netdev - Re: [PATCH net-next] tcp: accept RST if SEQ matches right edge of right-most SACK block

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Date:	Wed, 08 Jun 2016 00:37:05 -0700 (PDT)
From:	David Miller <davem@...emloft.net>
To:	pau.espin@...sares.net
Cc:	netdev@...r.kernel.org, ncardwell@...gle.com,
	eric.dumazet@...il.com, ycheng@...gle.com
Subject: Re: [PATCH net-next] tcp: accept RST if SEQ matches right edge of
 right-most SACK block

From: Pau Espin Pedrol <pau.espin@...sares.net>
Date: Tue,  7 Jun 2016 16:30:34 +0200

> RFC 5961 advises to only accept RST packets containing a seq number
> matching the next expected seq number instead of the whole receive
> window in order to avoid spoofing attacks.
> 
> However, this situation is not optimal in the case SACK is in use at the
> time the RST is sent. I recently run into a scenario in which packet
> losses were high while uploading data to a server, and userspace was
> willing to frequently terminate connections by sending a RST. In
> this case, the ACK sent on the receiver side (rcv_nxt) is frozen waiting
> for a lost packet retransmission and SACK blocks are used to let the
> client continue uploading data. At some point later on, the client sends
> the RST (snd_nxt), which matches the next expected seq number of the
> right-most SACK block on the receiver side which is going forward
> receiving data.
> 
> In this scenario, as RFC 5961 defines, the RST SEQ doesn't match the
> frozen main ACK at receiver side and thus gets dropped and a challenge
> ACK is sent, which gets usually lost due to network conditions. The main
> consequence is that the connection stays alive for a while even if it
> made sense to accept the RST. This can get really bad if lots of
> connections like this one are created in few seconds, allocating all the
> resources of the server easily.
> 
> For security reasons, not all SACK blocks are checked (there could be a
> big amount of SACK blocks => acceptable SEQ numbers). Furthermore, it
> wouldn't make sense to check for RST in blocks other than the right-most
> received one because the sender is not expected to be sending new data
> after the RST. For simplicity, only up to the 4 most recently updated
> SACK blocks (selective_acks[4] field) are compared to find the
> right-most block, as usually those are the ones with bigger probability
> to contain it.
> 
> This patch was tested in a 3.18 kernel and probed to improve the
> situation in the scenario described above.
> 
> Signed-off-by: Pau Espin Pedrol <pau.espin@...sares.net>

Applied.