Date:	Fri, 13 Sep 2013 08:42:25 -0400
From:	Neil Horman <nhorman@...driver.com>
To:	Daniel Borkmann <dborkman@...hat.com>
Cc:	davem@...emloft.net, netdev@...r.kernel.org,
	linux-sctp@...r.kernel.org, adobriyan@...il.com,
	Steffen Klassert <steffen.klassert@...unet.com>,
	Hannes Frederic Sowa <hannes@...essinduktion.org>
Subject: Re: [PATCH net v2] net: sctp: fix ipv6 ipsec encryption bug in
 sctp_v6_xmit

On Wed, Sep 11, 2013 at 04:58:36PM +0200, Daniel Borkmann wrote:
> Alan Chester reported an issue with IPv6 on SCTP that IPsec traffic is not
> being encrypted, whereas on IPv4 it is. Setting up an AH + ESP transport
> does not seem to have the desired effect:
> 
> SCTP + IPv4:
> 
>   22:14:20.809645 IP (tos 0x2,ECT(0), ttl 64, id 0, offset 0, flags [DF], proto AH (51), length 116)
>     192.168.0.2 > 192.168.0.5: AH(spi=0x00000042,sumlen=16,seq=0x1): ESP(spi=0x00000044,seq=0x1), length 72
>   22:14:20.813270 IP (tos 0x2,ECT(0), ttl 64, id 0, offset 0, flags [DF], proto AH (51), length 340)
>     192.168.0.5 > 192.168.0.2: AH(spi=0x00000043,sumlen=16,seq=0x1):
> 
> SCTP + IPv6:
> 
>   22:31:19.215029 IP6 (class 0x02, hlim 64, next-header SCTP (132) payload length: 364)
>     fe80::222:15ff:fe87:7fc.3333 > fe80::92e6:baff:fe0d:5a54.36767: sctp
>     1) [INIT ACK] [init tag: 747759530] [rwnd: 62464] [OS: 10] [MIS: 10]
> 
> Moreover, Alan says:
> 
>   This problem was seen with both Racoon and Racoon2. Other people have seen
>   this with OpenSwan. When IPsec is configured to encrypt all upper layer
>   protocols the SCTP connection does not initialize. After using Wireshark to
>   follow packets, this is because the SCTP packet leaves Box A unencrypted and
>   Box B believes all upper layer protocols are to be encrypted so it drops
>   this packet, causing the SCTP connection to fail to initialize. When IPsec
>   is configured to encrypt just SCTP, the SCTP packets are observed unencrypted.
> 
> In fact, using `socat sctp6-listen:3333 -` on one end and transferring "plaintext"
> string on the other end, results in cleartext on the wire where SCTP eventually
> does not report any errors, thus in the latter case that Alan reports, the
> non-paranoid user might think he's communicating over an encrypted transport on
> SCTP although he's not (tcpdump ... -X):
> 
>   ...
>   0x0030: 5d70 8e1a 0003 001a 177d eb6c 0000 0000  ]p.......}.l....
>   0x0040: 0000 0000 706c 6169 6e74 6578 740a 0000  ....plaintext...
> 
> Only in /proc/net/xfrm_stat we can see XfrmInTmplMismatch increasing on the
> receiver side. Initial follow-up analysis from Alan's bug report was done by
> Alexey Dobriyan. Also thanks to Vlad Yasevich for feedback on this.
> 
> SCTP has its own implementation of sctp_v6_xmit() not calling inet6_csk_xmit().
> This has the implication that it probably never really got updated along with
> changes in inet6_csk_xmit() and therefore does not seem to invoke xfrm handlers.
> 
> SCTP's IPv4 xmit however, properly calls ip_queue_xmit() to do the work. Since
> a call to inet6_csk_xmit() would solve this problem, but result in unecessary
> route lookups, let us just use the cached flowi6 instead that we got through
> sctp_v6_get_dst(). Since all SCTP packets are being sent through sctp_packet_transmit(),
> we do the route lookup / flow caching in sctp_transport_route(), hold it in
> tp->dst and skb_dst_set() right after that. If we would alter fl6->daddr in
> sctp_v6_xmit() to np->opt->srcrt, we possibly could run into the same effect
> of not having xfrm layer pick it up, hence, use fl6_update_dst() in sctp_v6_get_dst()
> instead to get the correct source routed dst entry, which we assign to the skb.
> 
> Also source address routing example from 625034113 ("sctp: fix sctp to work with
> ipv6 source address routing") still works with this patch! Nevertheless, in RFC5095
> it is actually 'recommended' to not use that anyway due to traffic amplification [1].
> So it seems we're not supposed to do that anyway in sctp_v6_xmit(). Moreover, if
> we overwrite the flow destination here, the lower IPv6 layer will be unable to
> put the correct destination address into IP header, as routing header is added in
> ipv6_push_nfrag_opts() but then probably with wrong final destination. Things aside,
> result of this patch is that we do not have any XfrmInTmplMismatch increase plus on
> the wire with this patch it now looks like:
> 
> SCTP + IPv6:
> 
>   08:17:47.074080 IP6 2620:52:0:102f:7a2b:cbff:fe27:1b0a > 2620:52:0:102f:213:72ff:fe32:7eba:
>     AH(spi=0x00005fb4,seq=0x1): ESP(spi=0x00005fb5,seq=0x1), length 72
>   08:17:47.074264 IP6 2620:52:0:102f:213:72ff:fe32:7eba > 2620:52:0:102f:7a2b:cbff:fe27:1b0a:
>     AH(spi=0x00003d54,seq=0x1): ESP(spi=0x00003d55,seq=0x1), length 296
> 
> This fixes Kernel Bugzilla 24412. This security issue seems to be present since
> 2.6.18 kernels. Lets just hope some big passive adversary in the wild didn't have
> its fun with that. lksctp-tools IPv6 regression test suite passes as well with
> this patch.
> 
>  [1] http://www.secdev.org/conf/IPv6_RH_security-csw07.pdf
> 
> Reported-by: Alan Chester <alan.chester@...elec.com>
> Reported-by: Alexey Dobriyan <adobriyan@...il.com>
> Signed-off-by: Daniel Borkmann <dborkman@...hat.com>
> Cc: Steffen Klassert <steffen.klassert@...unet.com>
> Cc: Hannes Frederic Sowa <hannes@...essinduktion.org>
> ---
>  v1->v2:
>   - use cached flow
>   - improved commit msg
> 
>  net/sctp/ipv6.c | 42 +++++++++++++-----------------------------
>  1 file changed, 13 insertions(+), 29 deletions(-)
> 
Acked-by: Neil Horman <nhorman@...driver.com>

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