Trigger rollback check even without local snapshot

If you do the following steps:
1. local root is loaded, root is updated until final
2. local timestamp is loaded
3. timestamp is updated (meta (version, hashes, length) for snapshot
changes here)
4. local snapshot is loaded (hashes and length do not match the data in
timestamp, preventing loading) remote snapshot is loaded
5. remote snapshot is loaded
then when executing step 5 the rollback checks will not be done because
the old snapshot was not loaded.

Similar issues for version numbers and expiry are being fixed by
delaying the checks until we know we have the "final" snapshot.
We don't want to do the exact same thing here as the hashes are meant
to prevent even parsing data we don't trust...
This seems to be a case where "trusted local metadata" and
"untrusted data from the network" is the deciding factor.

TrustedMetadataSet does not have this information currently so possibly
the best solution is to add a new argument "trusted" to update_snapshot.
Even though this is ugly as the rest of the update functions doesn't
have such an argument, it seems the best solution as it seems to work
in all cases:
- when loading a local snapshot, we know the data has at some point been
trusted (signatures have been checked): it doesn't need to match hashes
now
- if there is no local snapshot and we're updating from remote, the
remote data must match meta hashes in timestamp
- f there is a local snapshot and we're updating from remote, the remote
data must match meta hashes in timestamp

Signed-off-by: Martin Vrachev <mvrachev@vmware.com>

tests/test_updater_with_simulator.py

@@ -6,18 +6,18 @@
 """Test ngclient Updater using the repository simulator
 """
 
-import logging
 import os
 import sys
 import tempfile
 from typing import Optional
-from tuf.exceptions import UnsignedMetadataError
+from tuf.exceptions import UnsignedMetadataError, BadVersionNumberError
 import unittest
 
 from tuf.ngclient import Updater
 
 from tests import utils
 from tests.repository_simulator import RepositorySimulator
+from securesystemslib import hash as sslib_hash
 
 class TestUpdater(unittest.TestCase):
     # set dump_dir to trigger repository state dumps
@@ -152,6 +152,33 @@ def test_keys_and_signatures(self):
 
         self._run_refresh()
 
+    def test_snapshot_rollback_check_with_missing_local_snapshot(self):
+        # Test triggering snapshot rollback check on a newly downloaded snapshot
+        # when the local snapshot is not loaded due to hash mismatch.
+        # Initialize all metadata and assign targets version higher than 1.
+        self.sim.targets.version = 2
+        self.sim.update_snapshot()
+        self._run_refresh()
+
+        # The new targets should have a lower version than the local trusted one.
+        self.sim.targets.version = 1
+        self.sim.update_snapshot()
+
+        # Calculate the hash of the new modified version of snapshot.
+        digest_object = sslib_hash.digest("sha256")
+        digest_object.update(self.sim._fetch_metadata("snapshot"))
+        new_snapshot_hash = digest_object.hexdigest()
+        # Assign the new hash to meta in timestamp, so that loading the local
+        # snapshot will fail, but loading the next snapshot version will succeed.
+        self.sim.timestamp.snapshot_meta.hashes = {"sha256": new_snapshot_hash}
+        self.sim.update_timestamp()
+
+        # Should fail as a new version of snapshot will be fetched which lowers
+        # the snapshot.meta["targets.json"] version by 1 and throws an error.
+        with self.assertRaises(BadVersionNumberError):
+            self._run_refresh()
+
+
 if __name__ == "__main__":
     if "--dump" in sys.argv:
         TestUpdater.dump_dir = tempfile.mkdtemp()

tuf/ngclient/_internal/trusted_metadata_set.py

@@ -258,7 +258,9 @@ def _check_final_timestamp(self) -> None:
         if self.timestamp.signed.is_expired(self.reference_time):
             raise exceptions.ExpiredMetadataError("timestamp.json is expired")
 
-    def update_snapshot(self, data: bytes) -> None:
+    def update_snapshot(
+        self, data: bytes, trusted: Optional[bool] = False
+    ) -> None:
         """Verifies and loads 'data' as new snapshot metadata.
 
         Note that an intermediate snapshot is allowed to be expired and version
@@ -271,6 +273,8 @@ def update_snapshot(self, data: bytes) -> None:
 
         Args:
             data: unverified new snapshot metadata as bytes
+            trusted: whether to consider the given data as trusted. Default is
+                False.
 
         Raises:
             RepositoryError: data failed to load or verify as final snapshot.
@@ -288,13 +292,15 @@ def update_snapshot(self, data: bytes) -> None:
 
         snapshot_meta = self.timestamp.signed.snapshot_meta
 
-        # Verify against the hashes in timestamp, if any
-        try:
-            snapshot_meta.verify_length_and_hashes(data)
-        except exceptions.LengthOrHashMismatchError as e:
-            raise exceptions.RepositoryError(
-                "Snapshot length or hashes do not match"
-            ) from e
+        # Verify non-trusted data against the hashes in timestamp, if any.
+        # Trusted snapshot data has already been verified once.
+        if not trusted:
+            try:
+                snapshot_meta.verify_length_and_hashes(data)
+            except exceptions.LengthOrHashMismatchError as e:
+                raise exceptions.RepositoryError(
+                    "Snapshot length or hashes do not match"
+                ) from e
 
         try:
             new_snapshot = Metadata[Snapshot].from_bytes(data)

tuf/ngclient/updater.py

@@ -345,7 +345,7 @@ def _load_snapshot(self) -> None:
         """Load local (and if needed remote) snapshot metadata"""
         try:
             data = self._load_local_metadata("snapshot")
-            self._trusted_set.update_snapshot(data)
+            self._trusted_set.update_snapshot(data, trusted=True)
             logger.debug("Local snapshot is valid: not downloading new one")
         except (OSError, exceptions.RepositoryError) as e:
             # Local snapshot does not exist or is invalid: update from remote
@@ -359,7 +359,7 @@ def _load_snapshot(self) -> None:
                 version = snapshot_meta.version
 
             data = self._download_metadata("snapshot", length, version)
-            self._trusted_set.update_snapshot(data)
+            self._trusted_set.update_snapshot(data, trusted=False)
             self._persist_metadata("snapshot", data)
 
     def _load_targets(self, role: str, parent_role: str) -> None: