Missing sched_priority bounds check in SCHED_OTHER + signed-overflow UB in p4prio_to_rtpprio
Summary
ksched_setscheduler validates sched_priority for RR/FIFO(:169-170) but NOT SCHED_OTHER(:181-184). p4prio_to_rtpprio(P)=RTP_PRIO_MAX-P=31-P. Extreme P -> signed overflow UB -> arbitrary u_short in lwp_rtprio.prio. NORMAL scheduler ignores prio currently. Latent.
Discussion (0)
PoC verification
Evidence pack
findings/poc/DF-0223 · 5 files| File | Type | Description | Size | |
|---|---|---|---|---|
| VERDICT.md | verdict | source-trace confirmation + fix rationale | 1.5 KB | ↓ raw |
| fix.diff | suggested-fix | git-apply-able fix for DF-0223 | 406 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 |
DF-0223 — Missing sched_priority bounds check in SCHED_OTHER + signed-overflow UB
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. ksched_setscheduler validates sched_priority for RR/FIFO(:169-170) but NOT SCHED_OTHER(:181-184). p4prio_to_rtpprio(P)=RTP_PRIO_MAX-P. Extreme P → signed overflow UB → arbitrary u_short in lwp_rtprio.prio. NORMAL scheduler ignores prio currently.
Kernel references (confirmed)
Fix
Bounds-check sched_priority for SCHED_OTHER, matching RR/FIFO. 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-0223/../../_combined_build.log(reference; full log at audit time). - Combined kernel.stripped sha256:
9337c4e114e3a91edc02fee6d9eff48799b3c0926c1151d642b4573cb7911000 - Build completed: 2026-07-22T22:33:21Z
Fix verification
fixedVALIDATED via combined build (rc=0).
NK_DONE rc=0; errors:0
Confirmed kernel references
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Detail
Exploit chain
none — Low-severity source-only confirmation.
Evidence (decisive lines)
Source-trace confirmed at sys/kern/kern_sched.c:169, sys/kern/kern_sched.c:181, sys/kern/kern_sched.c:183. Combined build rc=0.
PoC changes
Authored fix.diff in findings/poc/DF-0223/.
Verified recommended fix
Bounds-check sched_priority for SCHED_OTHER; supersedes finding proposal.
Verdict
CONFIRMED. ksched_setscheduler validates priority for RR/FIFO(:169) not SCHED_OTHER(:181). p4prio_to_rtpprio(P)=MAX-P; extreme P -> signed overflow UB -> arbitrary prio. NORMAL ignores prio.
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