Summary

Interrupt-IN callback reassembles partial touchscreen packets from 17-byte UNINITIALIZED stack buffer buf[17]. reassembly memcpy at :226 copies res=pkt_len-sc->buf_len bytes out of buf without checking current transfer delivered res valid bytes. Short transfer arrives(len==1 legal short_xfer_ok=1 :283) res exceeds len memcpy reads uninitialized kernel stack. Packet unconditionally processed 5 raw bytes pushed to RX FIFO(:391) readable by any user opening /dev/uep*(mode 0644 group operator :331). Attacker device sends two 1-byte transfers first 0x80 sets sc->buf_len=1 second triggers reassembly res=4 reads buf[0-3] only buf[0] initialized leaks 3 stack bytes per cycle. REPORT path(0x80/0x81) leaks to userspace DIAG(0x0a) rejected.

PoC verification

Evidence pack

findings/poc/DF-2608 · 5 files

File	Type	Description	Size
VERDICT.md	verdict	analysis: code bug confirmed, gate confirmed, fix authored	1.6 KB	↓ raw
fix.diff	suggested-fix	git-apply-able defense-in-depth fix (verified --check clean)	467 B	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-2608 — PoC verdict

File: sys/bus/u4b/input/uep.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)

The interrupt-IN callback reassembles packets from a 17-byte UNINITIALIZED stack buffer buf[17]. On reassembly, res=pkt_len-sc->buf_len bytes are memcpy'd out of buf; a legal short transfer (len==1) makes res exceed the bytes actually delivered by usbd_copy_out, so uninitialized kernel stack is copied in and pushed to the RX FIFO, readable via /dev/uep*.

Cited lines

Why it does not reproduce on this guest

No USB eGalax touchscreen on the QEMU guest (usbconfig list shows no devices; no /dev/uep*).

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

Defense-in-depth fix

Clamp the reassembly count to what was actually received: if res>len, goto tr_setup (drop) instead of reading uninitialized stack.

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-2608/fix.diff -> OK. No runtime test possible (HW/ACPI/device-gated).

↓ fix.diffper-fix-DF-2608

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-2608/{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-2608/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): uep uninitialized kernel stack memory leaked to user (no USB eGalax touchscreen). 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.

Uninitialized kernel stack memory leaked to userspace via USB FIFO in packet reassembly