chore(tools): validate-before-allocate ceiling in frame-crosscheck lz4BlockDecompress (LAB-1202) - #55
chore(tools): validate-before-allocate ceiling in frame-crosscheck lz4BlockDecompress (LAB-1202)#5527Bslash6 wants to merge 2 commits into
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…4BlockDecompress (LAB-1202) Deferred follow-up from the LAB-903 crypto/protocol panel (protocol#47). lz4BlockDecompress allocated the output buffer straight from original_size — a value read off the wire — before any bound applied. Not exploitable in this repo (the driver matches original_size against the committed vector first), but this reader is the porting template for SDK envelope readers that WILL face attacker-controlled envelopes, and the ratified rule is validate-before-allocate. Ceiling choice (the one decision the ticket delegates): ratio-derived, not an absolute cap. The LZ4 block format cannot expand more than 255x — a match sequence costs at least 3 input bytes (token + 2-byte offset) and each match-length extension byte adds at most 255 output bytes, so per-sequence output L+19+255b <= 255*(3+a+L+b) input. expectedSize > 255*src.length is therefore impossible for well-formed input, self-scales with the fixture, and needs no revisiting when larger vectors land. Non-integer and negative sizes are refused by the same guard. tools/test-frame-crosscheck-guard.mjs pins the guard load-bearing per the LAB-903 mutation-evidence convention: baseline exit-0 on committed vectors, mutation no-op checks, and a synthetic vector declaring 64 MiB from 3 compressed bytes. Exit codes cannot discriminate guarded from unguarded (the ticket-acknowledged post-allocation op !== expectedSize check fires either way), so the suite discriminates by failure ORDER: guarded fails with the pre-allocation ceiling refusal; guard-stripped fails with the post-allocation size mismatch, proving the oversized allocation succeeded first — the exact pre-fix vulnerability. The suite runs before the tool in verify.yml, same cannot-silently-degrade rule as the version-floors suite. test-vectors/*.json byte-untouched; no normative spec text moved (panel gate not re-triggered, per the panel's own deferral scope).
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WalkthroughThe LZ4 decompressor now rejects unsafe or excessive declared output sizes before allocation. A mutation test verifies guarded and unguarded behaviour. CI runs this test before the full JavaScript cross-check. ChangesLZ4 allocation guard
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: 🔵 Low · up to The change safely rejects implausible LZ4 output sizes before allocation and is mergeable, though invalid non-integer or negative declarations currently produce a misleading ceiling-error message that should be corrected. Sequence Diagram(s)sequenceDiagram
participant CI as verify.yml
participant GuardTest as test-frame-crosscheck-guard.mjs
participant CrossCheck as frame-crosscheck.mjs
CI->>GuardTest: Run mutation guard test
GuardTest->>CrossCheck: Test oversized frame with guard
GuardTest->>CrossCheck: Test oversized frame without guard
CI->>CrossCheck: Run full frame cross-check
🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
Full details: Docstring CoverageExplanation Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 2 functions across 2 files. (1 skipped: 1 unsupported.) ✨ Finishing Touches 💡 1📝 Generate docstrings 💡
🧪 Generate unit tests (beta)
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Inline comments:
In `@tools/frame-crosscheck.mjs`:
- Around line 164-168: Split the validation around expectedSize in the frame
reader: first reject non-safe-integer values with an error describing an invalid
declared original size, then separately reject valid values above the LZ4
expansion ceiling while preserving the existing “exceeds max expansion ceiling”
message. Keep the existing ceiling calculation and safe-integer semantics
unchanged.
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…in lz4 guard (LAB-1202) Accepted on merit: the combined condition reported a NaN/undefined/negative original_size as "exceeds max expansion ceiling", which misdescribes the fault. This reader is the porting template for SDK envelope readers facing attacker-controlled envelopes, where an accurate rejection cause matters for triage. Semantics unchanged; both branches still fire before allocation. Mutation suite 6/6, frame-crosscheck green. CodeRabbit-Resolved: tools/frame-crosscheck.mjs:168:Separate the type check
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Closes LAB-1202 — deferred follow-up 2 of 2 from the LAB-903 crypto/protocol expert panel (#47).
What
lz4BlockDecompressintools/frame-crosscheck.mjsallocated its output buffer straight fromoriginal_size— a value read off the wire — before any bound applied. Not exploitable here (the driver cross-checksoriginal_sizeagainst the committed vector first), but this reader is the porting template for SDK envelope readers that will face attacker-controlled envelopes, and the ratified rule is validate-before-allocate. The guard now refuses an implausibleexpectedSizebeforenew Uint8Array(...)is reached, and refuses non-integer/negative sizes with it.The ceiling (the decision the ticket delegates)
Ratio-derived, option 1 from the ticket:
expectedSize <= 255 * src.length. Derived from the LZ4 block format, not guessed — a match sequence costs at least 3 input bytes (token + 2-byte offset) and each match-length extension byte adds at most 255 output bytes, so per-sequence outputL+19+255b <= 255*(3+a+L+b)input; no well-formed block expands more than 255× (the lz4 project's documented maximum compression ratio). Self-scaling: no revisiting when larger vectors land. Derivation lives in the source comment at the guard.Mutation evidence (LAB-903 convention)
New zero-dep
tools/test-frame-crosscheck-guard.mjs, wired intoverify.ymlbefore the tool run (same cannot-silently-degrade rule as the version-floors suite):original_sizefrom 3 compressed bytes exits non-zero with the pre-allocation ceiling refusalop !== expectedSizecheck the ticket acknowledges fires either way), so the suite discriminates by failure order — the stripped tool's failure is the post-allocation size mismatch, proving the 64 MiB allocation succeeded before any validation. That is the pre-fix vulnerability, pinned.All 6 cases green locally;
frame-crosscheck.mjsandpython-frame-reference.py verifygreen.Scope guarantees
test-vectors/*.jsonbyte-untouched (python-frame.jsonsha256-pin unmoved)wire-format-reference.pyuntouched (no LZ4 there, per ticket non-goals)Summary by CodeRabbit
Bug Fixes
Tests