Luca Moro says:
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The issue lies in the filtering of ICMP and ICMPv6 errors that include an
inner IP datagram.
For these packets, icmp_error_message() extract the ICMP error and inner
layer to search of a known state.
If a state is found the packet is tagged as related (IP_CT_RELATED).

The problem is that there is no correlation check between the inner and
outer layer of the packet.
So one can encapsulate an error with an inner layer matching a known state,
while its outer layer is directed to a filtered host.
In this case the whole packet will be tagged as related.
This has various implications from a rule bypass (if a rule to related
trafic is allow), to a known state oracle.

Unfortunately, we could not find a real statement in a RFC on how this case
should be filtered.
The closest we found is RFC5927 (Section 4.3) but it is not very clear.

A possible fix would be to check that the inner IP source is the same than
the outer destination.

We believed this kind of attack was not documented yet, so we started to
write a blog post about it.
You can find it attached to this mail (sorry for the extract quality).
It contains more technical details, PoC and discussion about the identified
behavior.
We discovered later that
https://www.gont.com.ar/papers/filtering-of-icmp-error-messages.pdf
described a similar attack concept in 2004 but without the stateful
filtering in mind.
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This implements above suggested fix:
In icmp(v6) error handler, take outer destination address, then pass
that into the common function that does the "related" association.

After obtaining the nf_conn of the matching inner-headers connection,
check that the destination address of the opposite direction tuple
is the same as the outer address and only set RELATED if thats the case.
Reported-by: Luca Moro <luca.moro@synacktiv.com>
Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>