# VERDICT — DF-1515: intel_overlay.c divide-by-zero via zero dst dimensions

## Verdict

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

The bug is real and present in master DEV source at `sys/dev/drm/i915/intel_overlay.c:916-929, 931-945`, but
the affected driver attaches only to hardware not present in the audit QEMU
guest (only virtio+PIIX3 PCI devices, no AMD/Intel GPUs, no ath NICs, no
AdvanSys SCSI, no mfi/tws/mrsas RAID, etc.), so it cannot be live-triggered
here. The `fix.diff` applies cleanly and the patched kernel compiles with
`-Werror` (combined build rc=0; see `fix_apply.log`).

## Mechanism (cited path → primitive → effect)

check_overlay_dst validates dst_x+dst_width <= pipe_src_w but never rejects zero width/height (0+0<=pipe_src_h always true). check_overlay_scaling at 936 then computes ((src_scan_height<<16)/rec->dst_height)>>16 with dst_height==0 -> x86 #DE trap -> kernel panic. Same for dst_width==0 at line 940.

## Reachability on this guest

No — `sys/dev/drm/i915/intel_overlay.c:916-929` is in a driver/module that only attaches
to hardware absent from the audit guest. The trigger requires the relevant
PCI device (or, for VBIOS-driven GPU paths, the actual GPU + a crafted VBIOS
loaded by root or via VFIO passthrough).

## Phase 6 — escalation potential

This is a Divide-by-zero 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).

Realistic impact ceiling on suitable HW: **kernel panic (DoS) via DRM ioctl from unprivileged DRI client on i915 GPU**.

## 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 the combined
  `make -j6 nativekernel KERNCONF=X86_64_GENERIC` build (kernel build rc=0;
  see `manifest.json`).
- For HW-gated findings the patched code path is not exercisable on this
  guest, so the fix is validated at the apply + compile level only.

Fix approach: Reject zero dst_width/dst_height in check_overlay_dst before the divide.

## 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.