# VERDICT — DF-1456: drm_mode_set_crtcinfo signed int overflow via unchecked vscan

## Verdict

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

The bug is real and present in master DEV source at `sys/dev/drm/drm_modes.c:1102-1130, 871-876`,
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)

drm_mode_validate_basic (1102-1130) validates h/v timing ordering but NOT vscan. drm_mode_set_crtcinfo (871-876) multiplies crtc_vdisplay/crtc_vsync_start/crtc_vsync_end/crtc_vtotal by `p->vscan` (u16, up to 65535). vscan=65535 * vtotal>=32769 overflows signed int → undefined behavior; downstream CRTC math reads garbage.

## Reachability on this guest

No — the drm core is built into radeon/amdgpu/i915 modules; needs a DRM-capable GPU. No GPU in the audit guest. Trigger is an attacker-supplied drm_mode_modeinfo with vscan up to 65535.

## Phase 6 — escalation potential

This is a signed integer overflow (UB) / OOB 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.

In drm_mode_validate_basic, reject modes where `vscan > 1 && vscan > UINT_MAX / max(vtotal,1)`.

## 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.