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

luoxuanqiang
2024-06-20T02:38:47Z

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?

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;
}