don't follow symlinks in the debugger tmp directory

ipykernel/debugger.py

@@ -2,6 +2,7 @@
 
 import os
 import re
+import stat
 import sys
 import typing as t
 from pathlib import Path
@@ -304,6 +305,18 @@ async def send_dap_request(self, msg):
         return rep
 
 
+def _is_own_directory(path):
+    """Whether path is a directory, and not a symlink, belonging to this user."""
+    try:
+        st = os.lstat(path)
+    except OSError:
+        return False
+    if not stat.S_ISDIR(st.st_mode):
+        return False
+    getuid = getattr(os, "getuid", None)
+    return getuid is None or st.st_uid == getuid()
+
+
 class Debugger:
     """The debugger class."""
 
@@ -427,8 +440,16 @@ def start(self):
         """Start the debugger."""
         if not self.debugpy_initialized:
             tmp_dir = get_tmp_directory()
-            if not Path(tmp_dir).exists():
+            try:
                 Path(tmp_dir).mkdir(mode=0o700, parents=True)
+            except FileExistsError:
+                if not _is_own_directory(tmp_dir):
+                    self.log.error(
+                        "Not starting the debugger: %s already exists and is not a"
+                        " directory belonging to this user",
+                        tmp_dir,
+                    )
+                    return False
             host, port = self.debugpy_client.get_host_port()
             code = "import debugpy;"
             code += 'debugpy.listen(("' + host + '",' + port + "))"
@@ -470,7 +491,9 @@ async def dumpCell(self, message):
         code = message["arguments"]["code"]
         file_name = get_file_name(code)
 
-        with open(file_name, "w", encoding="utf-8") as f:  # noqa: ASYNC230
+        flags = os.O_WRONLY | os.O_CREAT | os.O_TRUNC | getattr(os, "O_NOFOLLOW", 0)
+        fd = os.open(file_name, flags, 0o600)
+        with open(fd, "w", encoding="utf-8") as f:  # noqa: ASYNC230
             f.write(code)
 
         return {

tests/test_debugger.py

@@ -1,3 +1,4 @@
+import logging
 import sys
 
 import pytest
@@ -368,6 +369,41 @@ def test_convert_to_long_pathname():
         _convert_to_long_pathname(__file__)
 
 
+@pytest.mark.skipif(debugpy is None, reason="requires debugpy")
+@pytest.mark.skipif(sys.platform == "win32", reason="symlinks need privileges on windows")
+def test_start_rejects_foreign_tmp_directory(tmp_path, monkeypatch):
+    from ipykernel.debugger import Debugger
+
+    # <tempdir>/ipykernel_<pid> is predictable, so on a shared temp directory
+    # another user can get there first and point it at a path they own.
+    elsewhere = tmp_path / "elsewhere"
+    elsewhere.mkdir()
+    planted = tmp_path / "ipykernel_1"
+    planted.symlink_to(elsewhere, target_is_directory=True)
+    monkeypatch.setattr("ipykernel.debugger.get_tmp_directory", lambda: str(planted))
+
+    debugger = Debugger(logging.getLogger(__name__), None, lambda event: None, None, None, [])
+    assert debugger.start() is False
+
+
+@pytest.mark.skipif(debugpy is None, reason="requires debugpy")
+@pytest.mark.skipif(sys.platform == "win32", reason="O_NOFOLLOW is posix only")
+async def test_dump_cell_does_not_follow_symlink(tmp_path, monkeypatch):
+    from ipykernel.debugger import Debugger
+
+    outside = tmp_path / "outside.py"
+    outside.write_text("# untouched\n")
+    cell_file = tmp_path / "cell.py"
+    cell_file.symlink_to(outside)
+    monkeypatch.setenv("IPYKERNEL_CELL_NAME", str(cell_file))
+
+    debugger = Debugger(logging.getLogger(__name__), None, lambda event: None, None, None, [])
+    message = {"seq": 1, "command": "dumpCell", "arguments": {"code": "x = 1\n"}}
+    with pytest.raises(OSError, match="symbolic link"):
+        await debugger.dumpCell(message)
+    assert outside.read_text() == "# untouched\n"
+
+
 def test_copy_to_globals(kernel_with_debug):
     local_var_name = "var"
     global_var_name = "var_copy"