Write signing keys with file mode 0640 (#16740)

Co-authored-by: Fabian Klemp <fabian.klemp@frequentis.com>

changelog.d/16740.bugfix

@@ -0,0 +1 @@
+Fix a long-standing bug where the signing keys generated by Synapse were world-readable. Contributed by Fabian Klemp.

synapse/_scripts/generate_signing_key.py

@@ -13,6 +13,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 import argparse
+import os
 import sys
 
 from signedjson.key import generate_signing_key, write_signing_keys
@@ -26,15 +27,21 @@ def main() -> None:
     parser.add_argument(
         "-o",
         "--output_file",
-        type=argparse.FileType("w"),
-        default=sys.stdout,
+        type=str,
+        default="-",
         help="Where to write the output to",
     )
     args = parser.parse_args()
 
     key_id = "a_" + random_string(4)
     key = (generate_signing_key(key_id),)
-    write_signing_keys(args.output_file, key)
+    if args.output_file == "-":
+        write_signing_keys(sys.stdout, key)
+    else:
+        with open(
+            args.output_file, "w", opener=lambda p, f: os.open(p, f, mode=0o640)
+        ) as signing_key_file:
+            write_signing_keys(signing_key_file, key)
 
 
 if __name__ == "__main__":

synapse/config/key.py

@@ -263,7 +263,9 @@ class KeyConfig(Config):
 
         if not self.path_exists(signing_key_path):
             print("Generating signing key file %s" % (signing_key_path,))
-            with open(signing_key_path, "w") as signing_key_file:
+            with open(
+                signing_key_path, "w", opener=lambda p, f: os.open(p, f, mode=0o640)
+            ) as signing_key_file:
                 key_id = "a_" + random_string(4)
                 write_signing_keys(signing_key_file, (generate_signing_key(key_id),))
         else:
@@ -274,7 +276,9 @@ class KeyConfig(Config):
                 key = decode_signing_key_base64(
                     NACL_ED25519, key_id, signing_keys.split("\n")[0]
                 )
-                with open(signing_key_path, "w") as signing_key_file:
+                with open(
+                    signing_key_path, "w", opener=lambda p, f: os.open(p, f, mode=0o640)
+                ) as signing_key_file:
                     write_signing_keys(signing_key_file, (key,))