virtio_balloon: inverted bounds KASSERT in vtballoon_update_stat disables defense-in-depth check
Summary
vtballoon_update_stat at line 474: KASSERT(idx >= VTBALLOON_S_NR, ...) is inverted - KASSERT panics when expression FALSE so panics for valid idx 0..9 and only passes OOB idx>=10. Guard should be idx < VTBALLOON_S_NR. Callers produce idx in [0,8) so no OOB write today. Defense-in-depth check silently absent. INVARIANTS kernel panics on first stat write. Fix: invert comparison.
Discussion (0)
PoC verification
Evidence pack
findings/poc/DF-1705 · 2 files| File | Type | Description | Size | |
|---|---|---|---|---|
| fix.diff | suggested-fix | Invert comparison to KASSERT(idx < VTBALLOON_S_NR). | 614 B | view raw |
| VERDICT.md | verdict | full analysis | 1.1 KB | ↓ raw |
DF-1705 — Verdict
Severity: Low Status: REPRODUCED (source-only confirmation — driver/HW-gated, not runtime-triggered on QEMU guest) Impact: panic Confidence: certain
Verdict
REPRODUCED. The cited bug is confirmed real in the audited source at sys/dev/virtual/virtio/balloon/virtio_balloon.c:474.
Mechanism
vtballoon_update_stat KASSERT(idx >= VTBALLOON_S_NR) is inverted: panics for valid idx 0..8, passes OOB idx>=10. Currently dormant (stats path #if 0'd) but would panic on INVARIANTS kernel if re-enabled.
Fix
Invert comparison to KASSERT(idx < VTBALLOON_S_NR).
The full git-apply-able diff is in fix.diff.
Build validation
fix.diff applies cleanly and compiles with -Werror as part of the batch module build
(all 51 fixes applied to /usr/src, kernel+modules built).
Notes
Source-only confirmation: this finding is in a device driver code path that requires specific hardware not present in the QEMU guest. The bug is confirmed by source tracing (cited path:line verified against sys/), and the fix compiles clean. No runtime trigger was attempted as the relevant device/module is HW-gated.
Fix verification
fixedVALIDATED via batch build rc=0.
virtio_balloon.ko built with -Werror.
Confirmed kernel references
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Detail
Exploit chain
none
Evidence (decisive lines)
Source traced at sys/dev/virtual/virtio/balloon/virtio_balloon.c:474. Fix compiled clean.
PoC changes
authored fix.diff: invert to KASSERT(idx < VTBALLOON_S_NR)
Verified recommended fix
Invert KASSERT comparison. Matches finding proposal.
Verdict
REPRODUCED (source-only). Inverted KASSERT(idx >= VTBALLOON_S_NR) panics for valid idx, passes OOB. Dormant (#if 0 priming) but INVARIANTS panic if re-enabled.
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