# DF-2490 — ATAPI PIO heap OOB write/read via unvalidated device byte_count

## Verdict
NOT REPRODUCED (hardware / trigger gated). The cited vulnerable code path exists
in `sys/dev/disk/nata/ata-lowlevel.c` and is compiled into the X86_64_GENERIC kernel, but it is **not
exercisable at runtime on this QEMU guest** because the required HBA / device /
trigger is absent. Confidence in the source bug itself: **certain**.

## Why it cannot be reproduced here (the gate)
Trigger-gated: the path is live (nata drives acd0) but the bug requires a MALICIOUS ATAPI device reporting an oversized CYL_LSB/CYL_MSB byte count; the benign QEMU DVD-ROM never reports a count exceeding the buffer, so donecount cannot overshoot bytecount.

See `env.txt` for the full guest PCI/kldstat/dev-node inventory that proves the
gate.

## The bug is real in source (traced line-by-line)
Cited path:
- sys/dev/disk/nata/ata-lowlevel.c:329
- sys/dev/disk/nata/ata-lowlevel.c:362-366
- sys/dev/disk/nata/ata-lowlevel.c:378-379
- The vulnerable construct is present verbatim in the current master source
  (confirmed by direct read of the cited lines during verification).
- A defense-in-depth `fix.diff` that closes the path is included and was
  validated to apply (`git apply --check`) **and** to compile cleanly in a full
  X86_64_GENERIC kernel build with `-Werror` (build rc=0).

## Fix
Cap the device-supplied per-DRQ length at the remaining request buffer (bytecount - donecount) so donecount can never be driven past bytecount and wrap transfersize.

The standalone git-apply-able diff is `fix.diff`.

## Reproduce
`./build.sh && ./run.sh` — both are no-ops on this guest by design (the gate
holds). Exercising the path requires the corresponding HBA/device/trigger
(ATA disk / ATAPI floppy or tape / NVMe controller / AdvanSys HBA / AHCI
controller / a malicious ATAPI device / a CD burner / hot-unplug).

## Status
status: not_reproduced  | reproduced: 0 | impact: none (HW-gated)
fix_status: not_testable (path cannot run on this guest; diff applies + compiles)