# VERDICT — DF-1551: if_sn.c snread packet_length u_short underflow -> heap overflow

## Verdict

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

The bug is real and present in master DEV source at `sys/dev/netif/sn/if_sn.c:983,989,1005-1006,1016,1021,1037-1038`, but
the affected driver attaches only to hardware not present in the audit QEMU
guest (only virtio+PIIX3 PCI devices, no AMD/Intel GPUs, no ath NICs, no
AdvanSys SCSI, no mfi/tws/mrsas RAID, etc.), so it cannot be live-triggered
here. The `fix.diff` applies cleanly and the patched kernel compiles with
`-Werror` (combined build rc=0; see `fix_apply.log`).

## Mechanism (cited path → primitive → effect)

packet_length = inw(BASE+DATA_REG_W) & RLEN_MASK (0x07ff, 0..2047). packet_length -= 6; assigned back to u_short, values 0..5 wrap to 65530..65535. RS_ERRORS filter only checks status bits not length. ODDFRAME+1 may further wrap 65535->0. m_len=packet_length=65530. MCLGET=2048 bytes. insw(BASE+DATA_REG_W, data, packet_length>>1) writes 32765 words=65530 bytes into 2KB cluster -> heap overflow write.

## Reachability on this guest

No — `sys/dev/netif/sn/if_sn.c:983` is in a driver/module that only attaches
to hardware absent from the audit guest. The trigger requires the relevant
PCI device (or, for VBIOS-driven GPU paths, the actual GPU + a crafted VBIOS
loaded by root or via VFIO passthrough).

## Phase 6 — escalation potential

This is a Heap overflow write via unsigned wrap 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).

Realistic impact ceiling on suitable HW: **up to ~63KB kernel heap overflow write**.

## 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 the combined
  `make -j6 nativekernel KERNCONF=X86_64_GENERIC` build (kernel build rc=0;
  see `manifest.json`).
- For HW-gated findings the patched code path is not exercisable on this
  guest, so the fix is validated at the apply + compile level only.

Fix approach: After the -= 6 subtraction, reject any packet_length > MCLBYTES (catches the unsigned wrap).

## 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.