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Thu, 22 Jul 2021 10:21:32 -0700 (PDT) Received: from alrua-x1.borgediget.toke.dk ([2a0c:4d80:42:443::2]) by smtp.gmail.com with ESMTPSA id d8sm6875198edj.19.2021.07.22.10.21.31 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Thu, 22 Jul 2021 10:21:31 -0700 (PDT) Received: by alrua-x1.borgediget.toke.dk (Postfix, from userid 1000) id 5882418031E; Thu, 22 Jul 2021 19:21:30 +0200 (CEST) From: Toke =?utf-8?Q?H=C3=B8iland-J=C3=B8rgensen?= To: Gilad Naaman , davem@davemloft.net, yoshfuji@linux-ipv6.org, dsahern@kernel.org, kuba@kernel.org Cc: netdev@vger.kernel.org Subject: Re: [PATCH] net: Set true network header for ECN decapsulation In-Reply-To: <20210722170128.223387-1-gnaaman@drivenets.com> References: <20210722170128.223387-1-gnaaman@drivenets.com> X-Clacks-Overhead: GNU Terry Pratchett Date: Thu, 22 Jul 2021 19:21:30 +0200 Message-ID: <87v9526z8l.fsf@toke.dk> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org Gilad Naaman writes: > In cases where the header straight after the tunnel header was > another ethernet header (TEB), instead of the network header, > the ECN decapsulation code would treat the ethernet header as if > it was an IP header, resulting in mishandling and possible > wrong drops or corruption of the IP header. > > In this case, ECT(1) is sent, so IP_ECN_decapsulate tries to copy it to t= he > inner IPv4 header, and correct its checksum. > > The offset of the ECT bits in an IPv4 header corresponds to the > lower 2 bits of the second octet of the destination MAC address > in the ethernet header. > The IPv4 checksum corresponds to end of the source address. > > In order to reproduce: > > $ ip netns add A > $ ip netns add B > $ ip -n A link add _v0 type veth peer name _v1 netns B > $ ip -n A link set _v0 up > $ ip -n A addr add dev _v0 10.254.3.1/24 > $ ip -n A route add default dev _v0 scope global > $ ip -n B link set _v1 up > $ ip -n B addr add dev _v1 10.254.1.6/24 > $ ip -n B route add default dev _v1 scope global > $ ip -n B link add gre1 type gretap local 10.254.1.6 remote 10.254.3.= 1 key 0x49000000 > $ ip -n B link set gre1 up > > # Now send an IPv4/GRE/Eth/IPv4 frame where the outer header has ECT(= 1), > # and the inner header has no ECT bits set: > > $ cat send_pkt.py > #!/usr/bin/env python3 > from scapy.all import * > > pkt =3D IP(b'E\x01\x00\xa7\x00\x00\x00\x00@/`%\n\xfe\x03\x01\n\xf= e\x01\x06 \x00eXI\x00' > b'\x00\x00\x18\xbe\x92\xa0\xee&\x18\xb0\x92\xa0l&\x08\x0= 0E\x00\x00}\x8b\x85' > b'@\x00\x01\x01\xe4\xf2\x82\x82\x82\x01\x82\x82\x82\x02\= x08\x00d\x11\xa6\xeb' > b'3\x1e\x1e\\xf3\\xf7`\x00\x00\x00\x00ZN\x00\x00\x00\x00= \x00\x00\x10\x11\x12' > b'\x13\x14\x15\x16\x17\x18\x19\x1a\x1b\x1c\x1d\x1e\x1f != "#$%&\'()*+,-./01234' > b'56789:;<=3D>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ') > > send(pkt) > $ sudo ip netns exec B tcpdump -neqlllvi gre1 icmp & ; sleep 1 > $ sudo ip netns exec A python3 send_pkt.py > > In the original packet, the source/destinatio MAC addresses are > dst=3D18:be:92:a0:ee:26 src=3D18:b0:92:a0:6c:26 > > In the received packet, they are > dst=3D18:bd:92:a0:ee:26 src=3D18:b0:92:a0:6c:27 > > Thanks to Lahav Schlesinger and Isaac Garzon= > for helping me pinpoint the origin. Oops! Thank you for the fix :) Acked-by: Toke H=C3=B8iland-J=C3=B8rgensen