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Date:   Mon, 23 Nov 2020 19:43:40 +0100
From:   Eric Dumazet <>
To:     Ricardo Dias <>,,,,,
Subject: Re: [PATCH v8] tcp: fix race condition when creating child sockets
 from syncookies

On 11/20/20 12:11 PM, Ricardo Dias wrote:
> When the TCP stack is in SYN flood mode, the server child socket is
> created from the SYN cookie received in a TCP packet with the ACK flag
> set.
> The child socket is created when the server receives the first TCP
> packet with a valid SYN cookie from the client. Usually, this packet
> corresponds to the final step of the TCP 3-way handshake, the ACK
> packet. But is also possible to receive a valid SYN cookie from the
> first TCP data packet sent by the client, and thus create a child socket
> from that SYN cookie.
> Since a client socket is ready to send data as soon as it receives the
> SYN+ACK packet from the server, the client can send the ACK packet (sent
> by the TCP stack code), and the first data packet (sent by the userspace
> program) almost at the same time, and thus the server will equally
> receive the two TCP packets with valid SYN cookies almost at the same
> instant.
> When such event happens, the TCP stack code has a race condition that
> occurs between the momement a lookup is done to the established
> connections hashtable to check for the existence of a connection for the
> same client, and the moment that the child socket is added to the
> established connections hashtable. As a consequence, this race condition
> can lead to a situation where we add two child sockets to the
> established connections hashtable and deliver two sockets to the
> userspace program to the same client.
> This patch fixes the race condition by checking if an existing child
> socket exists for the same client when we are adding the second child
> socket to the established connections socket. If an existing child
> socket exists, we drop the packet and discard the second child socket
> to the same client.
> Signed-off-by: Ricardo Dias <>

Ok, lets keep this version, thanks !

Signed-off-by: Eric Dumazet <>

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