DF-0066

Undefined behavior: shift by sysctl-controlled token_window_shift in backoff spin

Summary

_lwkt_trytokref_spin (lwkt_token.c:400/:413): rdtsc()>>(int)token_window_shift; token_window_shift writable via SYSCTL_INT CTLFLAG_RW (:157-158). rdtsc returns tsc_uclock_t 64-bit unsigned. If root sets negative or >=64, shift is UB (C11 6.5.7). Impact: unpredictable backoff-window (scheduling fairness) only, NOT memory corruption/info leak. Root-only. Fix: clamp [0,63] or mask &63.

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Severity: Info
Status: new
Bucket: Kernel memory leaks / uninitialized disclosure
Confidence: speculative
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:N/A:L
CWE: CWE-758 Reliance on Undefined Behavior
File: sys/kern/lwkt_token.c
Lines: 398-419
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: speculative
Tested: 2026-07-23 18:17:01
Attempts: —
Runtime: —
Code hash: —
Guest: DragonFly dfbsd 6.5-DEVELOPMENT #0
Build cmd: —
Run cmd: —

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Fix verification

fixed

Fix status: fixed
Baseline reproduced: yes
Patched reproduced: no
Confidence: speculative
Tested: 2026-07-23 18:17:01
Runtime: —
Kernel: combined build rc=0
fix.diff: findings/poc/DF-0066/fix.diff
Runner: df-bsd-pocrunner

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PoC verification

Evidence pack

findings/poc/DF-0066 · 1 files

File	Type	Description	Size
fix.diff	suggested-fix	Undefined behavior: shift by sysctl-controlled token_window_shift in backoff spi	379 B	view raw

Fix verification

fixed

baseline reproduced→ patch + rebuild →patched clean

fix.diff applied + combined nativekernel build rc=0 (-Werror)

fix.diff applied + combined nativekernel build rc=0 (-Werror)

↓ fix.diffcombined build rc=0

Confirmed kernel references

—

Detail

Exploit chain

none (Info severity)

Evidence (decisive lines)

Source-confirmed at sys/kern/lwkt_token.c:398: UB shift by sysctl-controlled token_window_shift

Verified recommended fix

Source-confirmed at sys/kern/lwkt_token.c:398: UB shift by sysctl-controlled token_window_shift

Verdict

Source-confirmed at sys/kern/lwkt_token.c:398: UB shift by sysctl-controlled token_window_shift