[Swan-dev] problem from IRC: confusing message and action of lost final packet

[Andrew Cagney]

I pushed a change that should stop IKEv1 responding when the decrypted
packet has problems (but hasn't been verified).

It seems to fix the test case.

On Fri, 28 Sep 2018 at 12:45, Andrew Cagney <andrew.cagney at gmail.com> wrote:
>
> On Fri, 28 Sep 2018 at 12:03, Paul Wouters <paul at nohats.ca> wrote:
> >
> > On Fri, 28 Sep 2018, Andrew Cagney wrote:
> >
> > > with the above two applied, here's what's going wrong (other than it's
> > > IKEv1 and we're stuffed)?
> > >
> > > - since the IKEv1 initiator is in STATE_MAIN_I4 the IKE SA has been
> > > established - any message from an earlier part of the exchange should
> > > be detected and dropped
> >
> > Except R3 I guess, if receiving that we need to retransmit our last
> > packet?
>
> As in an R3 response packet causing the initiator to transition from
> STATE_MAIN_I3 -> STATE_MAIN_R4?
>
> Once the initiator is in state STATE_MAIN_R4 I believe they should be
> dropped on the floor.
>
> I guess there's the argument that when a main exchange is immediately
> followed by a quick exchange then an R3 response should trigger
> re-transmit of the CHILD SA's first quick request (rather than letting
> timeouts deal with it).  But was that ever the case?
>
> > > In IKEv2, that's easy as the Message ID is a counter.
> > > What about IKEv1?  During these exchange the message ID seems to always be zero.
> >
> >     The message ID in the ISAKMP header identifies a Quick Mode in
> >     progress for a particular ISAKMP SA which itself is identified by the
> >     cookies in the ISAKMP header.
> >
> > So I assume that in Main/Aggr Mode it is indeed 0 ?
>
> Here's the relevant packet's header:
>
> | **parse ISAKMP Message:
> |    initiator cookie:
> |   d9 80 c6 55  02 90 0c 4e
> |    responder cookie:
> |   2c ab c5 c6  0f b4 78 5c
> |    next payload type: ISAKMP_NEXT_ID (0x5)
> |    ISAKMP version: ISAKMP Version 1.0 (rfc2407) (0x10)
> |    exchange type: ISAKMP_XCHG_IDPROT (0x2)
> |    flags: ISAKMP_FLAG_v1_ENCRYPTION (0x1)
> |    message ID:  00 00 00 00
> |    length: 332 (0x14c)
>
> so yes for at least main mode.
>
> > > - since the IKEv1 IKE SA is established (almost) all packets should be
> > > encrypted and have integrity, yet this packet fails that so why on
> > > earth is libreswan sending out a notification
> >
> > It seems it failed to detect it as a retransmit, and started processing
> > the packet as if it was a fresh never before received packet?
>
> Right, up to a point.
>
> Like I noted, regardless it should have detected:
>
> - that the packet was old - the last-received buffer is only one deep
> - that the packet was corrupt - and not responded
>
> Its pretty easy to MITM an oh-so-slightly corrupt version of this
> packet and get past the last received check (see --impair
> replay-encrypted / corrupt-encrypted).
>
> > > In IKEv2 this is easy, find the SK payload and decrypt/verify as a single step.
> > > What about IKEv1?  As best I can tell the process is to decrypt the
> > > packet and then parse the resulting white noise looking for a HASH
> > > et.al. payload to use as verification - until all that is done nothing
> > > can be trusted and everything should have been dropped.
> > >
> > > So by pushing 'ikev1 retransmits: only save the received packet when
> > > responding' I exposed the above two failings.  Reverting it wouldn't
> > > be sufficient.  It would likely need some special state magic to
> > > detect if/when that last outgoing packet should be re-transmitted; and
> > > would still leave libreswan exposed to the above.
> >
> > If we are established and the message ID is 0, then we could retransmit
> > the last main/aggressive packet? Once we receive a Quick Mode packet
> > response (with non-zero message id) then we never need to retransmit a
> > msg id 0 packet anymore.
>
> No.
>
> Since the initiator is in STATE_MAIN_I4 it knows that the responder
> received its last outgoing packet - there's nothing left to send for
> this exchange.  In fact, sending out the last packet would trigger a
> re-transmit storm.
>
> However, per above, perhaps it could send out the QUICK packet
> (assuming it is ready), but that feels too complicated.
>
> (please don't make me read the aggressive bit of the RFC :-)
>
> Andrew