DF-4830

mdcreate unit allocation (mdunits++) is unlocked and non-atomic — concurrent mdopen auto-create of the top unit can mint two md devices with the same unit number/cdev name

Summary

No global lock around mdunits++; mdopen auto-create is the only multi-threaded caller - two CPUs opening the top unit can both read mdunits==N, producing two same-named /dev/mdN cdevs, duplicate devstat entries, and double dispose at cleanup. Operator-group open-only trigger, tiny window. Serialize-creation fix.

Discussion (0)

No comments yet.

Severity: Low
Status: new
Bucket: Base system — drivers
Confidence: likely
Pass: 2 · GLM 5.3 second pass
CVSS 3.1: CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:N/I:L/A:L
CWE: CWE-362
File: sys/dev/disk/md/md.c
Lines: 135-410
Area: dev
Discovered: 2026-09-24
Updated: 2026-09-24

Team triage

untriaged

priority: — · owner: —

PoC verification

untested

Status: untested
Reproduced: no
Impact: none
Confidence: likely
Tested: 2026-09-24 08:41:58
Attempts: 0
Runtime: —
Code hash: —
Guest: —
Build cmd: —
Run cmd: —

jump to evidence pack ↓

Fix verification

not_testable

Fix status: not_testable
Baseline reproduced: —
Patched reproduced: —
Confidence: likely
Tested: 2026-09-24 08:41:58
Runtime: —
Kernel: per-fix-DF-4830
fix.diff: findings/poc/DF-4830/fix.diff
Runner: df-bsd-reviewer

jump to fix verification ↓

PoC verification

Evidence pack

findings/poc/DF-4830 · 1 files

File	Type	Description	Size
verdict.json	file		1.1 KB	view raw

Fix verification

not_testable

per-fix-DF-4830

Confirmed kernel references

Detail

Exploit chain

N processes on separate CPUs loop open(/dev/md) -> duplicate-named units -> devstat duplication and double dispose at cleanup

Evidence (decisive lines)

findings/poc/DF-4830/ (this verdict)

Verdict

SMP race, low impact: sc->unit = mdunits++ at md.c:410 has no global lock - the per-unit token serializes nothing global, and mdopen's auto-create (:135-136) is the only multi-threaded caller. Two CPUs opening the top unit can both read mdunits==N, minting two md_s with the same unit/cdev name (devfs never dedups), duplicate devstat entries, and later double dispose of the shared name/minor space at cleanup. Operator-group trigger (open only), tiny window, uncertain downstream - Low. The growth aspect of auto-create was re-confirmed live on the guest (opening /dev/md0 created md1). Distinct from DF-2477/2478. Serialize-creation fix.