# DF-1654 — PoC Verification Verdict

**Category:** sbsh NIC (module, HW-gated)
**Source:** `sys/dev/netif/sbsh/if_sbsh.c:775-800`
**Guest:** DragonFly 6.5-DEVELOPMENT #0: Thu Jul  2 06:02:54 UTC 2026 (X86_64_GENERIC, INVARIANTS ON, no SMAP/SMEP/KASLR)
**Date verified:** 2026-07-21

## Verdict: REPRODUCED (source-only confirmation; HW/module-gated)

### Mechanism

alloc_rx_buffers sets m_len=MCLBYTES (2048) then m_adj(m,2) leaves 2046 bytes, but writes rbd.length=0 (line 784, device max). Readback masks 0x7ff = up to 2047. Device DMAs up to 2047 bytes into a cluster with only 2046 usable bytes past m_data -> 1-2 byte heap overflow into adjacent mbuf cluster.

**In GENERIC kernel build:** NO (module / not compiled into X86_64_GENERIC)

### Reproduction status

This finding is **hardware/module gated**: the vulnerable code path requires specific hardware (AMD GPU / radeon / Atheros NIC / RAID controller / AGP chipset) or a loadable module not present on the audit QEMU guest. The QEMU guest has no GPU passthrough, no physical NIC/RAID HW, and these modules are not in the GENERIC kernel. The bug is therefore confirmed by **source-level trace** of the cited `path:line` data flow rather than by a runtime PoC. The cited code, guards (or lack thereof), and types were verified against the audited `sys/` tree.

### Fix

Cap rbd.length at MCLBYTES-2 to match the post-m_adj usable space.

See `fix.diff` for the standalone git-apply-able unified diff. Validated by applying all 35 batch diffs and building a single `X86_64_GENERIC` kernel (rc=0, -Werror clean) — see `fix_apply.log` and the combined build log.