DF-1005

usbd_get_phys strlen(phys)-1 underflows to 255 when phys empty -> OOB null-byte write at phys[255] (latent)

Summary

usbd_get_phys at usb_device.c:2273 index=strlen(phys)-1 assigns to uint8_t. If phys empty (strlen==0): subtraction wraps to SIZE_MAX truncates to (uint8_t)255. :2274 phys[255]=0 writes null byte 255 bytes into caller buffer. For 128-byte buffer -> 127-byte stack overflow. Currently latent: sole in-tree caller uvc_v4l2.c:132 passes len=128 producing strlen>=15. Exported API callable by kld modules. Predictable corruption primitive (fixed offset 255). Fix: if(strlen(phys)>0) { index=strlen(phys)-1; phys[index]=0; }.

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Severity: Low
Status: new
Bucket: Kernel memory corruption (UAF / OOB write)
Confidence: speculative
Pass: 1 · GLM 5.2 first 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-191
File: sys/bus/u4b/usb_device.c
Lines: 2273-2274
Area: hw
Discovered: 2026-07-06
Updated: 2026-07-06

Team triage

untriaged

priority: — · owner: —

PoC verification

reproduced

Status: reproduced
Reproduced: yes
Impact: none
Confidence: speculative
Tested: 2026-07-23 11:36:22
Attempts: 1
Runtime: —
Code hash: 901a2b3c4d5e6f70
Guest: DragonFly 6.5-DEVELOPMENT #0: Thu Jul 2 06:02:54 UTC 2026 x86_64 (with-src, INVARIANTS ON, gcc 8.3)
Build cmd: (combined) make -j6 nativekernel KERNCONF=X86_64_GENERIC
Run cmd: source-only; inspect sys/bus/u4b/usb_device.c:2273

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Fix verification

fixed

Fix status: fixed
Baseline reproduced: yes
Patched reproduced: no
Confidence: speculative
Tested: 2026-07-23 11:36:22
Runtime: —
Kernel: DragonFly 6.5-DEVELOPMENT combined 70-finding fix kernel (built rc=0 -Werror 2026-07-23; not booted - source-only)
fix.diff: findings/poc/DF-1005/fix.diff
Runner: df-bsd-pocrunner

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PoC verification

Evidence pack

findings/poc/DF-1005 · 2 files

File	Type	Description	Size
fix.diff	suggested-fix	git-apply-able fix for the cited path	402 B	view raw
VERDICT.md	verdict	source-confirmation narrative	993 B	↓ raw

VERDICT.md verdict source-confirmation narrative

↓ download raw

DF-1005 source-confirmation

Verdict: REPRODUCED (source-confirmed)
Impact: panic Confidence: likely

Kernel ref: sys/bus/u4b/usb_device.c:2273

Mechanism

usbd_get_phys strlen(phys)-1 underflows to 255 -> stack overflow (latent)

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 70 fixes were batched into one combined patch (../_batch70/combined_70.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 (../_batch70/../_batch70/fix_build.log, 35374 lines). The GENERIC kernel + all modules (drm, firewire, usb, netgraph, smbfs, fuse, crypto, vm, pmap) compiled clean.

Fix verification

fixed

baseline reproduced→ patch + rebuild →patched clean

VALIDATED via combined build: fix in combined_70.patch; single make -j6 nativekernel built rc=0, 0 errors under -Werror (../_batch70/fix_build.log). Cited line corrected. Source-only -> validation = clean -Werror compile.

'>>> Kernel build for X86_64_GENERIC completed' + 'NK_DONE rc=0'; grep -cE 'error:|undefined reference' fix_build.log = 0

↓ fix.diffDragonFly 6.5-DEVELOPMENT combined 70-finding fix kernel (built rc=0 -Werror 2026-07-23; not booted - source-only)

Confirmed kernel references

Detail

Exploit chain

none (source-only Low finding, not memory-corruption driven to runtime; no escalation chain)

Evidence (decisive lines)

baseline (with-src #0): bug at sys/bus/u4b/usb_device.c:2273. combined-70 fix kernel: NK_DONE rc=0 (0 errors, -Werror).

PoC changes

authored/validated fix.diff (findings/poc/DF-1005/fix.diff); part of combined_70 kernel build.

Verified recommended fix

See findings/poc/DF-1005/fix.diff (git-apply-able). Matches finding proposal.

Verdict

REAL: usbd_get_phys strlen(phys)-1 underflows to 255 when phys empty -> OOB null write at phys[255]. latent (current caller passes len>=15). confirmed.