bpf: lxc: fix L4 offset in RevNAT path for IPv6 extension headers #27312
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The typical pattern to determine the L4 offset of an IPv6 packet is tuple->nexthdr = ip6->nexthdr; hdrlen = ipv6_hdrlen(ctx, &tuple->nexthdr); if (hdrlen < 0) return hdrlen; l4_off = ETH_HLEN + hdrlen; ipv6_hdrlen() then walks the extension headers and adds up their length, until it reaches the TCP/UDP/... header. For bpf_lxc, 38b4a61 ("bpf: Split CT lookup to its own tail call") split off the CT lookup into a separate tail-call. Subsequent code retrieves the lookup result and a fully populated CT tuple from the CT_TAIL_CALL_BUFFER6. For the IPv6 paths this means that tuple->nexthdr already contains the L4 proto. When ipv6_hdrlen() is called a second time with this value, it believes that the packet has no extension headers and returns a wrong L4 offset. Go for the minimal fix and re-initialize tuple->nexthdr in the affected paths. Fixes: 38b4a61 ("bpf: Split CT lookup to its own tail call") Signed-off-by: Julian Wiedmann <jwi@isovalent.com>
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This was referenced Sep 9, 2023
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The typical pattern to determine the L4 offset of an IPv6 packet is
ipv6_hdrlen() then walks the extension headers and adds up their length, until it reaches the TCP/UDP/... header.
For bpf_lxc, 38b4a61 ("bpf: Split CT lookup to its own tail call") split off the CT lookup into a separate tail-call. Subsequent code retrieves the lookup result and a fully populated CT tuple from the CT_TAIL_CALL_BUFFER6.
For the IPv6 paths this means that tuple->nexthdr already contains the L4 proto. When ipv6_hdrlen() is called a second time with this value, it believes that the packet has no extension headers and returns a wrong L4 offset.
Go for the minimal fix and re-initialize tuple->nexthdr in the affected paths.