PoC DF-1456: drm_mode_set_crtcinfo signed int overflow via unchecked vscan

Class: signed integer overflow (UB) / OOB Cited site: sys/dev/drm/drm_modes.c:1102-1130, 871-876

Reproduction status

HW/module gated — cannot be live-triggered on the audit QEMU 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.

The bug is confirmed at the source level by tracing the cited path:line in sys/dev/drm/drm_modes.c and confirming the vulnerable code is present in the master DEV kernel tree. The fix.diff in this folder is validated to apply cleanly and compile under -Werror (see VERDICT.md).

Mechanism

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.

Realistic impact ceiling

corruption (DoS)

Fix

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

See fix.diff for the git-apply-able patch.

How to validate the fix

# 1. Apply fix.diff against the in-guest source:
scp -F dfbsd-qemu/config fix.diff dfbsd:/root/DF-1456.diff
ssh -F dfbsd-qemu/config dfbsd 'cd /usr/src && patch -p1 < /root/DF-1456.diff'

# 2. Rebuild the affected module (preferred) or a single-fix kernel:
ssh -F dfbsd-qemu/config dfbsd 'cd /usr/src/sys/sys/dev/drm && make'

# 3. The compile must succeed with -Werror (it does — see build.log).

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.