# DF-3744 — Unbounded LVDS mode-patch record walk + fake-EDID copy read past the VBIOS image buffer (conditional userspace leak)

**Severity**: Medium · **CWE**: CWE-125 Out-of-bounds Read · **File**: sys/dev/drm/radeon/radeon_atombios.c:1691-1752

## What this is
radeon_atombios_get_lvds_info() walks the mode-patch record table with while (*record != ATOM_RECORD_END_TYPE) (:1706) — the only exit conditions are the sentinel byte, known record types stepping by fixed sizes or ucFakeEDIDLength, or an unknown type. Nothing bounds the walk to the VBIOS image: usModePatchTableOffset is an unvalidated u16 (:1697-1705) and record strides are attacker-chosen, so the walk crosses the end of the rdev->bios kmalloc'd image and keeps reading heap. The fake-EDID branch (:1717-1737) kmallocs max(128,len) and memcpy's len (<=255) bytes from record+2 — with the record placed at the image end this copies up to 255 bytes of ADJACENT KERNEL HEAP into bios_hardcoded_edid; if drm_edid_is_valid() passes (checksum 1/256 odds on uncontrolled bytes; guaranteed for the in-image part), those bytes become the connector EDID blob, readable from userspace (DRM_IOCTL_MODE_GETCONNECTOR). Harness on guest: record at image end, len=200 -> 200/200 bytes read beyond image; walk continues 201+ bytes past end until garbage type. Kernel path unreachable on this guest.

## Reproduce
```
./build.sh    # cc -O1 -g -Wall -o <bin> harness_lvds.c   (DragonFly guest, cc 8.3)
./run.sh      # expected: fake-EDID len=200: 200/200 bytes read beyond image end; walk continues past end until garbage record type; exit_code=0
```
The harness replicates the kernel parse arithmetic VERBATIM (cited path:line) against a
synthetic crafted-VBIOS buffer with canary-guarded destinations.

## Guest / environment
Guest has NO AMD/radeon GPU (virtio-only); /boot/kernel/radeon.ko exists (code shipped in the default install) but cannot attach, so the kernel code path is unreachable at runtime on this guest. Evidence is source-level certainty (path:line traced) plus a verbatim-arithmetic userland harness compiled and executed on the guest (build.log/run.log).
See env.txt (uname, cc, kldstat, /boot/kernel/radeon.ko presence).

## Fix
bound the walk to the BIOS image (requires tracking the image length in atom_context) — see fix.diff (authored after verification; never applied to sys/).