From 39fa55874151267a86b55095a4c83333d4fa2bc6 Mon Sep 17 00:00:00 2001
From: "Brian S. Stephan" <bss@incorporeal.org>
Date: Wed, 28 Jun 2023 14:34:28 -0500
Subject: [PATCH] check the config's CRC32 checksum while reading

---
 gp2040ce_bintools/storage.py | 14 ++++++++++++--
 tests/test_storage.py        |  8 ++++++++
 2 files changed, 20 insertions(+), 2 deletions(-)

diff --git a/gp2040ce_bintools/storage.py b/gp2040ce_bintools/storage.py
index 79169e1..eea6d53 100644
--- a/gp2040ce_bintools/storage.py
+++ b/gp2040ce_bintools/storage.py
@@ -1,5 +1,6 @@
 """Interact with the protobuf config from a picotool flash dump of a GP2040-CE board."""
 import argparse
+import binascii
 import logging
 
 from google.protobuf.json_format import MessageToJson
@@ -21,6 +22,10 @@ FOOTER_MAGIC = b'\x65\xe3\xf1\xd2'
 #################
 
 
+class ConfigCrcError(ValueError):
+    """Exception raised when the CRC checksum in the footer doesn't match the actual content's."""
+
+
 class ConfigLengthError(ValueError):
     """Exception raised when a length sanity check fails."""
 
@@ -52,7 +57,7 @@ def get_config_footer(content: bytes) -> tuple[int, int, str]:
     Args:
         content: bytes from a GP2040-CE board's storage section
     Returns:
-        the discovered config size, config CRC, and magic from the config footer
+        the discovered config size, config CRC checksum, and magic from the config footer
     Raises:
         ConfigLengthError, ConfigMagicError: if the provided bytes are not a config footer
     """
@@ -70,10 +75,15 @@ def get_config_footer(content: bytes) -> tuple[int, int, str]:
     config_crc = int.from_bytes(reversed(footer[4:8]), 'big')
     config_magic = f'0x{footer[8:12].hex()}'
 
-    # one last sanity check
+    # more sanity checks
     if len(content) < config_size + FOOTER_SIZE:
         raise ConfigLengthError("provided content is not large enough according to the config footer!")
 
+    content_crc = binascii.crc32(content[-(config_size + 12):-12])
+    if config_crc != content_crc:
+        raise ConfigCrcError(f"provided content CRC checksum {content_crc} does not match footer's expected CRC "
+                             f"checksum {config_crc}!")
+
     logger.debug("detected footer (size:%s, crc:%s, magic:%s", config_size, config_crc, config_magic)
     return config_size, config_crc, config_magic
 
diff --git a/tests/test_storage.py b/tests/test_storage.py
index 7c4666b..c3e6d39 100644
--- a/tests/test_storage.py
+++ b/tests/test_storage.py
@@ -56,6 +56,14 @@ def test_config_footer_bad_magic(storage_dump):
         _, _, _ = storage.get_config_footer(unmagical)
 
 
+def test_config_footer_bad_crc(storage_dump):
+    """Test that a config footer isn't detected if the CRC checksums don't match."""
+    corrupt = bytearray(storage_dump)
+    corrupt[-50:-40] = bytearray(0*10)
+    with pytest.raises(storage.ConfigCrcError):
+        _, _, _ = storage.get_config_footer(corrupt)
+
+
 def test_config_fails_without_pb2s(storage_dump):
     """Test that we need the config_pb2 to exist/be compiled for reading the config to work."""
     with pytest.raises(ModuleNotFoundError):