# DF-1143 — divide-by-zero in amdgpu `ci_thermal_setup_fan_table`

## Result
**NOT REPRODUCED** on this guest. Real source-level defense-in-depth bug;
fix authored.  See `VERDICT.md` for the full analysis.

## Build & run
```
./build.sh    # documents why no executable PoC is possible
./run.sh      # documents why runtime trigger is impossible
```

## Reachability summary
- amdgpu is **not** in `X86_64_GENERIC` (0 symbols in `kernel.debug`)
- No AMD GPU on the QEMU guest
- Attacker model is malicious VBIOS, not unprivileged local

## Fix
`fix.diff` — guard zero `t_diff1`/`t_diff2` before the divide at
`sys/dev/drm/amd/amdgpu/ci_dpm.c:1128-1129`.  Applies cleanly with
`git apply`.