Repository: quantumlib/ReCirq

Issue:
Unsafe writing of tar file in recirq/documentation_utils.py
CodeQL scanning alerts flagged recirq/documentation_utils.py:72 for an unsafe action: it downloads a tar file from a 3rd-party site and then extracts it without validating the paths inside the tar file.

From the alert:

> Extracting files from a malicious tar archive without validating that the destination file path is within the destination directory can cause files outside the destination directory to be overwritten, due to the possible presence of directory traversal elements (..) in archive paths.

The code in question:

```python
 stream = urllib.request.urlopen("https://ndownloader.figshare.com/files/25541666")

 with tarfile.open(fileobj=stream, mode='r|xz') as tf:
     tf.extractall(path=base_dir)
```


Discussion hints:

Solution patch:
diff --git a/recirq/documentation_utils.py b/recirq/documentation_utils.py
index c6428ed..f3d0029 100644
--- a/recirq/documentation_utils.py
+++ b/recirq/documentation_utils.py
@@ -13,6 +13,7 @@
 # limitations under the License.
 
 import inspect
+import io
 import os
 import re
 import tarfile
@@ -67,6 +68,14 @@ def fetch_guide_data_collection_data(base_dir=None):
 
     print("Downloading guide/data_collection data.")
     stream = urllib.request.urlopen("https://ndownloader.figshare.com/files/25541666")
-
-    with tarfile.open(fileobj=stream, mode='r|xz') as tf:
-        tf.extractall(path=base_dir)
+    # Read into a BytesIO object to make it seekable
+    stream_bytes = io.BytesIO(stream.read())
+
+    with tarfile.open(fileobj=stream_bytes, mode='r|xz') as tf:
+        for member in tf.getmembers():
+            # Ensure the path being extracted is safe.
+            if not os.path.abspath(os.path.join(base_dir, member.name)).startswith(
+                os.path.abspath(base_dir)
+            ):
+                raise ValueError(f"Encountered untrusted path {member.name}")
+            tf.extract(member, path=base_dir)


Test patch:
diff --git a/recirq/documentation_utils_test.py b/recirq/documentation_utils_test.py
index d203452..209a6f2 100644
--- a/recirq/documentation_utils_test.py
+++ b/recirq/documentation_utils_test.py
@@ -12,6 +12,11 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import os
+import io
+import tarfile
+from unittest import mock
+
+import pytest
 
 import recirq
 from recirq.readout_scan.tasks import ReadoutScanTask
@@ -35,6 +40,23 @@ which is primarily affected by readout error.
 """
 
 
-def test_fetch_guide_data_collection_data(tmpdir):
-    recirq.fetch_guide_data_collection_data(base_dir=tmpdir)
-    assert os.path.exists(f'{tmpdir}/2020-02-tutorial')
+@mock.patch('urllib.request.urlopen')
+def test_fetch_guide_data_collection_data_traversal(mock_urlopen, tmpdir):
+    # Create a malicious tarball in memory.
+    malicious_tar_stream = io.BytesIO()
+    with tarfile.open(fileobj=malicious_tar_stream, mode='w:xz') as tf:
+        # Add a file that tries to write outside the target directory
+        malicious_info = tarfile.TarInfo(name="../../tmp/pwned")
+        tf.addfile(malicious_info, io.BytesIO(b"pwned"))
+    malicious_tar_stream.seek(0)
+
+    # Read the stream into a BytesIO object so that the mock should return a
+    # response object whose read() method returns the tarball content.
+    mock_response = mock.Mock()
+    mock_response.read.return_value = malicious_tar_stream.getvalue()
+    mock_urlopen.return_value = mock_response
+
+    with pytest.raises(ValueError, match="Encountered untrusted path"):
+        recirq.fetch_guide_data_collection_data(base_dir=tmpdir)
+
+    assert not os.path.exists('/tmp/pwned')