CHECK-fail in tf.raw_ops.EncodePng

Impact

An attacker can trigger a CHECK fail in PNG encoding by providing an empty input tensor as the pixel data:

import tensorflow as tf

image = tf.zeros([0, 0, 3])
image = tf.cast(image, dtype=tf.uint8) 
tf.raw_ops.EncodePng(image=image) 

This is because the implementation only validates that the total number of pixels in the image does not overflow. Thus, an attacker can send an empty matrix for encoding. However, if the tensor is empty, then the associated buffer is nullptr. Hence, when calling png::WriteImageToBuffer, the first argument (i.e., image.flat<T>().data()) is NULL. This then triggers the CHECK_NOTNULL in the first line of png::WriteImageToBuffer.

template <typename T>
bool WriteImageToBuffer(
    const void* image, int width, int height, int row_bytes, int num_channels,
    int channel_bits, int compression, T* png_string,
    const std::vector<std::pair<std::string, std::string> >* metadata) {
  CHECK_NOTNULL(image);
  ...
}

Since image is null, this results in abort being called after printing the stacktrace. Effectively, this allows an attacker to mount a denial of service attack.

Patches

We have patched the issue in GitHub commit 26eb323554ffccd173e8a79a8c05c15b685ae4d1.