gh-139400: Make sure that parent parsers outlive their subparsers in `pyexpat` by hartwork · Pull Request #139403 · python/cpython

hartwork · 2025-09-28T22:21:08Z

Issue: Heap-use-after-free issue in pyexpat related to .ExternalEntityParserCreate #139400

Within libexpat, a parser created via XML_ExternalEntityParserCreate is relying on its parent parser throughout its entire lifetime. Prior to this fix, is was possible for the parent parser to be garbage-collected by CPython. This fixes related reference counting, to stop that from happening.

picnixz

Now, this looks like Tcl/Tk and their callbacks. If you want to check for reference leaks, you should also run the tests with -R (python -m test -R the_test)

Modules/pyexpat.c

@picnixz

Idea by @picnixz

@picnixz

Idea by @picnixz

hartwork · 2025-09-28T23:12:13Z

Now, this looks like Tcl/Tk and their callbacks.

@picnixz I'm not sure what that means 😃

If you want to check for reference leaks, you should also run the tests with -R (python -m test -R the_test)

I cannot figure out why or what to do with python -m test -R the_test. Could you spell it out for me? 🙏

picnixz · 2025-09-28T23:26:35Z

I cannot figure out why or what to do with python -m test -R the_test. Could you spell it out for me? 🙏

Sorry, to be precise the -R : option checks if there are reference leaks. In your case run:

$ ./python -m test test_pyexpat -m test_parent_parser_outlives_its_subparsers -R :

picnixz · 2025-09-28T23:27:40Z

I'm not sure what that means

Tcl/Tk is a graphic library that allows to have delayed callbacks and the callback data is the Python object. Here, for Expat, the handler data is the XML parser itself as well. I wanted to make a parallel between those two constructions.

picnixz

I'm a bit tired (it's 01:33 AM here) so the review might not be perfect. I'll have another look tomorrow.

Modules/pyexpat.c

Lib/test/test_pyexpat.py

Modules/pyexpat.c

hartwork · 2025-09-28T23:44:46Z

I cannot figure out why or what to do with python -m test -R the_test. Could you spell it out for me? 🙏

Sorry, to be precise the -R : option checks if there are reference leaks. In your case run:
$ ./python -m test test_pyexpat -m test_parent_parser_outlives_its_subparsers -R :

@picnixz after a recompile with --with-pydebug that worked — thank you! 🙏

@picnixz

Idea by @picnixz

@picnixz

Idea by @picnixz

As requested.

hartwork · 2025-09-28T23:54:25Z

I'm a bit tired (it's 01:33 AM here) so the review might not be perfect. I'll have another look tomorrow.

@picnixz no worries, same UTC+2 here. Thank you! 👍 👍

hartwork · 2025-09-28T23:57:42Z

Related: libexpat/libexpat#1066

Misc/NEWS.d/next/Core_and_Builtins/2025-09-29-00-01-28.gh-issue-139400.X2T-jO.rst

Modules/pyexpat.c

@picnixz

As suggested by @picnixz

@picnixz

Suggested by @picnixz

Modules/pyexpat.c

@picnixz

Kudos to @picnixz

@picnixz

As suggested by @picnixz

picnixz · 2025-10-05T15:38:34Z

Lucky me, 3.14 has a clean backport :)

picnixz · 2025-10-05T15:46:01Z

@hartwork Could you make the backports for 3.10-3.13 please? I'm not on my dev session now (sorry for bothering you with this).

hartwork · 2025-10-05T15:46:36Z

@picnixz thanks for your help getting this over the finish line 🙏 🎉 I can look into the backports for this and report back if I hit any roadblocks.

picnixz · 2025-10-05T15:50:18Z

Thank you for writing libexpat :')

…rs in `pyexpat` (python#139403) * Modules/pyexpat.c: Disallow collection of in-use parent parsers. Within libexpat, a parser created via `XML_ExternalEntityParserCreate` is relying on its parent parser throughout its entire lifetime. Prior to this fix, is was possible for the parent parser to be garbage-collected too early. (cherry picked from commit 6edb2dd)

bedevere-app · 2025-10-05T16:01:28Z

GH-139608 is a backport of this pull request to the 3.13 branch.

