# DF-2064 — VERDICT

**Verdict:** REPRODUCED (source-confirmed; HW/module-gated — not runtime-triggerable on this guest)
**Class:** Integer overflow / underflow (CWE-190)
**Impact ceiling:** none
**Confidence:** certain

## Reproduction status

This finding is **HW/module-gated** and could not be triggered at runtime on the
audit guest. The guest has **no Atheros NIC, no AdvanSys HBA, no Intel i915 GPU,
no AMD Southern-Islands GPU, no LSI MegaRAID controller, and no loaded dm
device** (only `vtnet0`). The PoC therefore cannot reach the vulnerable path at
runtime here. The bug is instead **confirmed by line-by-line source tracing**
against the audited `sys/` tree (master DEV, the same commit the kernel was built
from) — the defect is real and deterministic *given the hardware/module*, which is
the standard bar for HW-gated driver findings.

## Mechanism (source-confirmed)

uc_fw->header_size, ucode_size and rsa_size are u32 (intel_uc_fw.h:69,71,73). At intel_uc_fw.c:109 the sum `size = header_size + ucode_size + rsa_size` is computed in 32-bit (unsigned int) arithmetic and only THEN zero-extended into size_t, so it wraps at 2^32 BEFORE the truncation guard `if (fw->datasize < size)` (:110). Example: header_size=0x80, ucode_size=0xFFFFFF00, rsa_size=0x100 sum to 0x100000180 which wraps to 0x80 in u32, passing the guard for any blob >= 128 bytes. The u32 subtractions at :83-85 and :96 are likewise unguarded. Impact is bounded: the GEM object uses the real fw->datasize and RSA is clamped by sg_pcopy_to_buffer, so the malformed fw just fails RSA verification and falls back gracefully -- a truncation-validation bypass / hardening gap, not memory corruption.

**Gating precondition:** Intel Skylake/Kabylake/Broxton iGPU + write access to the firmware blob path (root /boot module param at mode 0400)

## Exploit chain

Not applicable — this finding is HW/module-gated on the audit guest (no
triggering hardware/module present), so no userspace-driven escalation chain can
be exercised here. Per AGENT.md Phase 6 the valid hard blocker that applies is
"vulnerable code path is reachable only on hardware/module absent from this
guest." The realistic impact ceiling for the bug itself is `none` as
documented above (panic for the corruption-class bugs on default GENERIC with
INVARIANTS ON; dos for the I/O-wedge/stall bugs; none/graceful-fallback for the
validation-bypass and pure missing-check / resource-leak findings).

## Fix (authored, git-apply-able)

Promote the :109 sum to size_t via explicit casts before adding, and add underflow guards on the u32 subtractions at :83-85 (modulus+key+exponent <= header_size_dw) and :96 (size_dw >= header_size_dw), matching upstream Linux hardening.

The standalone diff is `fix.diff` in this directory. It applies cleanly to the
pristine audited source (`git apply --check` verified).

## Phase 8 — combined fix-build validation

All nine findings' fixes were applied together to a single in-guest source tree
and built with `make -j6 nativekernel KERNCONF=X86_64_GENERIC`. The DragonFly
kernel compile invokes `cc ... -Werror` on **every** translation unit, so this
is a true **-Werror** build.

- Result: **`=== NK_DONE rc=0 ===`** (full log: `fix_build.log`)
- Compiler errors in changed files: **0**
- Compiler warnings (entire tree): **0**
- Patched `kernel.stripped` sha256: `6086a989213ef7be890180f60edc0fd9827bf60efbc3ed8c969724746e250df6`
- Baseline (unpatched) kernel: `DragonFly 6.5-DEVELOPMENT #0: Thu Jul  2 06:02:54 UTC 2026`

Because the PoC is HW/module-gated, the fix could not be runtime A/B tested on
this guest (no way to trigger the bad behaviour on either the baseline or the
patched kernel). The fix is therefore classified **fix_status = not_testable**
(compile-validated + source-traced to close the cited path), which is the honest
result for HW-gated findings.

## Kernel references (verified during this trace)

- `sys/dev/drm/i915/intel_uc_fw.c:83`
- `sys/dev/drm/i915/intel_uc_fw.c:96`
- `sys/dev/drm/i915/intel_uc_fw.c:109`
- `sys/dev/drm/i915/intel_uc_fw.c:110`
- `sys/dev/drm/i915/intel_uc_fw.h:69`