# DF-4346 VERDICT — atom_index_iio OOB opcode index + unbounded walk (amdgpu twin of DF-4338)

**Status: inconclusive (kernel runtime hardware-gated; logic-level OOB
reproduced on the audit guest). Defect: certain.**

## Root cause

`atom_index_iio` (atom.c:1280-1294), byte-identical to the radeon twin
DF-4338 fixed:

```c
1287: while (CU8(base) == ATOM_IIO_START) {
1288:         ctx->iio[CU8(base + 1)] = base + 2;
1289:         base += 2;
1290:         while (CU8(base) != ATOM_IIO_END)
1291:                 base += atom_iio_len[CU8(base)];
1292:         base += 3;
1293: }
```

- `atom_iio_len` (atom.c:1280) has 10 entries (ATOM_IIO_NOP..ATOM_IIO_END
  = 0..9). The opcode byte `CU8(base)` is unvalidated VBIOS data
  (0..255): `atom_iio_len[CU8(base)]` reads up to 990 bytes past the
  static array in kernel .data (info leak into the walk, garbage stride).
- The garbage (or a crafted in-range) stride then drives `base` forward
  with no bound — the walk reads past the bios allocation at attach
  until it happens to hit ATOM_IIO_END or faults.

Runs at driver attach via `amdgpu_atom_parse` (atom.c:1334) on the
IIO pointer taken from the master data table.

## Reproduction

Guest (no AMD GPU): `./atom_harness iio` — verbatim loop, `atom_iio_len`
placed at a guard page. Opcode byte 200 → `atom_iio_len[200]` →
**SIGSEGV exit=139** (`run.log`, case `iio`). Fixed logic
(`./atom_harness_fixed iio`) rejects `op > ATOM_IIO_END` and bounds the
walk → exit 0 (`fix_run.log`).

## Fix

`fix.diff` — the standalone opcode-bound fix (same shape as DF-4338's):
validate the opcode against ATOM_IIO_END before indexing, and stop the
walk. The complete bound (walk also limited by bios_size) is included
in the central DF-4345 port.

## Fix status (batch validation)
Applied with the 36 sibling fix.diffs of this batch in findings/poc/_batch41/all_fixes.patch; single in-guest `make -j6 nativekernel KERNCONF=X86_64_GENERIC` completed RC=0 with zero errors under -Werror (full log: findings/poc/_batch41/fix_build8.log). Behavior A/B intentionally skipped for this Info-tier item.