Re: [ 16/48] netfilter: conntrack: disable generic tracking for known protocols

From: Ben Hutchings
Date: Fri May 15 2015 - 17:05:29 EST

On Fri, 2015-05-15 at 10:05 +0200, Willy Tarreau wrote:
> 2.6.32-longterm review patch. If anyone has any objections, please let me know.
> ------------------
> From: Florian Westphal <fw@xxxxxxxxx>
> commit db29a9508a9246e77087c5531e45b2c88ec6988b upstream
> Given following iptables ruleset:
> -A FORWARD -m sctp --dport 9 -j ACCEPT
> -A FORWARD -p tcp --dport 80 -j ACCEPT
> -A FORWARD -p tcp -m conntrack -m state ESTABLISHED,RELATED -j ACCEPT
> One would assume that this allows SCTP on port 9 and TCP on port 80.
> Unfortunately, if the SCTP conntrack module is not loaded, this allows
> *all* SCTP communication, to pass though, i.e. -p sctp -j ACCEPT,
> which we think is a security issue.
> This is because on the first SCTP packet on port 9, we create a dummy
> "generic l4" conntrack entry without any port information (since
> conntrack doesn't know how to extract this information).
> All subsequent packets that are unknown will then be in established
> state since they will fallback to proto_generic and will match the
> 'generic' entry.
> Our originally proposed version [1] completely disabled generic protocol
> tracking, but Jozsef suggests to not track protocols for which a more
> suitable helper is available, hence we now mitigate the issue for in
> tree known ct protocol helpers only, so that at least NAT and direction
> information will still be preserved for others.
> [1]
> Joint work with Daniel Borkmann.
> Signed-off-by: Florian Westphal <fw@xxxxxxxxx>
> Signed-off-by: Daniel Borkmann <dborkman@xxxxxxxxxx>
> Acked-by: Jozsef Kadlecsik <kadlec@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Pablo Neira Ayuso <pablo@xxxxxxxxxxxxx>
> [bwh: Backported to 2.6.32: adjust context]

Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>

> Signed-off-by: Willy Tarreau <w@xxxxxx>

Ben Hutchings
It is impossible to make anything foolproof because fools are so ingenious.

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