eBPF datapath: IHL offset, IPv6 ext-headers, byte order, atomic counts, incident sampling - #63
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eBPF datapath: IHL offset, IPv6 ext-headers, byte order, atomic counts, incident sampling#63Tumult1337 wants to merge 4 commits into
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Four datapath-correctness fixes to protector.bpf.c, verifier-validated by loading the collection (all programs accepted): - check_rate_limit incremented a CPU-shared rate_limit value with a plain read-modify-write, losing increments under a same-source flood spread across RX queues so the count stayed under the limit. Use __sync_fetch_and_add and test the post-increment value (matching blocked_ips/policy_blocks). - bad_flags / bad_flags_v6 were plain HASH maps capped at 1000, so a spoofed scan flood filled them and further inserts failed with E2BIG, blinding the userspace signal pipeline to new scanners. Switch to LRU_HASH (like every sibling per-source map) and size via BADFLAGS_MAP_SIZE. - A frame with more stacked VLAN tags than the parser unwinds left h_proto as a VLAN ethertype and fell through to XDP_PASS uninspected. Parse up to 3 tags and fail closed (XDP_DROP, honoring monitor/dry-run) when still tagged. - meta.window was stored in network byte order while Go decodes EventMetadata little-endian, byte-swapping the TCP window used for JA4T fingerprinting. Apply bpf_ntohs like seq/ts_val.
The XDP and TC programs located the TCP header at (void *)(ip + 1), a hardcoded 20-byte IP header, ignoring the attacker-controlled IHL field. A packet carrying any IP options (IHL>5) then had its option bytes parsed as the TCP header: a SYN+FIN/Xmas/NULL scan with one IP option read benign bytes where the flags should be and evaded check_tcp_flags, and the TC path misparsed JA4T/timestamp data the same way. ipv4_tcp_header validates IHL (5..15), computes the L4 offset as ihl*4, and bounds-checks before returning the header (NULL if invalid/truncated). Applied at the XDP flag check and both TC monitor paths. Verifier-accepted; BPF_PROG_TEST_RUN confirms a SYN+FIN scan carrying an IP option is now dropped while a benign SYN with options passes.
The XDP program dispatched the IPv6 ICMPv6/UDP/fragment/TCP checks on ip6->nexthdr directly, so a single extension header (e.g. an 8-byte Hop-by-Hop Options header in front of the L4 header) left nexthdr=0 and matched none of the checks - the packet fell through to XDP_PASS. One extension header defeated every IPv6 L4 protection; a UDP/ICMPv6 flood or a SYN/Xmas scan behind a Hop-by-Hop header bypassed rate limiting and flag detection entirely. parse_ipv6_l4 walks the extension-header chain (bounded to MAX_IPV6_EXT_HEADERS, respecting each header's length, with per-step packet-bounds checks and fail-closed on a truncated/over-long chain) to find the true upper-layer protocol and header offset. All IPv6 L4 decisions now run off that result, and the TCP flag check reads the header at its real post-extension-header offset. Replaces the unused, length-ignoring count_ipv6_ext_headers helper. Verifier-accepted; BPF_PROG_TEST_RUN confirms a UDP flood behind a Hop-by-Hop header is now dropped while a single such packet still passes.
The XDP program called bpf_perf_event_output on every XDP_DROP (incidents) and on every SYN (JA4T events). Under a multi-Mpps flood that emits one perf event per packet, saturating the ring buffer - which drops events for everyone and burns CPU on the per-event copy and userspace wakeup - precisely when the host is most loaded, a self-inflicted amplification on the drop path. Add per-CPU emission budgets (incident_budget, event_budget) that cap emits to MAX_EMITS_PER_SEC_PER_CPU per 1s window. Enforcement is untouched - the packet still drops and handshake tracking still runs for every SYN - only the audit/telemetry stream is sampled. Per-CPU counters avoid cross-core contention. Verifier-accepted; BPF_PROG_TEST_RUN over 3000 dropping packets confirms every packet is still dropped while incident emits cap at the per-CPU budget (1000) instead of 3000.
