docs(security): document the ByteStorage decompression bound (LAB-2504) - #75
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The threat-model bullet claimed "size limits + ratio validation" with no numbers and no statement of reach, which left three things a reader has to rediscover from source: what the limits actually are, that original_size is attacker-controlled and deliberately not trusted, and that xxHash3-64 is unkeyed and therefore not a control against forgery at all. Also states the ceiling's blast radius honestly. 512 MiB assumes a host that can absorb a 512 MiB allocation; a Workers isolate has ~128 MiB, so a payload well inside these limits can still OOM it. Sizing is LAB-2505's call, so this records the constraint rather than changing it. Records that compression_bomb runs in the quick-fuzz matrix on every push and PR, not as an ad hoc script — the guard is only worth citing if a reader can tell it gates merges.
…-2504) Expert-panel findings on the previous commit. Three claims were wrong or overstated and a security doc that overstates its own guarantees is worse than one that says nothing. CI coverage: 'runs on every push and pull request' was false for pushes -- security.yml is on: push: branches: [main], so feature-branch pushes never run quick-fuzz. Worse, fuzz/.gitignore excludes corpus/*/, so on a fresh CI checkout the corpus is empty and 'cargo fuzz run -runs=0' generates nothing: at PR time the job proves the target BUILDS and exercises no input. The real input coverage is the weekly deep-fuzz run plus the unit tests and Kani proofs, so those are what the section now points at. The 120 s figure is gone; it was a timeout, not a measure of coverage. Fuzz-target contract: the target also accepts DecompressionFailed, and its compressed_size is a u16 -- so it caps compressed input at 64 KiB and cannot reach the 512 MiB boundary the old text cited. It exercises the ratio bound, not the absolute one. 'original_size is not trusted' was too strong and papered over the interesting part. It is not trusted as a BOUND, but it does size the allocation within that bound, so a forged envelope can still make a reader allocate up to 1000x its wire size before the LZ4 stream is validated -- an eager memory.grow on wasm32 needing no valid stream behind it. That is the LAB-2505 sizing question, now named as such instead of hidden behind a reassuring phrase.
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Estimated code review effort: 1 (Trivial) | ~2 minutes Merge Risk: 🔵 Low · up to The change documents decompression safeguards without changing runtime behavior, but two descriptions of verification coverage remain inaccurate. The PR is mergeable with explicit owner awareness and follow-up to correct those bounded security-documentation issues. 🚥 Pre-merge checks | ✅ 5✅ Passed checks (5 passed)
Full details: Docstring CoverageExplanation No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 0 files. (1 skipped: 1 unsupported.) ✨ Finishing Touches🧪 Generate unit tests (beta)
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Inline comments:
In `@SECURITY.md`:
- Around line 74-75: Update the SECURITY.md description of zero-length
compressed data to state that StorageEnvelope::extract rejects every input with
compressed_data.len() == 0, including when original_size == 0, while preserving
the existing DecompressionBomb behavior.
- Around line 103-104: Update the SECURITY.md fuzz-coverage statement to say
compression_bomb.rs does not cover the 512 MiB compressed-size boundary, while
noting that its u16 compressed_size exercises the ratio bound and its u32
original_size can test values above the 512 MiB uncompressed limit.
- Around line 112-113: Update the Kani coverage statement in SECURITY.md to
describe Kani proofs as bounded formal checks of selected arithmetic predicates,
not execution or input coverage for StorageEnvelope::extract; reserve “input
coverage” claims for unit-test and fuzz executions.
- Around line 109-110: Update the SECURITY.md description of cargo fuzz run
-runs=0 to state that the empty-corpus path executes the initialization callback
and newline seed before the run-limit check, while no mutation-based fuzzing
occurs; avoid claiming that no inputs execute or implying a fixed libFuzzer
version, since the workflow uses the rolling nightly toolchain.
