>>>
>>> drop_and_free:
>>> diff --git a/net/ipv4/inet_connection_sock.c b/net/ipv4/inet_connection_sock.c
>>> index d81f74ce0f02..7dd6892b10b9 100644
>>> --- a/net/ipv4/inet_connection_sock.c
>>> +++ b/net/ipv4/inet_connection_sock.c
>>> @@ -1122,25 +1122,33 @@ static void reqsk_timer_handler(struct timer_list *t)
>>> inet_csk_reqsk_queue_drop_and_put(oreq->rsk_listener, oreq);
>>> }
>>>
>>> -static void reqsk_queue_hash_req(struct request_sock *req,
>>> +static bool reqsk_queue_hash_req(struct request_sock *req,
>>> unsigned long timeout)
>>> {
>>> + bool found_dup_sk;
>>> +
>>> + if (!inet_ehash_insert(req_to_sk(req), NULL, &found_dup_sk))
>>> + return false;
>>> +
>>> timer_setup(&req->rsk_timer, reqsk_timer_handler, TIMER_PINNED);
>>> mod_timer(&req->rsk_timer, jiffies + timeout);
>>>
>>> - inet_ehash_insert(req_to_sk(req), NULL, NULL);
>>> /* before letting lookups find us, make sure all req fields
>>> * are committed to memory and refcnt initialized.
>>> */
>>> smp_wmb();
>>> refcount_set(&req->rsk_refcnt, 2 + 1);
>>> + return true;
>>> }
>>>
>>> -void inet_csk_reqsk_queue_hash_add(struct sock *sk, struct request_sock *req,
>>> +bool inet_csk_reqsk_queue_hash_add(struct sock *sk, struct request_sock *req,
>>> unsigned long timeout)
>>> {
>>> - reqsk_queue_hash_req(req, timeout);
>>> + if (!reqsk_queue_hash_req(req, timeout))
>>> + return false;
>>> +
>>> inet_csk_reqsk_queue_added(sk);
>>> + return true;
>>> }
>>> EXPORT_SYMBOL_GPL(inet_csk_reqsk_queue_hash_add);
>>>
>>> ---8<---
>> Thank you for your patient explanation. I understand
>> your point and will send out the V4 version, looking
>> forward to your review.
>>
>> Also, I'd like to confirm a detail with you. For the DCCP part, is it
>> sufficient to simply call reqsk_free() for the return value, or should
>> we use goto drop_and_free? The different return values here will
>> determine whether a reset is sent, and I lack a comprehensive
>> understanding of DCCP. so could you please help me confirm this
>> from a higher-level perspective?
> I think we need not send RST because in this case there's another
> establishing request already in ehash and we should not abort it
> as we don't do so in tcp_conn_request().
>
> Thanks!

Thank you for confirming. Best wishes! :)

>
>> diff --git a/net/dccp/ipv4.c b/net/dccp/ipv4.c
>> index ff41bd6f99c3..5926159a6f20 100644
>> --- a/net/dccp/ipv4.c
>> +++ b/net/dccp/ipv4.c
>> @@ -657,8 +657,11 @@ int dccp_v4_conn_request(struct sock *sk, struct sk_buff *skb)
>> if (dccp_v4_send_response(sk, req))
>> goto drop_and_free;
>>
>> - inet_csk_reqsk_queue_hash_add(sk, req, DCCP_TIMEOUT_INIT);
>> - reqsk_put(req);
>> + if (unlikely(!inet_csk_reqsk_queue_hash_add(sk, req, DCCP_TIMEOUT_INIT)))
>> + reqsk_free(req); // or goto drop_and_free: <============
>> + else
>> + reqsk_put(req);
>> +
>> return 0;
>>
>> drop_and_free:
>> reqsk_free(req);
>> drop:
>> __DCCP_INC_STATS(DCCP_MIB_ATTEMPTFAILS);
>> return -1;
>> }