DF-2339

Stack buffer overflow in cue_setmulti: 8-byte hashtbl indexed by 9-bit multicast hash

Summary

cue_setmulti sizes local hash table to 8 bytes (uint8_t hashtbl[8] at if_cue.c:311) but CATC EL1210A uses 512-bit/64-byte multicast hash table (if_cuereg.h:97) and CUE_BITS is 9 (if_cue.c:273) so cue_mchash returns 0..511. Statements hashtbl[h>>3]|=1<<(h&0x7) (:329 :338) index byte 0..63 into 8-byte stack array writing up to 56 bytes past end. Broadcast ff:ff:ff:ff:ff:ff hashes to 255 (h>>3=31) so overflow triggered deterministically and unconditionally on every cue_setmulti while IFF_BROADCAST set (default for Ethernet). cue_setmulti reached from cue_init on ifconfig up and from SIOCADDMULTI/SIOCDELMULTI ioctls via any local user setsockopt(IP_ADD_MEMBERSHIP). |= bit-set at attacker-influenced byte offsets [8..63] and bit positions [0..7]. By choosing multicast IP whose MAC hashes to desired 9-bit value local attacker selects corrupted stack offset and bit. On stack-protected kernel immediate panic DoS. Without canary corrupts saved frame pointer/return address local unpriv->kernel code execution. IP_ADD_MEMBERSHIP needs no privilege.

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Severity: High
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:L/UI:N/S:U/C:H/I:H/A:H
CWE: CWE-787 Out-of-bounds Write
File: sys/bus/u4b/net/if_cue.c
Lines: 273-341
Area: hw
Discovered: 2026-07-28
Updated: 2026-07-28

Team triage

untriaged

priority: — · owner: —

PoC verification

not_reproduced

Status: not_reproduced
Reproduced: no
Impact: none
Confidence: certain
Tested: 2026-08-09 14:46:05
Attempts: 1
Runtime: 1s
Code hash: —
Guest: DragonFly dfbsd 6.5-DEVELOPMENT #0: Thu Jul 2 06:02:54 UTC 2026 root@dfbsd:/usr/obj/usr/src/sys/X86_64_GENERIC x86_64
Build cmd: n/a (HW/ACPI/device-gated)
Run cmd: ./run.sh # gate check; target device absent

jump to evidence pack ↓

Fix verification

not_testable

Fix status: not_testable
Baseline reproduced: no
Patched reproduced: no
Confidence: certain
Tested: 2026-08-09 14:46:05
Runtime: —
Kernel: per-fix-DF-2339
fix.diff: findings/poc/DF-2339/fix.diff
Runner: df-bsd-pocrunner

jump to fix verification ↓

PoC verification

Evidence pack

findings/poc/DF-2339 · 5 files

File	Type	Description	Size
VERDICT.md	verdict	analysis: code bug confirmed, gate confirmed, fix authored	1.7 KB	↓ raw
fix.diff	suggested-fix	git-apply-able defense-in-depth fix (verified --check clean)	1.1 KB	view raw
env.txt	environment	guest gate-proof: usbconfig/pciconf/ifconfig/devinfo/sysctl output	2.0 KB	view raw
build.sh	build-script	no-op (no live-device PoC)	255 B	view raw
run.sh	run-script	gate check (required device is absent)	333 B	view raw

VERDICT.md verdict analysis: code bug confirmed, gate confirmed, fix authored

↓ download raw

DF-2339 — PoC verdict

File: sys/bus/u4b/net/if_cue.c

Verdict: NOT REPRODUCED on this QEMU guest — confirmed HW/ACPI/device-gated; the code bug is REAL and a defense-in-depth fix.diff is attached.

Mechanism (confirmed in source)

cue_setmulti sizes the local hash table to 8 bytes (uint8_t hashtbl[8]) but CUE_BITS=9 makes cue_mchash return 0..511 for the device 512-bit/64-byte table (if_cuereg.h:96). hashtbl[h>>3] therefore indexes 0..63 into an 8-byte stack array (up to 56 bytes past end); broadcast hashes to 255, so it overflows deterministically.

Cited lines

Why it does not reproduce on this guest

No CATC EL1210A USB-ethernet adapter on the QEMU guest (usbconfig list shows no devices; only ehci/xhci host-controller modules loaded; no /dev/cue*).

Guest gate-proof (full usbconfig/pciconf/ifconfig/devinfo/sysctl/kldstat output) is in env.txt.

Defense-in-depth fix

Add CUE_HASH_TABLE_LEN=64, size hashtbl[CUE_HASH_TABLE_LEN], fill 64 bytes in the ALLMULTI branch, and write 64 bytes via cue_mem.

The git-apply-able diff is in fix.diff (verified git apply --check clean).

Classification

status: not_reproduced
reproduced: 0
impact: none (not reachable on this guest; latent code bug confirmed in source)
confidence: certain (code bug + gate both confirmed by direct source trace and guest enumeration)
fix_status: not_testable (patch applies + compiles-correct by inspection, but no live device to exercise on this guest)

Fix verification

not_testable

baseline no→ patch + rebuild →patched clean

not_testable: target device absent on this guest. fix.diff applies clean and is source-correct; no live device to exercise.

git apply --check findings/poc/DF-2339/fix.diff -> OK. No runtime test possible (HW/ACPI/device-gated).

↓ fix.diffper-fix-DF-2339

Confirmed kernel references

Detail

Exploit chain

none — valid hard blocker (driver/device path dead at runtime on this guest: no target HW/ACPI/device). No unprivileged->root path.

Evidence (decisive lines)

usbconfig list -> No device match; pciconf -l -> no target HW/capability; ifconfig -> vtnet0 lo0; kldstat -> kernel/ehci/xhci only; sysctl/devinfo -> no target OIDs. Source confirmed at cited lines.

PoC changes

Created findings/poc/DF-2339/{VERDICT.md,fix.diff,manifest.json,env.txt,build.sh,run.sh}. No PoC source (HW/ACPI/device-gated).

Verified recommended fix

Defense-in-depth fix.diff closes the cited path (see findings/poc/DF-2339/fix.diff; git apply --check OK).

Verdict

NOT REPRODUCED (HW/ACPI/device-gated on this guest). The bug is REAL in source (traced line-by-line): cue_setmulti hashtbl[8] indexed by 9-bit hash -> stack overflow (no CATC USB-ethernet). Gate confirmed via usbconfig list (no devices), pciconf -l (no target HW/capability), ifconfig (vtnet0 lo0 only), kldstat (no target module), sysctl/devinfo (no target ACPI/device OIDs). The benign QEMU environment cannot produce the malicious device/ACPI/descriptor the bug requires.