netdev - Re: [PATCH v2 net-next] tcp: avoid reusing FIN_WAIT2 when trying to find port in connect() process

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20240815200629.45526-1-kuniyu@amazon.com>
Date: Thu, 15 Aug 2024 13:06:29 -0700
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <kerneljasonxing@...il.com>
CC: <davem@...emloft.net>, <dsahern@...nel.org>, <edumazet@...gle.com>,
	<jadedong@...cent.com>, <kernelxing@...cent.com>, <kuba@...nel.org>,
	<kuniyu@...zon.com>, <ncardwell@...gle.com>, <netdev@...r.kernel.org>,
	<pabeni@...hat.com>
Subject: Re: [PATCH v2 net-next] tcp: avoid reusing FIN_WAIT2 when trying to find port in connect() process

From: Jason Xing <kerneljasonxing@...il.com>
Date: Thu, 15 Aug 2024 19:37:45 +0800
> From: Jason Xing <kernelxing@...cent.com>
> 
> We found that one close-wait socket was reset by the other side
> which is beyond our expectation, so we have to investigate the
> underlying reason. The following experiment is conducted in the
> test environment. We limit the port range from 40000 to 40010
> and delay the time to close() after receiving a fin from the
> active close side, which can help us easily reproduce like what
> happened in production.
> 
> Here are three connections captured by tcpdump:
> 127.0.0.1.40002 > 127.0.0.1.9999: Flags [S], seq 2965525191
> 127.0.0.1.9999 > 127.0.0.1.40002: Flags [S.], seq 2769915070
> 127.0.0.1.40002 > 127.0.0.1.9999: Flags [.], ack 1
> 127.0.0.1.40002 > 127.0.0.1.9999: Flags [F.], seq 1, ack 1
> // a few seconds later, within 60 seconds
> 127.0.0.1.40002 > 127.0.0.1.9999: Flags [S], seq 2965590730
> 127.0.0.1.9999 > 127.0.0.1.40002: Flags [.], ack 2
> 127.0.0.1.40002 > 127.0.0.1.9999: Flags [R], seq 2965525193
> // later, very quickly
> 127.0.0.1.40002 > 127.0.0.1.9999: Flags [S], seq 2965590730
> 127.0.0.1.9999 > 127.0.0.1.40002: Flags [S.], seq 3120990805
> 127.0.0.1.40002 > 127.0.0.1.9999: Flags [.], ack 1
> 
> As we can see, the first flow is reset because:
> 1) client starts a new connection, I mean, the second one
> 2) client tries to find a suitable port which is a timewait socket
>    (its state is timewait, substate is fin_wait2)
> 3) client occupies that timewait port to send a SYN
> 4) server finds a corresponding close-wait socket in ehash table,
>    then replies with a challenge ack
> 5) client sends an RST to terminate this old close-wait socket.
> 
> I don't think the port selection algo can choose a FIN_WAIT2 socket
> when we turn on tcp_tw_reuse because on the server side there
> remain unread data. If one side haven't call close() yet, we should
> not consider it as expendable and treat it at will.
> 
> Even though, sometimes, the server isn't able to call close() as soon
> as possible like what we expect, it can not be terminated easily,
> especially due to a second unrelated connection happening.
> 
> After this patch, we can see the expected failure if we start a
> connection when all the ports are occupied in fin_wait2 state:
> "Ncat: Cannot assign requested address."
> 
> Reported-by: Jade Dong <jadedong@...cent.com>
> Signed-off-by: Jason Xing <kernelxing@...cent.com>

Reviewed-by: Kuniyuki Iwashima <kuniyu@...zon.com>

Thanks!