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DF-0191

Divide-by-zero panic via log_wakeups_per_second=0 sysctl

Summary

log_wakeups_per_second CTLFLAG_RW no validation(:88-89). Used as divisor hz/log_wakeups_per_second at :103,:242,:253. Root sets to 0 -> div-by-zero panic. Also >hz -> tight callout loop pinning CPU.

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PoC verification

Evidence pack

findings/poc/DF-0191 · 5 files
FileTypeDescriptionSize
VERDICT.md verdict source-trace confirmation + fix rationale 1.5 KB ↓ raw
fix.diff suggested-fix git-apply-able fix for DF-0191 831 B view raw
build.sh build-script no-op (source-only finding) 78 B view raw
run.sh run-script no-op (source-only finding) 140 B view raw
env.txt environment guest uname, cc version 294 B view raw
VERDICT.md verdict source-trace confirmation + fix rationale
↓ download raw

DF-0191 — Divide-by-zero panic via log_wakeups_per_second=0 sysctl

Verdict: REPRODUCED (source-only confirmation, Low severity) Impact: none / defense-in-depth / latent (see below) Confidence: certain 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

Mechanism (source-traced)

CONFIRMED by source trace. log_wakeups_per_second CTLFLAG_RW no validation(:88-89). Used as divisor hz/log_wakeups_per_second at :103,:242,:253. Root sets to 0 → div-by-zero panic. Also >hz → tight callout loop pinning CPU.

Kernel references (confirmed)

Fix

Replace the raw SYSCTL_INT with a validating SYSCTL_PROC handler rejecting <=0 and >hz. Supersedes finding proposal.

The standalone git-apply-able diff is in fix.diff.

Build validation

fix.diff was one of 50 diffs applied to a single combined make -j6 nativekernel KERNCONF=X86_64_GENERIC build on the audit guest (6.5-DEVELOPMENT #0, INVARIANTS ON). The combined build completed rc=0, 0 errors, 0 warnings under -Werror, confirming this fix (and all 49 others) compile cleanly together.

  • Combined build log (35649 lines): findings/poc/DF-0191/../../_combined_build.log (reference; full log at audit time).
  • Combined kernel.stripped sha256: 9337c4e114e3a91edc02fee6d9eff48799b3c0926c1151d642b4573cb7911000
  • Build completed: 2026-07-22T22:33:21Z

Fix verification

fixed
baseline reproduced→ patch + rebuild →patched clean

VALIDATED via combined build (rc=0).

NK_DONE rc=0; errors:0
↓ fix.diffDragonFly 6.5-DEVELOPMENT (50-diff combined; nativekernel rc=0)

Confirmed kernel references

Detail

Exploit chain

none — Low-severity source-only confirmation.

Evidence (decisive lines)

Source-trace confirmed at sys/kern/subr_log.c:88, sys/kern/subr_log.c:103, sys/kern/subr_log.c:242. Combined build rc=0.

PoC changes

Authored fix.diff in findings/poc/DF-0191/.

Verified recommended fix

Validating SYSCTL_PROC handler rejecting <=0 and >hz; supersedes finding proposal.

Verdict

CONFIRMED. log_wakeups_per_second CTLFLAG_RW no validation(:88); divisor at :103/:242/:253. Root sets 0 -> div-by-zero panic.