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…e decompression-limits section All four CodeRabbit findings verified against source and applied; an expert-panel pass on the result found five more of the same class, also applied. Corrected here: - zero-length compressed data is rejected unconditionally, not only when original_size is non-zero (byte_storage.rs:122 has no original_size term) - the compression_bomb target reaches MAX_UNCOMPRESSED_SIZE's rejection branch (u32 original_size) but never MAX_COMPRESSED_SIZE (u16 compressed_size) - its 512 MiB output assertion is vacuous for the same reason, and its error-variant assertions are guarded, not universal - -runs=0 executes libFuzzer's newline seed before the run-limit check; it does no mutation rather than no execution - the Kani harnesses are tautologies (assert_eq!(P, P)) and run only on schedule/dispatch, so they are neither verification of the bound nor a merge-time gate Also stated: deep-fuzz does not persist its corpus, validate() reaches the full extract allocation, the pre-deserialization bound lives in ByteStorage rather than on the public StorageEnvelope, the OutputTooSmall bound depends on lz4_flex's default safe-decode, and wasm32 linear memory never shrinks. CodeRabbit-Resolved: SECURITY.md:75:Document the unconditional zero-length rej CodeRabbit-Resolved: SECURITY.md:104:Correct the fuzz coverage boundary claim CodeRabbit-Resolved: SECURITY.md:110:Describe the -runs=0 coverage accurately CodeRabbit-Resolved: SECURITY.md:113:Qualify the Kani coverage statement
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Inline comments:
In `@SECURITY.md`:
- Around line 159-164: Update the SECURITY.md discussion of the size/ratio
harnesses to identify only verify_input_size_limits and
verify_compressed_size_limits as containing literal self-comparisons; describe
verify_decompression_bomb_protection as comparing checked and direct
multiplication, and verify_compression_ratio_calculation_safety as deriving
is_bomb and checking its branches.
- Around line 147-148: Update the quick-fuzz job description in SECURITY.md to
characterize it as single-seed smoke coverage: state that it can catch failures
on the initial seed but does not perform mutation-based fuzzing or generate
boundary coverage, and identify unit tests as providing merge-time coverage for
boundary cases.
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…riptions Both from CodeRabbit's re-review of a5f051a, both correct. quick-fuzz: "proves little beyond building" undersold it. The seed does execute, so the job is single-seed smoke coverage — it catches a build break or a panic on that one input. Says so, and points at the unit tests as the merge-time boundary coverage. Kani: my previous text said three of four harnesses assign the same predicate to two bindings. Only two do that literally. The other two are equally vacuous by different routes — checked_mul compared against the same product under an assume that makes overflow impossible, and a branch that restates its own definition. All four are now described by their actual pattern rather than lumped under one. CodeRabbit-Resolved: SECURITY.md:148:Describe the pull-request fuzz job as sin CodeRabbit-Resolved: SECURITY.md:164:Correct the Kani harness count and patter
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Docs-only. Part of the LAB-2504 cross-SDK decompression-bomb audit. Merge this before the py and ts PRs — both link the
#decompression-limitsanchor this adds.Why
SECURITY.mdclaimed "Decompression bombs: Size limits + ratio validation" with no numbers, which left a reader to rediscover from source what the limits are, thatoriginal_sizeis attacker-controlled and deliberately not used as a bound, and that xxHash3-64 is unkeyed and therefore not a control against forgery at all.Audit outcome for this repo: the bound is sound, no code change
StorageEnvelope::extractbounds decompression before callinglz4_flex::decompress, and the published 0.5.0 that all three SDKs pin is byte-identical tomainon this file. Verified during the audit:u64 checked_mulratio → decompress → checksum → post-decompress length re-check.u64, so LAB-1135 item 7 / LAB-2594's wasm32usizewrap concern does not apply here.safe-decodeis alz4_flexdefault feature and is not disabled, so output cannot over-run the allocation — and the post-decompress length re-check catches thetruncate()under-run even when an attacker recomputes the unkeyed checksum. That is LAB-1751's reusable lesson (a decompressor's size argument sizes a buffer, it never asserts the decoded length) satisfied in both directions.rmp_serde::from_slicedoes not pre-allocate from a declaredbin32length, so there is no envelope-deserialization bomb upstream ofextract.What this PR corrects about its own first draft
The expert panel found three claims in the initial commit that were wrong or overstated, and a security doc that overstates its guarantees is worse than one that says nothing:
security.ymlison: push: branches: [main]. Andfuzz/.gitignoreexcludescorpus/*/, so on a fresh CI checkout the corpus is empty andcargo fuzz run … -runs=0generates no inputs: at PR time the job proves the target builds and exercises no bomb. The section now points at the weekly deep-fuzz run and the in-tree unit tests + Kani proofs as the actual input coverage. The "120 s per target" figure is gone — it was a timeout, not coverage.compression_bombalso acceptsDecompressionFailed, and itscompressed_sizeis au16, capping compressed input at 64 KiB — so it cannot reach the 512 MiB boundary the draft cited. It exercises the ratio bound, not the absolute one.original_sizeis not trusted" was too strong and hid the interesting part. It is not trusted as a bound, but it does size the allocation within that bound, so a forged envelope can make a reader allocate up to 1000× its wire size before the LZ4 stream is validated — an eagermemory.growonwasm32needing no valid stream behind it. Now named as the LAB-2505 sizing question rather than smoothed over.Scope
No code, no constants changed. Sizing (512 MiB is server-class; a Workers isolate is ~128 MiB) belongs to LAB-2505 and is recorded here, not decided.
Closes LAB-2504 for this repo.
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