Re: [PATCH] tcp: Restore RFC5961-compliant behavior for SYN packets

From: Eric Dumazet
Date: Thu Nov 20 2014 - 18:42:28 EST

On Thu, 2014-11-20 at 15:09 -0800, Calvin Owens wrote:
> Commit c3ae62af8e755 ("tcp: should drop incoming frames without ACK
> flag set") was created to mitigate a security vulnerability in which a
> local attacker is able to inject data into locally-opened sockets by
> using TCP protocol statistics in procfs to quickly find the correct
> sequence number.
> This broke the RFC5961 requirement to send a challenge ACK in response
> to spurious RST packets, which was subsequently fixed by commit
> 7b514a886ba50 ("tcp: accept RST without ACK flag").
> Unfortunately, the RFC5961 requirement that spurious SYN packets be
> handled in a similar manner remains broken.
> RFC5961 section 4 states that:
> ... the handling of the SYN in the synchronized state SHOULD be
> performed as follows:
> 1) If the SYN bit is set, irrespective of the sequence number, TCP
> MUST send an ACK (also referred to as challenge ACK) to the remote
> peer:
> After sending the acknowledgment, TCP MUST drop the unacceptable
> segment and stop processing further.
> By sending an ACK, the remote peer is challenged to confirm the loss
> of the previous connection and the request to start a new connection.
> A legitimate peer, after restart, would not have a TCB in the
> synchronized state. Thus, when the ACK arrives, the peer should send
> a RST segment back with the sequence number derived from the ACK
> field that caused the RST.
> This RST will confirm that the remote peer has indeed closed the
> previous connection. Upon receipt of a valid RST, the local TCP
> endpoint MUST terminate its connection. The local TCP endpoint
> should then rely on SYN retransmission from the remote end to
> re-establish the connection.
> This patch lets SYN packets through the discard added in c3ae62af8e755,
> so that spurious SYN packets are properly dealt with as per the RFC.
> The challenge ACK is sent unconditionally and is rate-limited, so the
> original vulnerability is not reintroduced by this patch.

I think this patch makes sense. But I wonder if the rate limiting wont
hurt anyway, as I presume you need that after some server being
rebooted, and if many connections are attempted in a small amount of
time, some of them wont get any answer ?

Thanks !

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