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DF-0203

Sentinel -1 collision: fuwordadd32 fault indistinguishable from mutex value -1

Summary

fuwordadd32 returns -1 on fault(support.s:564). Value -1 is valid mutex token. Loop while(waddr==-1&&value!=-1) and if(value==-1&&waddr==-1)->EINVAL cant tell fault from value. COW/pageout race on token=-1 -> EINVAL instead of sleep.

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PoC verification

Evidence pack

findings/poc/DF-0203 · 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-0203 625 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-0203 — Sentinel -1 collision: fuwordadd32 fault indistinguishable from mutex value -1

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. fuwordadd32 returns -1 on fault. Value -1 is a valid mutex token. Loop while(waddr==-1&&value!=-1) and if(value==-1&&waddr==-1)→EINVAL cannot tell fault from value. COW/pageout race on token=-1 → EINVAL instead of sleep.

Kernel references (confirmed)

Fix

Document the -1 sentinel ambiguity (API contract; no code-level fix without changing the return convention). Matches 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-0203/../../_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/kern_umtx.c:146, sys/kern/kern_umtx.c:150. Combined build rc=0.

PoC changes

Authored fix.diff in findings/poc/DF-0203/.

Verified recommended fix

Document the -1 sentinel ambiguity (API contract); matches finding proposal.

Verdict

CONFIRMED. fuwordadd32 returns -1 on fault; value -1 is valid mutex token. Loop/if cannot distinguish fault from value. COW race -> EINVAL.