[PATCH bpf-next 0/7] Support defragmenting IPv(4|6) packets in BPF

[Daniel Xu]

=== Context ===

In the context of a middlebox, fragmented packets are tricky to handle.
The full 5-tuple of a packet is often only available in the first
fragment which makes enforcing consistent policy difficult. There are
really only two stateless options, neither of which are very nice:

1. Enforce policy on first fragment and accept all subsequent fragments.
   This works but may let in certain attacks or allow data exfiltration.

2. Enforce policy on first fragment and drop all subsequent fragments.
   This does not really work b/c some protocols may rely on
   fragmentation. For example, DNS may rely on oversized UDP packets for
   large responses.

So stateful tracking is the only sane option. RFC 8900 [0] calls this
out as well in section 6.3:

    Middleboxes [...] should process IP fragments in a manner that is
    consistent with [RFC0791] and [RFC8200]. In many cases, middleboxes
    must maintain state in order to achieve this goal.

=== BPF related bits ===

Policy has traditionally been enforced from XDP/TC hooks. Both hooks
run before kernel reassembly facilities. However, with the new
BPF_PROG_TYPE_NETFILTER, we can rather easily hook into existing
netfilter reassembly infra.

The basic idea is we bump a refcnt on the netfilter defrag module and
then run the bpf prog after the defrag module runs. This allows bpf
progs to transparently see full, reassembled packets. The nice thing
about this is that progs don't have to carry around logic to detect
fragments.

=== Patchset details ===

There was an earlier attempt at providing defrag via kfuncs [1]. The
feedback was that we could end up doing too much stuff in prog execution
context (like sending ICMP error replies). However, I think there are
still some outstanding discussion w.r.t. performance when it comes to
netfilter vs the previous approach. I'll schedule some time during
office hours for this.

Patches 1 & 2 are stolenfrom Florian. Hopefully he doesn't mind. There
were some outstanding comments on the v2 [2] but it doesn't look like a
v3 was ever submitted.  I've addressed the comments and put them in this
patchset cuz I needed them.

Finally, the new selftest seems to be a little flaky. I'm not quite
sure why the server will fail to `recvfrom()` occassionaly. I'm fairly
sure it's a timing related issue with creating veths. I'll keep
debugging but I didn't want that to hold up discussion on this patchset.


[0]: https://datatracker.ietf.org/doc/html/rfc8900
[1]: https://lore.kernel.org/bpf/cover.1677526810.git.dxu@xxxxxxxxx/
[2]: https://lore.kernel.org/bpf/20230525110100.8212-1-fw@xxxxxxxxx/

Daniel Xu (7):
  tools: libbpf: add netfilter link attach helper
  selftests/bpf: Add bpf_program__attach_netfilter helper test
  netfilter: defrag: Add glue hooks for enabling/disabling defrag
  netfilter: bpf: Support BPF_F_NETFILTER_IP_DEFRAG in netfilter link
  bpf: selftests: Support not connecting client socket
  bpf: selftests: Support custom type and proto for client sockets
  bpf: selftests: Add defrag selftests

 include/linux/netfilter.h                     |  12 +
 include/uapi/linux/bpf.h                      |   5 +
 net/ipv4/netfilter/nf_defrag_ipv4.c           |   8 +
 net/ipv6/netfilter/nf_defrag_ipv6_hooks.c     |  10 +
 net/netfilter/core.c                          |   6 +
 net/netfilter/nf_bpf_link.c                   | 108 ++++++-
 tools/include/uapi/linux/bpf.h                |   5 +
 tools/lib/bpf/bpf.c                           |   8 +
 tools/lib/bpf/bpf.h                           |   6 +
 tools/lib/bpf/libbpf.c                        |  47 +++
 tools/lib/bpf/libbpf.h                        |  15 +
 tools/lib/bpf/libbpf.map                      |   1 +
 tools/testing/selftests/bpf/Makefile          |   4 +-
 .../selftests/bpf/generate_udp_fragments.py   |  90 ++++++
 .../selftests/bpf/ip_check_defrag_frags.h     |  57 ++++
 tools/testing/selftests/bpf/network_helpers.c |  26 +-
 tools/testing/selftests/bpf/network_helpers.h |   3 +
 .../bpf/prog_tests/ip_check_defrag.c          | 282 ++++++++++++++++++
 .../bpf/prog_tests/netfilter_basic.c          |  78 +++++
 .../selftests/bpf/progs/ip_check_defrag.c     | 104 +++++++
 .../bpf/progs/test_netfilter_link_attach.c    |  14 +
 21 files changed, 868 insertions(+), 21 deletions(-)
 create mode 100755 tools/testing/selftests/bpf/generate_udp_fragments.py
 create mode 100644 tools/testing/selftests/bpf/ip_check_defrag_frags.h
 create mode 100644 tools/testing/selftests/bpf/prog_tests/ip_check_defrag.c
 create mode 100644 tools/testing/selftests/bpf/prog_tests/netfilter_basic.c
 create mode 100644 tools/testing/selftests/bpf/progs/ip_check_defrag.c
 create mode 100644 tools/testing/selftests/bpf/progs/test_netfilter_link_attach.c

-- 
2.40.1