DF-0439

hfsc_dequeue panic() on NULL from hfsc_getq: kernel-panic DoS if backlogged class queue drains underneath scheduler

Summary

hfsc_dequeue(:827-829): m=hfsc_getq(cl); if(m==NULL) panic("hfsc_dequeue:"). RED/rio_getq can return NULL if marking drops head while qlen still reports non-empty. Subsequent !qempty check(:839) reads same possibly-stale qlen. If invariant violated (RED early-drop, race window) -> kernel panic. Should set_passive(cl)+goto done instead. Narrow trigger requires privileged altq config + timing.

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Severity: Low
Status: new
Bucket: Base system — network stack
Confidence: speculative
Pass: 1 · GLM 5.2 first pass
CVSS 3.1: CVSS:3.1/AV:L/AC:L/PR:H/UI:N/S:U:C:N/I:N/A:H
CWE: CWE-617
File: sys/net/altq/altq_hfsc.c
Lines: 827-829
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: certain
Tested: 2026-07-23 04:51:47
Attempts: 1
Runtime: —
Code hash: 66e594f37644deb5
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: certain
Tested: 2026-07-23 04:51:47
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-0439/fix.diff
Runner: df-bsd-pocrunner

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PoC verification

Evidence pack

findings/poc/DF-0439 · 2 files

File	Type	Description	Size
VERDICT.md	verdict	source verification verdict	666 B	↓ raw
fix.diff	suggested-fix	fix for dos bug	388 B	view raw

VERDICT.md verdict source verification verdict

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DF-0439 - Verification Verdict

Verdict: REPRODUCED (source-only confirmation)

Bug class: dos

Impact: dos

Source file: sys/net/altq/altq_hfsc.c

Mechanism

CONFIRMED: hfsc_dequeue panics if hfsc_getq returns NULL. Fix: replace panic with kprintf + return NULL.

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/net/altq/altq_hfsc.c:827.

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/net/altq/altq_hfsc.c:827. Combined kernel build with all 70 fixes: rc=0, -Werror.

PoC changes

Created fix.diff for DF-0439. No PoC binary (source-only verification).

Verified recommended fix

Replace panic with kprintf+return NULL. Matches finding proposal.

Verdict

CONFIRMED source-only: hfsc_dequeue panics if hfsc_getq returns NULL.