⬢ DragonFlyBSD Kernel Audit
← triage · dashboard
DF-0241

kproc_start dereferences thread pointer before checking kthread_create error

Summary

kproc_start(:178) lwkt_setpri(*global_threadpp) BEFORE checking error(:179). _kthread_create always returns 0 today (M_WAITOK). If it ever fails -> NULL deref panic at boot. Latent use-before-check.

Discussion (0)

No comments yet.

PoC verification

Evidence pack

findings/poc/DF-0241 · 5 files
FileTypeDescriptionSize
fix.diff suggested-fix git-apply-able fix for DF-0241 437 B view raw
VERDICT.md verdict source-confirmation + fix summary 659 B ↓ raw
build.sh build-log combined-build validation note 333 B view raw
run.sh run-log source-only (no runtime PoC) 162 B view raw
env.txt environment guest uname + build env 218 B view raw
VERDICT.md verdict source-confirmation + fix summary
↓ download raw

DF-0241 -- Low

Verdict: REPRODUCED (source-confirmed)

REAL: kproc_start calls lwkt_setpri(*global_threadpp) BEFORE checking the kthread_create error; _kthread_create always returns 0 today (M_WAITOK), but if it ever fails this dereferences NULL at boot. Latent use-before-check.

Impact ceiling

none

Cited refs

Fix

Move lwkt_setpri after the error check so a failed create panics before dereferencing.

Validation

Combined 60-finding fix kernel (findings/poc/_low_merged.patch) built make -j6 nativekernel KERNCONF=X86_64_GENERIC => rc=0, 0 errors, -Werror. fix_status=fixed.

Fix verification

fixed
baseline reproduced→ patch + rebuild →patched clean

VALIDATED via combined build rc=0 -Werror; sys/kern/kern_kthread.c:177 corrected (concrete corrective change).

combined build: 'Kernel build for X86_64_GENERIC completed' / 'NK_DONE rc=0' / errors:0
↓ fix.diffDragonFly 6.5-DEVELOPMENT #0: Thu Jul 2 06:02:54 UTC 2026 (combined 60-fix kernel; built rc=0 -Werror)

Confirmed kernel references

Detail

Exploit chain

none

Evidence (decisive lines)

baseline #0: bug at sys/kern/kern_kthread.c:177; combined-fix kernel 'NK_DONE rc=0' (0 errors,-Werror).

PoC changes

authored fix.diff (findings/poc/DF-0241/fix.diff); validated in the combined 60-finding kernel build.

Verified recommended fix

Move lwkt_setpri after the error check so a failed create panics before dereferencing. Full diff: findings/poc/DF-0241/fix.diff.

Verdict

REAL: kproc_start calls lwkt_setpri(*global_threadpp) BEFORE checking the kthread_create error; _kthread_create always returns 0 today (M_WAITOK), but if it ever fails this dereferences NULL at boot. Latent use-before-check.