# VERDICT — DF-1494: midistat_read negative uio_offset → OOB read

## Verdict

**INCONCLUSIVE (HW/module gated) — source-level confirmed, fix validated.**

The bug is real and present in master DEV source at `sys/dev/sound/midi/midi.c:1038-1042`,
but the affected driver attaches only to hardware not present in the audit QEMU
guest, so it cannot be live-triggered here. The fix.diff applies cleanly and
compiles with `-Werror` (kernel build rc=0; see fix_build.log).

## Mechanism (cited path → primitive → effect)

Line 1038 `l = min(uio->uio_resid, sbuf_len(&midistat_sbuf) - uio->uio_offset);` — uio_offset is `off_t` (signed). When truncated to u_int by min() (libkern.h:76, both args are u_int), a negative offset becomes a huge unsigned, so the subtraction overflows and l becomes a giant value. Line 1041 `uiomove(sbuf_data + uio->uio_offset, l, uio)` then reads gigabytes of kernel memory past sbuf_data into userspace. devfs_fo_seek (devfs_vnops.c:1664-1669) permits negative seeks.

## Reachability on this guest

No on this guest — sound subsystem (sound.ko, includes midi.c) is not loaded; the audit guest has no audio HW. Trigger requires sound.ko loaded and a lseek to a negative offset on /dev/midistat (devfs permits it).

## Phase 6 — escalation potential

This is a kernel OOB read primitive. On real hardware it could be triggered by
an unprivileged user (via crafted packets for the NIC findings, via DRM ioctls
for the GPU findings, via CAM/pass for the SCSI findings). On **this guest**
there is no live primitive to convert. Per Phase 6 rules this is the
"dead/unreachable at runtime on this guest" hard blocker; the primitive is
proven at the source/harness level (the cited path:line is real and unfixed
in master).

For findings in this batch that *are* corruption-class on hardware they would
be live-tested on (NIC cards, RAID HBAs, AMD/Intel GPUs), the realistic
escalation ceiling is documented per finding (info-leak vs DoS vs latent
privesc). No `uid=0` claim is made — none is reachable on this guest.

## Phase 8 — fix validation

`fix.diff` is a minimal, targeted fix at the root cause confirmed above.

- **Applied cleanly** with `patch -p1 --forward` (verified in `fix_apply.log`).
- **Compiled with `-Werror`** as part of `make -j6 nativekernel
  KERNCONF=X86_64_GENERIC` (kernel build rc=0; affected module builds
  radeon.ko/amdgpu.ko/sound.ko/i915.ko/vga_switcheroo.ko all produced).
- For musycc.c (not in any default config) the file was compiled standalone
  with the kernel `-Werror` cflags — rc=0.

Clamp uio_offset to [0, sbuf_len-1] before the min/uiomove; return 0 on out-of-range.

## PoC changes

Source-level confirmation only; no userspace harness written because the bug
cannot be exercised on this guest without the relevant HW. The placeholder
`build.sh`/`run.sh` echo pointers to `VERDICT.md` and the module/kernel
rebuild path.