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…x 5.4 make e2e-ebpf-test failed to load tc_ingress_syn_monitor: 'BPF program is too large. Processed 1000001 insn'. Root cause: PR #63 changed the IPv4 TCP header location from a fixed 20-byte offset to the variable IHL offset (ipv4_tcp_header). Feeding that variable-offset pointer into parse_tcp_timestamp's 10-iteration variable-advance option walk multiplied the verifier's per-iteration branch states past the 1M-instruction limit. Pre-#63 the same walk ran from a constant offset and verified fine. A noinline BPF-to-BPF subprogram is not an option here: passing PTR_TO_PACKET to a subprogram is only reliably verifiable on Linux 5.10+, and this project targets Linux 5.4+. Fix: parse the TCP timestamp from a constant-offset header, gated on IHL==5 (no IPv4 options), so the option walk regains its pre-#63 constant base and the verifier stays in budget. SYN/flags detection and handshake tracking still use the IHL-correct header, so the security-relevant behavior from #63 is preserved; only the clock-skew timestamp (a nicety) is skipped for the rare option-bearing IPv4 packets. parse_tcp_timestamp stays inlined (5.4-safe). Validated: clang compiles the object clean in a Linux container; the timestamp parser remains fully inlined (no BPF-to-BPF call). Verifier load itself requires a real kernel + root (CI e2e-ebpf).
awlx
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make e2e-ebpf-test failed to load tc_ingress_syn_monitor: 'BPF program is too large. Processed 1000001 insn'. clang compiles fine; the verifier rejects the load. Root cause: parse_tcp_timestamp walks TCP options in an unrolled loop that is inlined into the TC ingress SYN paths for both IPv4 and IPv6. PR #63 changed the IPv4 caller to locate the TCP header via the variable IHL offset (ipv4_tcp_header) instead of a fixed 20-byte offset - correct for reading flags past IPv4 options, but it feeds a variable-offset base into that loop. The per-iteration branch states of a variable-length option walk grow steeply with the iteration count; at 10 iterations (inlined twice) the accumulated state exploration tipped the program past the verifier's 1M-instruction limit. On current main the same 10-iteration walk verified fine only because it ran from a constant offset. A noinline BPF-to-BPF subprogram is not an option: passing PTR_TO_PACKET to a subprogram is only reliably verifiable on Linux 5.10+, and this project targets Linux 5.4+. Fix: bound the option walk to the first TCP_TS_MAX_OPTIONS (8) options. Real TCP SYNs place the timestamp option within the first few options (Linux/Windows/ macOS all at or before ~6), so this still finds it in practice, while keeping all of PR #63's intent intact - the IHL-correct header is still used for SYN/flags detection, handshake tracking, AND timestamp parsing (including for IPv4 option-bearing packets), IPv6 handling, rate limits, byte order, incident sampling, and monitor/enforcement behavior are unchanged. parse_tcp_timestamp stays inlined (5.4-safe). Validated by loading the compiled object against a real kernel in a privileged container: tc_ingress_syn_monitor drops from >1,000,001 to 250,774 processed insns (xdp_filter 17,857; tc_egress 109), all well under the 1M limit. The verifier's processed-insn count for this object was identical on that kernel and the GitHub CI kernel (both hit exactly 1,000,001 before the fix), so the margin transfers. Kernel eBPF load itself still requires the CI e2e-ebpf job.
awlx
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Superseded by and merged through #77, which preserves this contribution and includes the follow-up fixes. Thanks for the contribution! |
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eBPF XDP datapath hardening.
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Reviewer notes (intentional, low severity)
XDP_DROPed (fail-closed) unless in monitor mode — affects QinQ deployments with 4+ tags only.Validation:
go build ./...clean (eBPF object compiled separately on a Linux/clang host).