…ers in `pyexpat` (GH-139403) (#139606) Co-authored-by: Sebastian Pipping <sebastian@pipping.org>

…rs in `pyexpat` (python#139403) * Modules/pyexpat.c: Disallow collection of in-use parent parsers. Within libexpat, a parser created via `XML_ExternalEntityParserCreate` is relying on its parent parser throughout its entire lifetime. Prior to this fix, is was possible for the parent parser to be garbage-collected too early. (cherry picked from commit 6edb2dd)

bedevere-app · 2025-10-05T16:08:16Z

GH-139609 is a backport of this pull request to the 3.12 branch.

…rs in `pyexpat` (python#139403) * Modules/pyexpat.c: Disallow collection of in-use parent parsers. Within libexpat, a parser created via `XML_ExternalEntityParserCreate` is relying on its parent parser throughout its entire lifetime. Prior to this fix, is was possible for the parent parser to be garbage-collected too early. (cherry picked from commit 6edb2dd)

bedevere-app · 2025-10-05T16:21:49Z

GH-139612 is a backport of this pull request to the 3.11 branch.

bedevere-app · 2025-10-05T16:22:13Z

GH-139608 is a backport of this pull request to the 3.13 branch.

bedevere-app · 2025-10-05T16:22:19Z

GH-139609 is a backport of this pull request to the 3.12 branch.

bedevere-app · 2025-10-05T16:22:25Z

GH-139612 is a backport of this pull request to the 3.11 branch.

…rs in `pyexpat` (python#139403) * Modules/pyexpat.c: Disallow collection of in-use parent parsers. Within libexpat, a parser created via `XML_ExternalEntityParserCreate` is relying on its parent parser throughout its entire lifetime. Prior to this fix, is was possible for the parent parser to be garbage-collected too early. (cherry picked from commit 6edb2dd)

bedevere-app · 2025-10-05T16:27:07Z

GH-139613 is a backport of this pull request to the 3.10 branch.

…rs in `pyexpat` (python#139403) * Modules/pyexpat.c: Disallow collection of in-use parent parsers. Within libexpat, a parser created via `XML_ExternalEntityParserCreate` is relying on its parent parser throughout its entire lifetime. Prior to this fix, is was possible for the parent parser to be garbage-collected too early. (cherry picked from commit 6edb2dd)

bedevere-app · 2025-10-05T16:35:02Z

GH-139614 is a backport of this pull request to the 3.9 branch.

…ers in `pyexpat` (GH-139403) (GH-139608) Within libexpat, a parser created via `XML_ExternalEntityParserCreate` is relying on its parent parser throughout its entire lifetime. Prior to this fix, is was possible for the parent parser to be garbage-collected too early. (cherry picked from commit 6edb2dd)

…ers in `pyexpat` (GH-139403) (#139612) * gh-139400: Make sure that parent parsers outlive their subparsers in `pyexpat` (#139403) * Modules/pyexpat.c: Disallow collection of in-use parent parsers. Within libexpat, a parser created via `XML_ExternalEntityParserCreate` is relying on its parent parser throughout its entire lifetime. Prior to this fix, is was possible for the parent parser to be garbage-collected too early. (cherry picked from commit 6edb2dd) * Move news item from section "Core and Builtins" to section "Security"

…ers in `pyexpat` (GH-139403) (#139613) * gh-139400: Make sure that parent parsers outlive their subparsers in `pyexpat` (#139403) * Modules/pyexpat.c: Disallow collection of in-use parent parsers. Within libexpat, a parser created via `XML_ExternalEntityParserCreate` is relying on its parent parser throughout its entire lifetime. Prior to this fix, is was possible for the parent parser to be garbage-collected too early. (cherry picked from commit 6edb2dd) * Move news item to from section "Core and Builtins" to section "Security"

bedevere-bot · 2025-10-07T01:50:06Z

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 Fedora Stable Refleaks 3.11 (tier-1) has failed when building commit 1459d1f.

