# TensorFlow `tf.io.parse_example` / `parse_single_example` / `parse_sequence_example` — SIGFPE (division-by-zero) Denial of Service

**Status: gated PoC, private disclosure in progress via huntr.com (protectai). Do not use against production systems.**

## Summary

TensorFlow's fast Example/SequenceExample proto parser
(`tensorflow/core/util/example_proto_fast_parsing.cc`) computes an internal
per-row element count (`elements_per_stride`) directly from the *inner* shape
supplied for a variable-length dense feature
(`tf.io.FixedLenSequenceFeature(shape=..., allow_missing=True)`, or the
equivalent `dense_shapes` attribute on the raw `ParseExampleV2` /
`ParseSingleExample` / `ParseSequenceExample` ops). If that inner shape has a
`0` in it, `elements_per_stride` is `0`, and the parser later divides/modulos
by it with **no zero-check**, crashing the whole process with `SIGFPE`
(signal 8) — not a catchable Python exception, not a `tf.errors.*` status,
just an instant process kill.

The crash is **independent of the actual serialized `Example`/`SequenceExample`
bytes** — it fires even for a syntactically empty/valid proto, and even when
the feature in question is entirely *absent* from the data. It is purely a
function of the (attacker-controlled, when the parsing graph ships inside a
model file) feature-spec shape. Any model/graph that wires up
`ParseExampleV2`/`ParseSingleExample`/`ParseSequenceExample` with such a shape
turns *every future invocation* of that signature into an instant crash of the
hosting process — a persistent, config-driven Denial of Service against any
inference server that loads the model.

## Root cause

`tensorflow/core/util/example_proto_helper.cc`, `GetDenseShapes()`:
```cpp
elements_per_stride->push_back(dense_shape.num_elements());
```
`dense_shape` is the caller-supplied shape with the (optional) leading
variable-length dimension stripped. If the remaining static dims multiply to
`0` (e.g. `shape=[0]`), `elements_per_stride` is `0`. Nothing rejects this.

`tensorflow/core/util/example_proto_fast_parsing.cc`:

* Batch path, `FastParseExample()` → `MergeDenseVarLenMinibatches` lambda:
  ```cpp
  const size_t stride_size = config.dense[d].elements_per_stride;
  const size_t max_num_elements = max_num_features / stride_size;   // <-- div by 0
  ```
  This runs **unconditionally once per configured variable-length dense
  feature**, regardless of whether that feature appears anywhere in the
  batch's data (confirmed by crashing on a batch of *empty* `Example`
  bytes with the feature entirely missing).

* Per-example path, `FastParseSerializedExample()`:
  ```cpp
  if (out.int64_list.size() % num_elements != 0) { ... }   // num_elements == elements_per_stride
  ```
  (analogous lines for `float_list` / `bytes_list`).

* Single-example path, `FastParseSingleExample()`:
  ```cpp
  if (num_elements % num_elements_divisor != 0) { ... }   // num_elements_divisor == elements_per_stride
  ```

* `SequenceExample` dense path, `ParseSequenceDenseFeatures()`:
  ```cpp
  if (!c.shape.AsTensorShape(&row_shape) ||
      expected_max_elements != (expected_max_elements / row_shape.num_elements()) *
                                    row_shape.num_elements()) { ... }   // div by 0
  ...
  int64_t expected_max_rows = expected_max_elements / row_shape.num_elements();  // div by 0
  ```

None of the reachable call sites (`tensorflow/core/kernels/example_parsing_ops.cc`,
`tensorflow/core/ops/parsing_ops.cc`) validate that a configured dense shape
has a non-zero element count before it is used as a divisor.

## Verified impact

Tested against a freshly `pip install`-ed **`tensorflow-cpu==2.21.0`** (current
stable release at the time of testing, matching a fresh `git clone` of
`tensorflow/tensorflow` `master`). All three PoCs reproducibly kill the Python
interpreter with **signal 8 (SIGFPE)**:

```
$ python3 confirm_signal.py
returncode: -8
Killed by signal: 8 SIGFPE
```

* `poc_parse_example_minimal.py` — `tf.io.parse_example([b""], {...})` crashes
  immediately, no TFRecord file, no real feature data at all.
* `poc_parse_sequence_example.py` — same crash via `tf.io.parse_sequence_example`.
* `poc_tfrecord_sigfpe.py` — the realistic end-to-end path: writes one
  perfectly ordinary `tf.train.Example` (`{"label": int64_list([1])}`) to a
  real `.tfrecord` file with `tf.io.TFRecordWriter`, then reads it back with
  the canonical `tf.data.TFRecordDataset(...).batch(1).map(tf.io.parse_example)`
  pipeline. The *data* is 100% benign; only the feature-spec
  (`"extra": tf.io.FixedLenSequenceFeature(shape=[0], dtype=tf.float32,
  allow_missing=True)`) is malicious. The process is killed by SIGFPE the
  moment the first batch is materialized.

## Attacker-controlled surface (malicious model file)

`dense_shapes` / the analogous shape attrs are ordinary attributes of the
`ParseExampleV2`, `ParseSingleExample`, and `ParseSequenceExample` ops. A
`GraphDef`/`SavedModel` is free to embed a pre-processing subgraph (e.g. an
input-parsing signature) that calls one of these ops with a zero-size inner
shape on a variable-length dense feature. Any consumer that loads that model
and invokes the corresponding signature — regardless of what input tensor it
passes — will crash on first use. This is a persistent Denial-of-Service
purely from loading and calling a malicious model file; no separately
malicious dataset/TFRecord is required (though, as `poc_tfrecord_sigfpe.py`
shows, this also fires naturally in the ordinary
`TFRecordDataset → parse_example` data-loading idiom if the feature spec is
ever malformed in this specific way).

## Suggested fix

Reject a `dense_shapes` entry whose element count is `0` for a variable-length
(`allow_missing=True` / leading `-1`) dense feature in `GetDenseShapes()`
(`example_proto_helper.cc`), or guard every division/modulo by
`elements_per_stride` (and the analogous `row_shape.num_elements()` in the
`SequenceExample` dense path) with an explicit `!= 0` check that returns an
`InvalidArgumentError` instead of dividing.

## Files

- `poc_parse_example_minimal.py` — minimal repro via `tf.io.parse_example`.
- `poc_parse_sequence_example.py` — minimal repro via `tf.io.parse_sequence_example`.
- `poc_tfrecord_sigfpe.py` — realistic end-to-end repro starting from a real, valid `.tfrecord` file.
- `confirm_signal.py` — runs the end-to-end PoC as a subprocess and prints the confirmed signal (SIGFPE / 8).

## Environment used to verify

```
$ pip install tensorflow-cpu
$ python3 -c "import tensorflow as tf; print(tf.__version__)"
2.21.0
```