PoC DF-1453: radeon_legacy_tv_dac_mode_set NULL deref via unguarded enc_priv

Class: NULL pointer dereference (DoS) Cited site: sys/dev/drm/radeon/radeon_legacy_encoders.c:1159, 1192, 1207

Reproduction status

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

No — radeon DRM is a loadable module and only attaches to ATI/AMD Radeon GPUs. No Radeon HW in the audit guest; trigger requires a Radeon card with a missing/empty CompassionateData TV DAC table.

The bug is confirmed at the source level by tracing the cited path:line in sys/dev/drm/radeon/radeon_legacy_encoders.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

Line 1159 tv_dac = radeon_encoder->enc_priv; is assigned from radeon_atombios_get_tv_dac_info, which returns NULL when the CompassionateData table is absent or kzalloc fails. Lines 1192 (tv_dac->tv_std) and 1207 (tv_dac->ps2_tvdac_adj) deref tv_dac with no NULL guard. NULL deref → kernel panic on mode set.

Realistic impact ceiling

panic (DoS)

Fix

Add if (tv_dac == NULL) { DRM_ERROR(...); return; } immediately after the assignment.

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-1453.diff
ssh -F dfbsd-qemu/config dfbsd 'cd /usr/src && patch -p1 < /root/DF-1453.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/radeon && make'

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

VERDICT — DF-1453: radeon_legacy_tv_dac_mode_set NULL deref via unguarded enc_priv

Verdict

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

The bug is real and present in master DEV source at sys/dev/drm/radeon/radeon_legacy_encoders.c:1159, 1192, 1207, 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)

Line 1159 tv_dac = radeon_encoder->enc_priv; is assigned from radeon_atombios_get_tv_dac_info, which returns NULL when the CompassionateData table is absent or kzalloc fails. Lines 1192 (tv_dac->tv_std) and 1207 (tv_dac->ps2_tvdac_adj) deref tv_dac with no NULL guard. NULL deref → kernel panic on mode set.

Reachability on this guest

No — radeon DRM is a loadable module and only attaches to ATI/AMD Radeon GPUs. No Radeon HW in the audit guest; trigger requires a Radeon card with a missing/empty CompassionateData TV DAC table.

Phase 6 — escalation potential

This is a NULL pointer dereference (DoS) 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.

Add if (tv_dac == NULL) { DRM_ERROR(...); return; } immediately after the assignment.

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.