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Date:	Fri, 21 Nov 2014 13:55:12 -0500
From:	Neal Cardwell <>
To:	Calvin Owens <>
Cc:	"David S. Miller" <>,
	Alexey Kuznetsov <>,
	James Morris <>,
	Eric Dumazet <>,,
	Netdev <>,
	LKML <>
Subject: Re: [PATCH] tcp: Restore RFC5961-compliant behavior for SYN packets

On Thu, Nov 20, 2014 at 6:09 PM, 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.
> Signed-off-by: Calvin Owens <>

Acked-by: Neal Cardwell <>

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