⬢ DragonFlyBSD Kernel Audit
DF-4911 / verdict.json
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{
  "finding_id": "DF-4911",
  "status": "untested",
  "reproduced": 0,
  "impact": "none",
  "confidence": "certain",
  "verdict": "Certain code defect, untested at runtime (no AMD display hardware on the audit guest; raw AUX chardev compiled out \u2014 CONFIG_DRM_DP_AUX_CHARDEV not defined in sys/dev/drm/kconfig.h). dm_dp_aux_transfer discards the dc_link_aux_transfer() return on both write cases (amdgpu_dm_mst_types.c:105-117 NATIVE_WRITE, 133-148 I2C_WRITE) and returns msg->size unconditionally, so NACKed/timed-out/HPD-raced DPCD and I2C-over-AUX writes are reported to the DRM DP core as full success (dc_link_aux_transfer returns -1/0 on those failures, dc_link_ddc.c:669-683). MST sideband DOWN_REQ sequencing and payload-alloc programming then desync from reality on failing or hostile DP hardware \u2014 persistent display DoS; no memory-safety impact found (16-byte cap at :89 is correct).",
  "exploit_chain": "",
  "evidence": "VERDICT.md (source trace + consumer analysis), run.log (guest: NO_AMD_PCI_DEVICE / NO_DEV_DRI / no aux chardev), fix.diff (assign result on write paths).",
  "kernel_refs": [
    "sys/dev/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c:105",
    "sys/dev/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c:116",
    "sys/dev/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c:140",
    "sys/dev/drm/amd/display/amdgpu_dm/amdgpu_dm_mst_types.c:147",
    "sys/dev/drm/amd/display/dc/core/dc_link_ddc.c:635",
    "sys/dev/drm/amd/display/dc/core/dc_link_ddc.c:669",
    "sys/dev/drm/drm_dp_helper.c:334",
    "sys/dev/drm/kconfig.h"
  ],
  "poc_changes": "No seeded PoC; wrote aux_chardev_probe.c to document the (disabled) direct userspace handle.",
  "attempts": 1,
  "guest_uname": "DragonFly dfbsd 6.5-DEVELOPMENT DragonFly 6.5-DEVELOPMENT #0: Thu Jul 2 06:02:54 UTC 2026 root@dfbsd:/usr/obj/usr/src/sys/X86_64_GENERIC x86_64",
  "runtime_sec": 60,
  "guest_dirty": 0,
  "build_cmd": "cc -O2 -Wall -o aux_chardev_probe aux_chardev_probe.c",
  "run_cmd": "./run.sh",
  "code_hash": "f6648f00fb4fb0d796b25f3f52a1d4b31284c5c3af7dd9b5749183ef313ff978",
  "notes": "Reachability of the buggy lines is hardware-gated; severity Low because the impact is display/link DoS on failing DP hardware, not memory corruption.",
  "recommended_fix": "Propagate dc_link_aux_transfer()s return value on the NATIVE_WRITE and I2C_WRITE cases instead of unconditionally returning msg->size (see fix.diff).",
  "fix_status": "not_testable",
  "fix_baseline_reproduced": 0,
  "fix_patched_reproduced": 0,
  "fix_verdict": "Fix cannot be validated on this guest (baseline path needs AMD DP hardware); fix.diff is a trivial, behavior-preserving-on-success change mirroring the read cases.",
  "fix_evidence": "fix.diff"
}