strncpy on binary MAC bytes silently truncates at 0x00 byte — radix key collision rule bypass
Summary
ip_fw3_layer2.c:127 strncpy(sa.sa_data,eh->ether_shost,6) and :170 strncpy(sa.sa_data,eh->ether_dhost,6). MAC is binary not C string — strncpy stops at first 0x00 byte zero-pads remainder. MAC 00:1b:54:aa:bb:cc produces key {00,00,00,00,00,00}. Table-insert side has same bug ip_fw3_table.c:180 strncpy. Consistent truncation = deterministic radix collision: rule keyed on MAC A matches every MAC sharing prefix-before-first-zero. LATENT: eh always NULL in current tree matcher always returns NOT_MATCH. Becomes live when bridge/L2 support wired to ipfw3. Fix: memcpy replace strncpy at both lookup sites and table-insert.
Discussion (0)
PoC verification
Evidence pack
findings/poc/DF-0750 · 2 files| File | Type | Description | Size | |
|---|---|---|---|---|
| fix.diff | suggested-fix | git-apply-able fix for the cited path | 662 B | view raw |
| VERDICT.md | verdict | source-confirmation narrative | 978 B | ↓ raw |
DF-0750 source-confirmation
Verdict: REPRODUCED (source-confirmed)
Impact: none Confidence: likely
Kernel ref: sys/net/ipfw3_layer2/ip_fw3_layer2.c:127
Mechanism
strncpy on binary MAC truncates at 0x00 byte -> wrong lookup key
Confirmation method
Source-trace confirmed the cited code path matches the finding (exact line/condition
verified against sys/). Runtime PoC not exercised for this source-only Low-severity
item; confirmation is by code inspection.
Recommended fix
See fix.diff in this folder (git-apply-able unified diff).
Phase 8 (combined build)
All 64 fixes were batched into one combined patch
(../_batch/combined_64.patch) and applied to in-guest /usr/src. A single
make -j6 nativekernel KERNCONF=X86_64_GENERIC completed rc=0 with 0 errors under
-Werror (../_batch/fix_build.log, 35374 lines). The GENERIC kernel + all modules
(drm, firewire, usb, netgraph, smbfs, fuse, crypto, vm, pmap) compiled clean.
Fix verification
not_testablenot_testable (source-only): no runtime PoC exercised; fix.diff applies cleanly and the combined single nativekernel build of all 64 fixes completed rc=0 with 0 errors under -Werror (findings/poc/_batch/fix_build.log). Bug source-confirmed in baseline tree. Compile-validation of the fix is the requested Phase-8 deliverable for this batch.
combined build: '=== NK_DONE rc=0 ===' and '>>> Kernel build for X86_64_GENERIC completed on Thu Jul 23 09:17:55 UTC 2026' (0 'error:' lines in 35374-line log).
Confirmed kernel references
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Detail
Exploit chain
none (non-corruption / source-only confirmation; no memory-corruption escalation chain developed for this Low-severity item)
Evidence (decisive lines)
source-only: strncpy on binary MAC truncates at 0x00 byte -> wrong lookup key @ sys/net/ipfw3_layer2/ip_fw3_layer2.c:127. Combined fix build: '=== NK_DONE rc=0 ===' / 'Kernel build for X86_64_GENERIC completed', 0 errors under -Werror (findings/poc/_batch/fix_build.log).
PoC changes
authored git-apply-able fix.diff targeting the cited line; source-trace verified the vulnerable path. Evidence pack (VERDICT.md, manifest.json, env.txt, fix.diff) in findings/poc/DF-0750/.
Verified recommended fix
strncpy->memcpy (+bzero tail) for binary MAC bytes. matches finding proposal. Full diff in findings/poc/DF-0750/fix.diff.
Verdict
REPRODUCED (source-confirmed). Source-trace confirmed strncpy on binary MAC truncates at 0x00 byte -> wrong lookup key at sys/net/ipfw3_layer2/ip_fw3_layer2.c:127 against the audited sys/ tree; exact line/condition verified.
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