e8537460a3
- Introduced AGENTS.md, README.md, TASKS.md, and implementation_plan.md for Vexer, detailing mission, responsibilities, key components, and operational notes. - Established similar documentation structure for Vulnerability Explorer and Zastava modules, including their respective workflows, integrations, and observability notes. - Created risk scoring profiles documentation outlining the core workflow, factor model, governance, and deliverables. - Ensured all modules adhere to the Aggregation-Only Contract and maintain determinism and provenance in outputs.
123 lines
5.3 KiB
Markdown
123 lines
5.3 KiB
Markdown
# Entry-Point Static Analysis
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This guide captures the static half of Stella Ops’ entry-point detection pipeline: how we turn image metadata and filesystem contents into a resolved binary, an execution chain, and a confidence score.
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## 1) Loading OCI images
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### 1.1 Supported inputs
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- Registry references (`repo:tag@sha256:digest`) using the existing content store.
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- Local OCI/Docker v2 archives (`docker save` tarball, OCI layout directory with `index.json` + `blobs/sha256/*`).
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### 1.2 Normalised model
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```csharp
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sealed class OciImage {
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public required string Os;
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public required string Arch;
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public required string[] Entrypoint;
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public required string[] Cmd;
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public required string[] Shell; // Windows / powershell overrides
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public required string WorkingDir;
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public required string[] Env;
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public required string[] ExposedPorts;
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public required LabelMap Labels;
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public required LayerRef[] Layers; // ordered, compressed blobs
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}
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```
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Compose the runtime argv as `Entrypoint ++ Cmd`, honouring shell-form vs exec-form (see §2.3).
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## 2) Overlay virtual filesystem
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### 2.1 Whiteouts
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- Regular whiteout: `path/.wh.<name>` removes `<name>` from lower layers.
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- Opaque directory: `path/.wh..wh..opq` hides the directory entirely.
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### 2.2 Lazy extraction
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- First pass: build a tar index `(path → layer, offset, size, mode, isWhiteout, isDir)`.
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- Decompress only when reading a file; optionally support eStargz TOC to accelerate random access.
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### 2.3 Shell-form composition
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- Dockerfile shell form is serialised as `["/bin/sh","-c","…"]` (or `Shell[]` override on Windows).
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- Always trust `config.json`; no need to inspect the Dockerfile.
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- Working directory defaults to `/` if unspecified.
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## 3) Low-level primitives
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### 3.1 PATH resolution
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- Extract `PATH` from environment (fallback `/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin`).
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- If `argv[0]` is relative or lacks `/`, walk the PATH to resolve an absolute file.
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- Verify execute bit (or Windows ACL) before accepting.
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### 3.2 Shebang handling
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- For non-ELF/PE files: read first line; interpret `#!interpreter args`.
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- Replace `argv[0]` with the interpreter, prepend shebang args, append script path per kernel semantics.
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### 3.3 Binary probes
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- Identify ELF via magic `\x7FELF`, parse `.interp`, `.dynamic`, linked libs, `.note.go.buildid`, DWARF producer.
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- Identify PE (Windows) and detect .NET single-file bundles via CLI header.
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- Record features for runtime scoring (Go vs Rust vs glibc vs musl).
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## 4) Wrapper catalogue
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Collapse known wrappers before analysing the target command:
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- Init shims: `tini`, `dumb-init`, `s6-svscan`, `runit`, `supervisord`.
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- Privilege droppers: `gosu`, `su-exec`, `chpst`.
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- Shells: `sh`, `bash`, `dash`, BusyBox variants.
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- Package runners: `npm`, `yarn`, `pnpm`, `pip`, `pipenv`, `poetry`, `bundle`, `rake`.
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Rules:
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- If wrapper contains a `--` sentinel (`tini -- app …`) drop the wrapper and record a reduction edge.
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- `gosu user cmd …` → collapse to `cmd …`.
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- For shell wrappers, delegate to the ShellFlow analyser (see separate guide).
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## 5) ShellFlow integration
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When the resolved command is a shell script, invoke the ShellFlow analyser to locate the eventual `exec` target. Key capabilities:
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- Parses POSIX sh (and common Bash extensions).
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- Tracks environment mutations (`set`, `export`, `set --`).
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- Resolves `$@`, `$1..9`, `${VAR:-default}`.
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- Recognises idioms from official Docker images (`if [ "$1" = "server" ]; then …`).
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- Emits multiple branches when predicates depend on unknown data, but tags them with lower confidence.
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The analyser returns one or more candidate commands along with reasons, which feed into the reduction engine.
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## 6) Reduction algorithm
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1. Compose argv `ENTRYPOINT ++ CMD`.
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2. Collapse wrappers; append `ReductionEdge` entries documenting each step.
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3. Resolve argv0 to an absolute file and classify (ELF/PE/script).
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4. If script → run ShellFlow; replace current command with highest-confidence `exec` target while preserving alternates as evidence.
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5. Attempt to resolve application artefacts for VM hosts (JARs, DLLs, JS entry, Python module, etc.).
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6. Emit `EntryTraceResult` with candidate terminals ranked by confidence.
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## 7) Confidence scoring
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Use a simple logistic model with feature contributions captured for the evidence trail. Example features:
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| Id | Signal | Weight |
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| --- | --- | --- |
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| `f1` | Entrypoint already an executable (ELF/PE) | +0.18 |
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| `f2` | Observed chain ends in non-wrapper binary | +0.22 |
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| `f3` | VM host + resolvable artefact | +0.20 |
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| `f4` | Exposed ports align with runtime | +0.06 |
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| `f5` | Shebang interpreter matches runtime family | +0.05 |
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| `f6` | Language artefact validation succeeded | +0.15 |
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| `f8` | Multi-branch script unresolved (`$@` taint) | −0.20 |
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| `f9` | Target missing execute bit | −0.25 |
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| `f10` | Shell with no `exec` | −0.18 |
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Persist per-feature evidence strings so UI/CLI users can see **why** the scanner picked a given entry point.
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## 8) Outputs
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Return a populated `EntryTraceResult`:
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- `Terminals` contains the best candidate(s) and their runtime classification.
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- `Evidence` aggregates feature messages, ShellFlow reasoning, wrapper reductions, and runtime detector hints.
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- `Chain` shows the explainable path from initial Docker argv to the final binary.
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Static and dynamic reducers share this shape, enabling downstream modules to remain agnostic of the detection mode.
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