DF-0908

size_t underflow in trailing-slash strip causes OOB access on empty mount path

Summary

dirfs_vfsops.c:137 nlen=strnlen(dm_path,MAXPATHLEN). Empty path: nlen=0. :138 dm_path[nlen-1]=dm_path[SIZE_MAX] wraps to &dm_path[-1] = last byte of dm_gid. OOB read + conditional OOB write of NUL if byte==/. M_ZERO makes dm_gid=0 currently non-impactful but C UB latent. Fix: check nlen>0.

Discussion (0)

No comments yet.

Severity: Low
Status: new
Bucket: Kernel memory corruption (UAF / OOB write)
Confidence: certain
Pass: 1 · GLM 5.2 first pass
CVSS 3.1: CVSS:3.1/AV:L/AC:L/PR:H/UI:N/S:U:C:N/I:N/A:N
CWE: CWE-788 Out-of-bounds Write;CWE-125 Out-of-bounds Read
File: sys/vfs/dirfs/dirfs_vfsops.c
Lines: 137-139
Area: vfs
Discovered: 2026-07-05
Updated: 2026-07-05

Team triage

untriaged

priority: — · owner: —

PoC verification

reproduced

Status: reproduced
Reproduced: yes
Impact: none
Confidence: likely
Tested: 2026-07-23 09:36:06
Attempts: 1
Runtime: —
Code hash: aaa78a6b3a62d12d
Guest: DragonFly 6.5-DEVELOPMENT #0 master DEV (with-src, INVARIANTS ON, gcc 8.3) x86_64
Build cmd: n/a (source-only confirmation)
Run cmd: n/a (source-only confirmation)

jump to evidence pack ↓

Fix verification

not_testable

Fix status: not_testable
Baseline reproduced: yes
Patched reproduced: no
Confidence: likely
Tested: 2026-07-23 09:36:07
Runtime: —
Kernel: DragonFly 6.5-DEVELOPMENT single-fix combined kernel (nativekernel rc=0 -Werror, 2026-07-23 09:17:55 UTC)
fix.diff: findings/poc/DF-0908/fix.diff
Runner: df-bsd-pocrunner

jump to fix verification ↓

PoC verification

Evidence pack

findings/poc/DF-0908 · 2 files

File	Type	Description	Size
fix.diff	suggested-fix	git-apply-able fix for the cited path	447 B	view raw
VERDICT.md	verdict	source-confirmation narrative	959 B	↓ raw

VERDICT.md verdict source-confirmation narrative

↓ download raw

DF-0908 source-confirmation

Verdict: REPRODUCED (source-confirmed)
Impact: none Confidence: likely

Kernel ref: sys/vfs/dirfs/dirfs_vfsops.c:138

Mechanism

empty path: nlen=0 -> dm_path[SIZE_MAX] underflow OOB

Confirmation method

Source-trace confirmed the cited code path matches the finding (exact line/condition verified against sys/). Runtime PoC not exercised for this source-only Low-severity item; confirmation is by code inspection.

Recommended fix

See fix.diff in this folder (git-apply-able unified diff).

Phase 8 (combined build)

All 64 fixes were batched into one combined patch (../_batch/combined_64.patch) and applied to in-guest /usr/src. A single make -j6 nativekernel KERNCONF=X86_64_GENERIC completed rc=0 with 0 errors under -Werror (../_batch/fix_build.log, 35374 lines). The GENERIC kernel + all modules (drm, firewire, usb, netgraph, smbfs, fuse, crypto, vm, pmap) compiled clean.

Fix verification

not_testable

baseline reproduced→ patch + rebuild →patched clean

not_testable (source-only): no runtime PoC exercised; fix.diff applies cleanly and the combined single nativekernel build of all 64 fixes completed rc=0 with 0 errors under -Werror (findings/poc/_batch/fix_build.log). Bug source-confirmed in baseline tree. Compile-validation of the fix is the requested Phase-8 deliverable for this batch.

combined build: '=== NK_DONE rc=0 ===' and '>>> Kernel build for X86_64_GENERIC completed on Thu Jul 23 09:17:55 UTC 2026' (0 'error:' lines in 35374-line log).

↓ fix.diffDragonFly 6.5-DEVELOPMENT single-fix combined kernel (nativekernel rc=0 -Werror, 2026-07-23 09:17:55 UTC)

Confirmed kernel references

Detail

Exploit chain

none (non-corruption / source-only confirmation; no memory-corruption escalation chain developed for this Low-severity item)

Evidence (decisive lines)

source-only: empty path: nlen=0 -> dm_path[SIZE_MAX] underflow OOB @ sys/vfs/dirfs/dirfs_vfsops.c:138. Combined fix build: '=== NK_DONE rc=0 ===' / 'Kernel build for X86_64_GENERIC completed', 0 errors under -Werror (findings/poc/_batch/fix_build.log).

PoC changes

authored git-apply-able fix.diff targeting the cited line; source-trace verified the vulnerable path. Evidence pack (VERDICT.md, manifest.json, env.txt, fix.diff) in findings/poc/DF-0908/.

Verified recommended fix

guard dm_path[nlen-1] with nlen>0. matches finding proposal. Full diff in findings/poc/DF-0908/fix.diff.

Verdict

REPRODUCED (source-confirmed). Source-trace confirmed empty path: nlen=0 -> dm_path[SIZE_MAX] underflow OOB at sys/vfs/dirfs/dirfs_vfsops.c:138 against the audited sys/ tree; exact line/condition verified.