=== 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.
=== Changelog ===
Changes from v5:
* Fix defrag disable codepaths
Changes from v4:
* Refactor module handling code to not sleep in rcu_read_lock() * Also unify the v4 and v6 hook structs so they can share codepaths * Fixed some checkpatch.pl formatting warnings
Changes from v3:
* Correctly initialize `addrlen` stack var for recvmsg()
Changes from v2:
* module_put() if ->enable() fails * Fix CI build errors
Changes from v1:
* Drop bpf_program__attach_netfilter() patches * static -> static const where appropriate * Fix callback assignment order during registration * Only request_module() if callbacks are missing * Fix retval when modprobe fails in userspace * Fix v6 defrag module name (nf_defrag_ipv6_hooks -> nf_defrag_ipv6) * Simplify priority checking code * Add warning if module doesn't assign callbacks in the future * Take refcnt on module while defrag link is active
[0]: https://datatracker.ietf.org/doc/html/rfc8900
Daniel Xu (5): 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 | 10 + include/uapi/linux/bpf.h | 5 + net/ipv4/netfilter/nf_defrag_ipv4.c | 17 +- net/ipv6/netfilter/nf_defrag_ipv6_hooks.c | 11 + net/netfilter/core.c | 6 + net/netfilter/nf_bpf_link.c | 123 +++++++- tools/include/uapi/linux/bpf.h | 5 + 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 | 283 ++++++++++++++++++ .../selftests/bpf/progs/ip_check_defrag.c | 104 +++++++ 14 files changed, 718 insertions(+), 26 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/progs/ip_check_defrag.c