avbroot/boot.py: Check KMI version when using prepatched image

This ensures that the user isn't replacing the kernel image with another
one that's not binary compatible (based on the GKI kernel module
interface version).

Signed-off-by: Andrew Gunnerson <accounts+github@chiller3.com>

avbroot/boot.py

@@ -1,6 +1,7 @@
 import hashlib
 import io
 import lzma
+import re
 import shutil
 import zipfile
 
@@ -235,6 +236,9 @@ class PrepatchedImage(BootImagePatch):
     be higher than the original image.
     '''
 
+    VERSION_REGEX = re.compile(
+        b'Linux version (\d+\.\d+).\d+-(android\d+)-(\d+)-')
+
     def __init__(self, prepatched):
         self.prepatched = prepatched
 
@@ -272,6 +276,14 @@ class PrepatchedImage(BootImagePatch):
                           f'{len(boot_image.ramdisks)} -> '
                           f'{len(prepatched_image.ramdisks)}')
 
+        if boot_image.kernel is not None:
+            old_kmi = self._get_kmi_version(boot_image)
+            new_kmi = self._get_kmi_version(prepatched_image)
+
+            if old_kmi != new_kmi:
+                errors.append('Kernel module interface version changed: '
+                              f'{old_kmi} -> {new_kmi}')
+
         if errors:
             raise ValueError('The prepatched boot image is not compatible '
                              'with the original:\n' +
@@ -279,6 +291,23 @@ class PrepatchedImage(BootImagePatch):
 
         return prepatched_image
 
+    @classmethod
+    def _get_kmi_version(cls, boot_image):
+        try:
+            with (
+                io.BytesIO(boot_image.kernel) as f_raw,
+                compression.CompressedFile(f_raw, 'rb') as f,
+            ):
+                decompressed = f.fp.read()
+        except ValueError:
+            decompressed = boot_image.kernel
+
+        m = cls.VERSION_REGEX.search(decompressed)
+        if not m:
+            return None
+
+        return b'-'.join(m.groups()).decode('ascii')
+
 
 def patch_boot(avb, input_path, output_path, key, passphrase,
                only_if_previously_signed, patch_funcs):