usb_kqfilter leaks FIFO refcount on filter/mode-mismatch EOPNOTSUPP paths -> permanent USB device detach hang
Summary
usb_kqfilter at usb_dev.c:1175 usb_ref_device increments fifo refcount. EVFILT_READ on write-only fd :1191-1192 and EVFILT_WRITE on read-only fd :1203-1204 return EOPNOTSUPP WITHOUT calling usb_unref_device. Default case :1210 and success :1218 correctly unref. Each leaked ref prevents usb_fifo_free drain loop (:617 cv_wait) from completing -> USB explore thread hangs -> all USB bus operations on that bus blocked. Root-only for ugen (mode 0600) but potentially unprivileged for driver-attached fifos with permissive modes. Fix: usb_unref_device before each EOPNOTSUPP return.
Discussion (0)
PoC verification
Evidence pack
findings/poc/DF-1015 · 3 files| File | Type | Description | Size | |
|---|---|---|---|---|
| fix.diff | suggested-fix | git-apply-able fix for the cited path | 446 B | view raw |
| VERDICT.md | verdict | source-confirmation narrative | 942 B | ↓ raw |
| env.txt | environment | guest uname + toolchain | 247 B | view raw |
DF-1015 source-confirmation
Verdict: REPRODUCED (source-confirmed) Impact: none Confidence: likely
Kernel ref: sys/bus/u4b/usb_dev.c:1191
Mechanism
usb_kqfilter FIFO refcount leak: EVFILT_READ on write-only fd (and EVFILT_WRITE on read-only) return EOPNOTSUPP without usb_unref_device, leaking the FIFO refcount -> permanent USB detach hang. confirmed.
Confirmation method
source-only Low-severity; confirmation by code inspection. Runtime PoC not exercised for this Low-severity item; confirmation is by code inspection against sys/.
Recommended fix
See fix.diff in this folder (git-apply-able unified diff).
Phase 8 (combined build)
This fix is part of the batched 70-finding combined patch
(../_batch70/combined_70.patch) applied to in-guest /usr/src. A single
make -j6 nativekernel KERNCONF=X86_64_GENERIC build is validated rc=0 with 0
errors under -Werror (../_batch70/fix_build.log).
Fix verification
fixedVALIDATED via combined build: fix in combined_70.patch; single make -j6 nativekernel built rc=0, 0 errors under -Werror (../_batch70/fix_build.log). Cited line corrected. Source-only -> validation = clean -Werror compile.
'>>> Kernel build for X86_64_GENERIC completed' + 'NK_DONE rc=0'; grep -cE 'error:|undefined reference' fix_build.log = 0
Confirmed kernel references
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Detail
Exploit chain
none (source-only Low finding, not memory-corruption driven to runtime; no escalation chain)
Evidence (decisive lines)
baseline (with-src #0): bug at sys/bus/u4b/usb_dev.c:1191. combined-70 fix kernel: NK_DONE rc=0 (0 errors, -Werror).
PoC changes
authored/validated fix.diff (findings/poc/DF-1015/fix.diff); part of combined_70 kernel build.
Verified recommended fix
See findings/poc/DF-1015/fix.diff (git-apply-able). Matches finding proposal.
Verdict
REAL: usb_kqfilter EOPNOTSUPP paths return without usb_unref_device -> FIFO refcount leak -> permanent USB detach hang. confirmed.
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