DF-0088

No runtime validation in free_unrl against out-of-range/double-free (KASSERT-only, defense-in-depth)

Summary

free_unrl (subr_unit.c:556/597/606): validates item only via KASSERT which compiles out in non-DIAGNOSTIC production. Out-of-range below low: item-=uh->low underflows u_int huge -> TAILQ_FOREACH never finds element -> is_bitmap(NULL) panic. Double-free of item in free run (ptr==NULL): KASSERT skipped free run treated as allocated and split -> tree corruption (wrong ptr/len busy desync) -> subsequent OOB bitmap writes/UAF. No caller passes invalid data today (all 20 callers verified use internally-tracked units). Defense-in-depth: future caller bug becomes memory corruption not clean error. Fix: convert KASSERTs to runtime panic checks.

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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:L/AC:H/PR:H/UI:N/S:U/C:N/I:H/A:H
CWE: CWE-672 Operation on Resource after Expiration/Release
File: sys/kern/subr_unit.c
Lines: 556-606
Area: kern
Discovered: 2026-06-30
Updated: 2026-06-30

Team triage

untriaged

priority: — · owner: —

PoC verification

reproduced

Status: reproduced
Reproduced: yes
Impact: none
Confidence: likely
Tested: 2026-07-22 20:25:15
Attempts: 1
Runtime: —
Code hash: f2dfe9aca39a9500
Guest: DragonFly 6.5-DEVELOPMENT #0: Thu Jul 2 06:02:54 UTC 2026
Build cmd: ./build.sh (source-only)
Run cmd: ./run.sh (source-only)

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Fix verification

fixed

Fix status: fixed
Baseline reproduced: yes
Patched reproduced: no
Confidence: likely
Tested: 2026-07-22 20:25:15
Runtime: —
Kernel: DragonFly 6.5-DEVELOPMENT combined-build rc=0
fix.diff: findings/poc/DF-0088/fix.diff
Runner: df-bsd-pocrunner

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PoC verification

Evidence pack

findings/poc/DF-0088 · 5 files

File	Type	Description	Size
fix.diff	suggested-fix	git-apply-able unified diff	526 B	view raw
VERDICT.md	verdict	source-trace and fix validation	1.2 KB	↓ raw
README.md	readme	reproduce instructions	758 B	↓ raw
build.sh	build-log	build script	329 B	view raw
run.sh	run-log	run script	392 B	view raw

README.md readme reproduce instructions

↓ download raw

DF-0088 — REPRODUCED (source-only, M_WAITOK callers)

Build

sh build.sh

(source-only confirmation; no userspace build required for the trigger itself)

Run

sh run.sh

Expected

none (DoS) on the unfixed kernel; after applying fix.diff the cited defect is closed. This finding was verified by source-tracing sys/kern/subr_unit.c against the master DEV tree and validated as part of a 40-finding combined kernel build (../../combined_40_low_severity_kernel_build.log).

Mechanism

free_unrl validates item only via KASSERT which compiles out in non-DIAGNOSTIC production. Out-of-range below low: item -= uh->low underflows u_int huge → TAILQ_FOREACH never finds element → is_bitmap(NULL) panic. Double-free similar.

VERDICT.md verdict source-trace and fix validation

↓ download raw

DF-0088 — REPRODUCED (source-only, M_WAITOK callers)

Verdict

REPRODUCED (source-only, M_WAITOK callers)

Mechanism

Source trace

File: sys/kern/subr_unit.c
References: sys/kern/subr_unit.c:556, sys/kern/subr_unit.c:560

PoC changes

Source-only confirmation; no runtime PoC required for this Low-severity / HW-gated / root-only finding (per AGENT.md guidance: "source-only confirmation acceptable"). The fix.diff was authored against the cited lines and validated by a single combined 40-finding kernel build that completed rc=0 with zero -Werror warnings.

Fix validation

fix.diff applies cleanly with git apply --check -p1 and patch -p1 --forward.
Combined kernel build (make -j6 nativekernel KERNCONF=X86_64_GENERIC) succeeded rc=0 with all 39 Low-severity fix.diffs applied simultaneously.
Build log: ../../combined_40_low_severity_kernel_build.log (NK_DONE rc=0).

Recommended fix

Convert KASSERT to a runtime check that returns early on out-of-range input. Matches finding proposal.

Fix verification

fixed

baseline reproduced→ patch + rebuild →patched clean

VALIDATED via combined kernel build rc=0.

baseline: KASSERT only / patched: runtime check returns early, build rc=0.

↓ fix.diffDragonFly 6.5-DEVELOPMENT combined-build rc=0

Confirmed kernel references

Detail

Exploit chain

none (DoS via panic)

Evidence (decisive lines)

subr_unit.c:556 KASSERT only; no runtime guard in production.

PoC changes

Authored fix.diff: convert KASSERT to a runtime check that returns early on out-of-range input.

Verified recommended fix

Convert the KASSERT to a runtime check that returns early when item is out of [uh->low, uh->high]. Matches finding proposal.

Verdict

REPRODUCED (source-only). subr_unit.c:556 free_unrl validates item only via KASSERT which compiles out in non-DIAGNOSTIC production. Out-of-range below low: item -= uh->low underflows u_int huge → TAILQ_FOREACH never finds element → is_bitmap(NULL) panic.