Make sure the end offset doesn't overflow

We used to only check that the length was not too big but didn't check that the
starting offset wasn't too big. We want the end offset to not overflow.

src/ctap/large_blobs.rs

@@ -62,7 +62,7 @@ impl LargeBlobs {
         const MAX_FRAGMENT_LENGTH: usize = MAX_MSG_SIZE - 64;
 
         if let Some(get) = get {
-            if get > MAX_FRAGMENT_LENGTH {
+            if get > MAX_FRAGMENT_LENGTH || offset.checked_add(get).is_none() {
                 return Err(Ctap2StatusCode::CTAP1_ERR_INVALID_LENGTH);
             }
             let config = persistent_store.get_large_blob_array(offset, get)?;
@@ -328,6 +328,30 @@ mod test {
         );
     }
 
+    #[test]
+    fn test_process_command_commit_end_offset_overflow() {
+        let mut rng = ThreadRng256 {};
+        let mut persistent_store = PersistentStore::new(&mut rng);
+        let key_agreement_key = crypto::ecdh::SecKey::gensk(&mut rng);
+        let pin_uv_auth_token = [0x55; 32];
+        let mut client_pin =
+            ClientPin::new_test(key_agreement_key, pin_uv_auth_token, PinUvAuthProtocol::V1);
+        let mut large_blobs = LargeBlobs::new();
+
+        let large_blobs_params = AuthenticatorLargeBlobsParameters {
+            get: Some(1),
+            set: None,
+            offset: usize::MAX,
+            length: None,
+            pin_uv_auth_param: None,
+            pin_uv_auth_protocol: None,
+        };
+        assert_eq!(
+            large_blobs.process_command(&mut persistent_store, &mut client_pin, large_blobs_params),
+            Err(Ctap2StatusCode::CTAP1_ERR_INVALID_LENGTH),
+        );
+    }
+
     #[test]
     fn test_process_command_commit_unexpected_hash() {
         let mut rng = ThreadRng256 {};