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Commit: #5900ecf

Parent: #43a6927

ipv6: Skip XFRM lookup if dst_entry in socket cache is valid

commit 00bc0ef5880dc7b82f9c320dead4afaad48e47be upstream.

At present we perform an xfrm_lookup() for each UDPv6 message we send. The lookup involves querying the flow cache (flow_cache_lookup) and, in case of a cache miss, creating an XFRM bundle.

If we miss the flow cache, we can end up creating a new bundle and deriving the path MTU (xfrm_init_pmtu) from on an already transformed dst_entry, which we pass from the socket cache (sk->sk_dst_cache) down to xfrm_lookup(). This can happen only if we're caching the dst_entry in the socket, that is when we're using a connected UDP socket.

To put it another way, the path MTU shrinks each time we miss the flow cache, which later on leads to incorrectly fragmented payload. It can be observed with ESPv6 in transport mode:

1) Set up a transformation and lower the MTU to trigger fragmentation # ip xfrm policy add dir out src ::1 dst ::1 \ tmpl src ::1 dst ::1 proto esp spi 1 # ip xfrm state add src ::1 dst ::1 \ proto esp spi 1 enc 'aes' 0x0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b0b # ip link set dev lo mtu 1500

2) Monitor the packet flow and set up an UDP sink # tcpdump -ni lo -ttt & # socat udp6-listen:12345,fork /dev/null &

3) Send a datagram that needs fragmentation with a connected socket # perl -e 'print "@" x 1470 | socat - udp6:[::1]:12345 2016/06/07 18:52:52 socat[724] E read(3, 0x555bb3d5ba00, 8192): Protocol error 00:00:00.000000 IP6 ::1 > ::1: frag (0|1448) ESP(spi=0x00000001,seq=0x2), length 1448 00:00:00.000014 IP6 ::1 > ::1: frag (1448|32) 00:00:00.000050 IP6 ::1 > ::1: ESP(spi=0x00000001,seq=0x3), length 1272 (^ ICMPv6 Parameter Problem) 00:00:00.000022 IP6 ::1 > ::1: ESP(spi=0x00000001,seq=0x5), length 136

4) Compare it to a non-connected socket # perl -e 'print "@" x 1500' | socat - udp6-sendto:[::1]:12345 00:00:40.535488 IP6 ::1 > ::1: frag (0|1448) ESP(spi=0x00000001,seq=0x6), length 1448 00:00:00.000010 IP6 ::1 > ::1: frag (1448|64)

What happens in step (3) is:

1) when connecting the socket in __ip6_datagram_connect(), we perform an XFRM lookup, miss the flow cache, create an XFRM bundle, and cache the destination,

2) afterwards, when sending the datagram, we perform an XFRM lookup, again, miss the flow cache (due to mismatch of flowi6_iif and flowi6_oif, which is an issue of its own), and recreate an XFRM bundle based on the cached (and already transformed) destination.

To prevent the recreation of an XFRM bundle, avoid an XFRM lookup altogether whenever we already have a destination entry cached in the socket. This prevents the path MTU shrinkage and brings us on par with UDPv4.

The fix also benefits connected PINGv6 sockets, another user of ip6_sk_dst_lookup_flow(), who also suffer messages being transformed twice.

Joint work with Hannes Frederic Sowa.

Reported-by: Jan Tluka <> Signed-off-by: Jakub Sitnicki <> Acked-by: Hannes Frederic Sowa <> Signed-off-by: David S. Miller <> Signed-off-by: Benedict Wong <> Signed-off-by: Greg Kroah-Hartman <>

avatar <gregkh @linuxfo…> (Committer) 21 days ago

avatar Jakub Sitnicki <jkbs @red…> (Author) 2016-06-08 16:13


1038 1038 const struct in6_addr *final_dst)
1039 1039 {
1040 1040 struct dst_entry *dst = sk_dst_check(sk, inet6_sk(sk)->dst_cookie);
1041 int err;
1042 1041
1043 1042 dst = ip6_sk_dst_check(sk, dst, fl6);
1043 if (!dst)
1044 dst = ip6_dst_lookup_flow(sk, fl6, final_dst);
1044 1045
1045 err = ip6_dst_lookup_tail(sk, &dst, fl6);
1046 if (err)
1047 return ERR_PTR(err);
1048 if (final_dst)
1049 fl6->daddr = *final_dst;
1051 return xfrm_lookup_route(sock_net(sk), dst, flowi6_to_flowi(fl6), sk, 0);
1046 return dst;
1052 1047 }
1053 1048 EXPORT_SYMBOL_GPL(ip6_sk_dst_lookup_flow);
1054 1049