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DF-0204

umtx_wakeup ignores count argument: over-wakeup / cross-process thundering herd

Summary

sys_umtx_wakeup(:297-302) count==1 -> wakeup_one, else -> wakeup_domain ALL. count>1 wakes unlimited. XXX comment: wakes them all up for now. Shared shm page -> attacker mass-wakes victim threads with single call.

Discussion (0)

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PoC verification

Evidence pack

findings/poc/DF-0204 · 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-0204 551 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-0204 — umtx_wakeup ignores count argument: over-wakeup / thundering herd

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. sys_umtx_wakeup(:297-302) count==1 → wakeup_one, else → wakeup_domain ALL. count>1 wakes unlimited. XXX comment: wakes them all up for now. Shared shm page → attacker mass-wakes victim threads with a single call.

Kernel references (confirmed)

Fix

Treat count<=1 uniformly (wakeup_one); document the bounded-wakeup gap (sleepq API doesn't expose count-bounded wake). 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-0204/../../_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:297, sys/kern/kern_umtx.c:301. Combined build rc=0.

PoC changes

Authored fix.diff in findings/poc/DF-0204/.

Verified recommended fix

Treat count<=1 uniformly; document bounded-wakeup gap; matches finding proposal.

Verdict

CONFIRMED. sys_umtx_wakeup count==1->wakeup_one else wakeup_domain ALL(:301). count>1 wakes unlimited. XXX comment admits it.