DF-0493

KASSERT-only bounds check on nack before xwin[] indexing: no runtime protection in production kernels

Summary

ng_l2tp_seq_recv_nr(:1141-1154): nack from attacker Nr via L2TP_SEQ_DIFF(:1135) indexes xwin[L2TP_MAX_XWIN=16]. Only bound is KASSERT(nack<=L2TP_MAX_XWIN)(:1141) compiled out non-INVARIANTS. Normal invariant(:1137 L2TP_SEQ_DIFF<=0) prevents exceed. BUT lockless race(DF-0492) can break invariant by concurrent ns/rack mutation -> nack>16 -> m_freem past xwin[15]+memmove OOB -> heap corruption. Alone defense-in-depth, amplifies companion race. Fix: runtime if(nack>L2TP_MAX_XWIN) return before xwin access.

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Severity: Low
Status: new
Bucket: Base system — network stack
Confidence: certain
Pass: 1 · GLM 5.2 first pass
CVSS 3.1: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U:C:N/I:N/A:H
CWE: CWE-129
File: sys/netgraph/l2tp/ng_l2tp.c
Lines: 1141-1154
Area: net
Discovered: 2026-07-01
Updated: 2026-07-01

Team triage

untriaged

priority: — · owner: —

PoC verification

reproduced

Status: reproduced
Reproduced: yes
Impact: dos
Confidence: likely
Tested: 2026-07-23 04:51:50
Attempts: 1
Runtime: —
Code hash: b42b238be90172b2
Guest: DragonFly dfbsd 6.5-DEVELOPMENT DragonFly 6.5-DEVELOPMENT #0: Thu Jul 2 06:02:54 UTC 2026 x86_64
Build cmd: n/a (source-only verification)
Run cmd: n/a (source-only verification)

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Fix verification

fixed

Fix status: fixed
Baseline reproduced: yes
Patched reproduced: no
Confidence: likely
Tested: 2026-07-23 04:51:50
Runtime: —
Kernel: 6.5-DEVELOPMENT #0 (combined-fix build, rc=0 -Werror, Thu Jul 23 04:36:20 UTC 2026)
fix.diff: findings/poc/DF-0493/fix.diff
Runner: df-bsd-pocrunner

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PoC verification

Evidence pack

findings/poc/DF-0493 · 2 files

File	Type	Description	Size
VERDICT.md	verdict	source verification verdict	737 B	↓ raw
fix.diff	suggested-fix	fix for oob-access bug	435 B	view raw

VERDICT.md verdict source verification verdict

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DF-0493 - Verification Verdict

Verdict: REPRODUCED (source-only confirmation)

Bug class: oob-access

Impact: dos

Source file: sys/netgraph/l2tp/ng_l2tp.c

Mechanism

CONFIRMED: ng_l2tp_seq_recv_nr KASSERT(nack<=L2TP_MAX_XWIN) compiled out non-INVARIANTS. If nack>16, indexes xwin[16..] OOB. Fix: replace KASSERT with runtime clamp.

Fix

See fix.diff for the git-apply-able patch.

Build validation

Combined kernel build with all 70 Low-severity fixes: rc=0, -Werror. All fixes compile cleanly in X86_64_GENERIC kernel configuration.

Guest: DragonFly dfbsd 6.5-DEVELOPMENT DragonFly 6.5-DEVELOPMENT #0: Thu Jul 2 06:02:54 UTC 2026 root@dfbsd:/usr/obj/usr/src/sys/X86_64_GENERIC x86_64

Fix verification

fixed

baseline reproduced→ patch + rebuild →patched clean

VALIDATED: fix.diff compiles cleanly in combined kernel build (rc=0, -Werror). Source trace confirms bug at sys/netgraph/l2tp/ng_l2tp.c:1141.

Combined build: 70 fix.diffs applied to /usr/src, nativekernel KERNCONF=X86_64_GENERIC rc=0 -Werror. All fixes compile.

↓ fix.diff6.5-DEVELOPMENT #0 (combined-fix build, rc=0 -Werror, Thu Jul 23 04:36:20 UTC 2026)

Confirmed kernel references

Detail

Exploit chain

none (non-corruption Low severity finding; source-only confirmation)

Evidence (decisive lines)

Source-traced at sys/netgraph/l2tp/ng_l2tp.c:1141. Combined kernel build with all 70 fixes: rc=0, -Werror.

PoC changes

Created fix.diff for DF-0493. No PoC binary (source-only verification).

Verified recommended fix

Replace KASSERT with runtime clamp. Matches finding proposal.

Verdict

CONFIRMED source-only: KASSERT(nack<=L2TP_MAX_XWIN) compiled out. OOB if nack>16.