Messages in this thread | | | Date | Thu, 20 Nov 2014 17:47:27 -0800 | From | Calvin Owens <> | Subject | Re: [PATCH] tcp: Restore RFC5961-compliant behavior for SYN packets |
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On Thursday 11/20 at 15:42 -0800, Eric Dumazet wrote: > 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: > > > > <SEQ=SND.NXT><ACK=RCV.NXT><CTL=ACK> > > > > 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 ?
That's actually not what led to finding this, but it's a good point. :)
What if the challenge-ACK counter were decremented in tcp_validate_incoming() when a valid RST packet is seen? That would allow legitimate remote hosts to reestablish connections without being ratelimited, and still prevent a malicious host from guessing sequence numbers.
There would need to be a way to tell if a challenge ACK had in fact been sent and only decrement in that case, since otherwise a local attacker could establish and immediately reset lots of connections to keep the counter below the ratelimit threshold and guess sequence numbers.
Simply adding a flag to struct tcp_sock would work: just set the flag whenever a challenge ACK is sent, and clear it and decrement the counter only if it is set when a valid RST packet is seen.
I suppose that should be a seperate patch?
Thanks, Calvin
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