What do you need to do:

Don't panic.
Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/1027/builds/823) and take a look at the build logs.
Check if the failure is related to this commit (1459d1f) or if it is a false positive.
If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/#/builders/1027/builds/823

Failed tests:

test_socket

Failed subtests:

test_aead_aes_gcm - test.test_socket.LinuxKernelCryptoAPI.test_aead_aes_gcm

Summary of the results of the build (if available):

==

Click to see traceback logs

Traceback (most recent call last):
  File "/home/buildbot/buildarea/3.11.cstratak-fedora-stable-x86_64.refleak/build/Lib/test/test_socket.py", line 6557, in test_aead_aes_gcm
    op.sendall(plain)
OSError: [Errno 22] Invalid argument

…rs in `pyexpat` (python#139403) * Modules/pyexpat.c: Disallow collection of in-use parent parsers. Within libexpat, a parser created via `XML_ExternalEntityParserCreate` is relying on its parent parser throughout its entire lifetime. Prior to this fix, is was possible for the parent parser to be garbage-collected too early.

…rs in `pyexpat` (GH-139403) (GH-139614) Within libexpat, a parser created via `XML_ExternalEntityParserCreate` is relying on its parent parser throughout its entire lifetime. Prior to this fix, is was possible for the parent parser to be garbage-collected too early. (cherry picked from commit 6edb2dd)

…ers in `pyexpat` (GH-139403) (GH-139609) Within libexpat, a parser created via `XML_ExternalEntityParserCreate` is relying on its parent parser throughout its entire lifetime. Prior to this fix, is was possible for the parent parser to be garbage-collected too early. (cherry picked from commit 6edb2dd)

hartwork added 3 commits September 29, 2025 00:17

Add a regression test for issue 139400

3b0afda

Add news item for issue 139400

8f70991

bedevere-app bot added the awaiting review label Sep 28, 2025

bedevere-app bot mentioned this pull request Sep 28, 2025

Heap-use-after-free issue in pyexpat related to .ExternalEntityParserCreate #139400

Closed

picnixz self-requested a review September 28, 2025 22:45

Improve test's docstring

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picnixz reviewed Sep 28, 2025

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hartwork added 2 commits September 29, 2025 01:04

Simplify xmlparseobject_ to xmlparseobject

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Idea by @picnixz

Leverage Py_CLEAR

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hartwork requested a review from picnixz September 28, 2025 23:13

picnixz reviewed Sep 28, 2025

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hartwork added 3 commits September 29, 2025 01:45

Simplify call to Py_CLEAR

f8c58a3

Idea by @picnixz

Streamline docsting of ParentParserLifetimeTest

f1e3e36

Idea by @picnixz

pyexpat.c: Adjust whitespace around pointer

1b2937d

As requested.

hartwork requested a review from picnixz September 29, 2025 14:06

picnixz reviewed Sep 29, 2025

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Re-write news item

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As suggested by @picnixz

hartwork requested a review from picnixz September 29, 2025 21:04

hartwork mentioned this pull request Sep 30, 2025

[3.14] gh-90949: add Expat API to prevent XML deadly allocations (CVE-2025-59375) (GH-139234) #139359

Merged

Turn parent into a PyObject pointer

a6f5520

Suggested by @picnixz

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hartwork added 2 commits September 30, 2025 15:49

Add missing parent visit to xmlparse_traverse

4470cea

Kudos to @picnixz

Re-write comment about mission of "PyObject *parent"

3759ac7

As suggested by @picnixz

bedevere-app bot removed the needs backport to 3.13 bugs and security fixes label Oct 5, 2025

hugovk pushed a commit that referenced this pull request Oct 5, 2025

[3.14] gh-139400: Make sure that parent parsers outlive their subpars…

66ca0d9

…ers in `pyexpat` (GH-139403) (#139606) Co-authored-by: Sebastian Pipping <sebastian@pipping.org>

bedevere-app bot removed the needs backport to 3.12 only security fixes label Oct 5, 2025

bedevere-app bot removed the needs backport to 3.11 only security fixes label Oct 5, 2025

bedevere-app bot removed the needs backport to 3.10 only security fixes label Oct 5, 2025

hartwork mentioned this pull request Oct 6, 2025

[3.9] gh-139400: Make sure that parent parsers outlive their subparsers in pyexpat (GH-139403) #139614

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