=== 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 ===
However, when policy is enforced through BPF, the prog is run before the kernel reassembles fragmented packets. This leaves BPF developers in a awkward place: implement reassembly (possibly poorly) or use a stateless method as described above.
Fortunately, the kernel has robust support for fragmented IP packets. This patchset wraps the existing defragmentation facilities in kfuncs so that BPF progs running on middleboxes can reassemble fragmented packets before applying policy.
=== Patchset details ===
This patchset is (hopefully) relatively straightforward from BPF perspective. One thing I'd like to call out is the skb_copy()ing of the prog skb. I did this to maintain the invariant that the ctx remains valid after prog has run. This is relevant b/c ip_defrag() and ip_check_defrag() may consume the skb if the skb is a fragment.
Originally I did play around with teaching the verifier about kfuncs that may consume the ctx and disallowing ctx accesses in ret != 0 branches. It worked ok, but it seemed too complex to modify the surrounding assumptions about ctx validity.
[0]: https://datatracker.ietf.org/doc/html/rfc8900
===
Changes from v1: * Add support for ipv6 defragmentation
Daniel Xu (8): ip: frags: Return actual error codes from ip_check_defrag() bpf: verifier: Support KF_CHANGES_PKT flag bpf, net, frags: Add bpf_ip_check_defrag() kfunc net: ipv6: Factor ipv6_frag_rcv() to take netns and user bpf: net: ipv6: Add bpf_ipv6_frag_rcv() kfunc bpf: selftests: Support not connecting client socket bpf: selftests: Support custom type and proto for client sockets bpf: selftests: Add defrag selftests
Documentation/bpf/kfuncs.rst | 7 + drivers/net/macvlan.c | 2 +- include/linux/btf.h | 1 + include/net/ip.h | 11 + include/net/ipv6.h | 1 + include/net/ipv6_frag.h | 1 + include/net/transp_v6.h | 1 + kernel/bpf/verifier.c | 8 + net/ipv4/Makefile | 1 + net/ipv4/ip_fragment.c | 15 +- net/ipv4/ip_fragment_bpf.c | 98 ++++++ net/ipv6/Makefile | 1 + net/ipv6/af_inet6.c | 4 + net/ipv6/reassembly.c | 16 +- net/ipv6/reassembly_bpf.c | 143 ++++++++ net/packet/af_packet.c | 2 +- tools/testing/selftests/bpf/Makefile | 3 +- .../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 | 327 ++++++++++++++++++ .../selftests/bpf/progs/bpf_tracing_net.h | 1 + .../selftests/bpf/progs/ip_check_defrag.c | 133 +++++++ 24 files changed, 931 insertions(+), 21 deletions(-) create mode 100644 net/ipv4/ip_fragment_bpf.c create mode 100644 net/ipv6/reassembly_bpf.c 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