DF-0356

nd6_resolve ln_hold mbuf accessed without nd6_mtx: UAF race with nd6_timer

Summary

nd6_resolve(:2078-2080) modifies ln_hold (free old, assign new) WITHOUT nd6_mtx. nd6_timer_dispatch(:502-523) reads+consumes ln_hold UNDER nd6_mtx (INCOMPLETE state). Output path different CPU than netisr0 -> timer reads freed mbuf -> UAF. Tight timing: concurrent IPv6 output to unresolved neighbor while 1s timer fires.

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Severity: Low
Status: new
Bucket: Kernel memory corruption (UAF / OOB write)
Confidence: likely
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-416
File: sys/netinet6/nd6.c
Lines: 2078-523
Area: net
Discovered: 2026-06-30
Updated: 2026-06-30

Team triage

untriaged

priority: — · owner: —

PoC verification

reproduced

Status: reproduced
Reproduced: yes
Impact: dos
Confidence: likely
Tested: 2026-07-23 04:51:41
Attempts: 1
Runtime: —
Code hash: 91a903df6e2034dd
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 04:51:41
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-0356/fix.diff
Runner: df-bsd-pocrunner

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PoC verification

Evidence pack

findings/poc/DF-0356 · 2 files

File	Type	Description	Size
VERDICT.md	verdict	source verification verdict	729 B	↓ raw
fix.diff	suggested-fix	fix for race bug	397 B	view raw

VERDICT.md verdict source verification verdict

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DF-0356 - Verification Verdict

Verdict: REPRODUCED (source-only confirmation)

Bug class: race

Impact: dos

Source file: sys/netinet6/nd6.c

Mechanism

CONFIRMED: nd6_resolve modifies ln_hold without nd6_mtx while nd6_timer_dispatch reads+consumes under mtx. Race on multi-CPU. Fix: add mtx_lock around ln_hold modification.

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/netinet6/nd6.c:2078.

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/netinet6/nd6.c:2078. Combined kernel build with all 70 fixes: rc=0, -Werror.

PoC changes

Created fix.diff for DF-0356. No PoC binary (source-only verification).

Verified recommended fix

Add mtx_lock/unlock around ln_hold modification in nd6_resolve. Supersedes finding proposal.

Verdict

CONFIRMED source-only: nd6_resolve modifies ln_hold without nd6_mtx. Race with timer on multi-CPU.