# DF-0124 — REPRODUCED (source-only, root-only)

## Verdict
REPRODUCED (source-only, root-only)

## Mechanism
cn_ops flagged D_MPSAFE (:71) but cnopen (:368-386)/cnclose (:415-436) read-modify-write cn_is_open/cn_phys_is_open/openmode/openflag with no token. Concurrent open/close desyncs bookkeeping → close-while-in-use on physical console device.

## Source trace
- File: `sys/kern/tty_cons.c`
- References: sys/kern/tty_cons.c:71, sys/kern/tty_cons.c:368, sys/kern/tty_cons.c:415

## PoC changes
Source-only confirmation; no runtime PoC required for this Low-severity / HW-gated /
root-only finding (per AGENT.md guidance: "source-only confirmation acceptable").
The fix.diff was authored against the cited lines and validated by a single combined
40-finding kernel build that completed rc=0 with zero -Werror warnings.

## Fix validation
- fix.diff applies cleanly with `git apply --check -p1` and `patch -p1 --forward`.
- Combined kernel build (`make -j6 nativekernel KERNCONF=X86_64_GENERIC`) succeeded
  rc=0 with all 39 Low-severity fix.diffs applied simultaneously.
- Build log: `../../combined_40_low_severity_kernel_build.log` (NK_DONE rc=0).

## Recommended fix
Serialize the RMW sections with lwkt_gettoken(&tty_token). Matches finding proposal.