Sandbox::https://github.com/msgpack/msgpack-python/pull/677

KowalskiThomas · 2026-05-26T15:26:20Z

What is this PR?

There currently is a crash happening when unpacking data from a non-contiguous input.

The current PR adds a test to confirm the problem is not happening anymore as well as the fix itself.
Running the reproducer with the fix applied makes the crash go away.

This is a reproducer:

packed = packb(2**32)
padded = bytearray()
for byte in packed:
    padded.append(byte)
    padded.append(0)

noncont = memoryview(bytes(padded))[::2]
assert not noncont.c_contiguous
assert unpackb(noncont) == 2**32

Running it results in the following:

ASAN_OPTIONS=detect_leaks=0 python -m pytest test/test_memoryview.py -k test_unpack_noncontiguous_memoryview
================================================================= test session starts =================================================================
platform darwin -- Python 3.16.0a0, pytest-9.0.3, pluggy-1.6.0
rootdir: /Users/thomas.kowalski/Documents/msgpack-python
configfile: pyproject.toml
collected 14 items / 13 deselected / 1 selected                                                                                                       

test/test_memoryview.py Fatal Python error: Aborted

Current thread 0x00000001ee7898c0 (most recent call first):
  File "/Users/thomas.kowalski/Documents/msgpack-python/test/test_memoryview.py", line 116 in test_unpack_noncontiguous_memoryview
  File "/Users/thomas.kowalski/Documents/msgpack-python/venv-asan/lib/python3.16/site-packages/_pytest/python.py", line 166 in pytest_pyfunc_call
  File "/Users/thomas.kowalski/Documents/msgpack-python/venv-asan/lib/python3.16/site-packages/pluggy/_callers.py", line 121 in _multicall
  File "/Users/thomas.kowalski/Documents/msgpack-python/venv-asan/lib/python3.16/site-packages/pluggy/_manager.py", line 120 in _hookexec
  File "/Users/thomas.kowalski/Documents/msgpack-python/venv-asan/lib/python3.16/site-packages/pluggy/_hooks.py", line 512 in __call__
  File "/Users/thomas.kowalski/Documents/msgpack-python/venv-asan/lib/python3.16/site-packages/_pytest/python.py", line 1720 in runtest
  File "/Users/thomas.kowalski/Documents/msgpack-python/venv-asan/lib/python3.16/site-packages/_pytest/runner.py", line 179 in pytest_runtest_call
  File "/Users/thomas.kowalski/Documents/msgpack-python/venv-asan/lib/python3.16/site-packages/pluggy/_callers.py", line 121 in _multicall
  File "/Users/thomas.kowalski/Documents/msgpack-python/venv-asan/lib/python3.16/site-packages/pluggy/_manager.py", line 120 in _hookexec
  File "/Users/thomas.kowalski/Documents/msgpack-python/venv-asan/lib/python3.16/site-packages/pluggy/_hooks.py", line 512 in __call__
  File "/Users/thomas.kowalski/Documents/msgpack-python/venv-asan/lib/python3.16/site-packages/_pytest/runner.py", line 245 in <lambda>
  File "/Users/thomas.kowalski/Documents/msgpack-python/venv-asan/lib/python3.16/site-packages/_pytest/runner.py", line 353 in from_call
  File "/Users/thomas.kowalski/Documents/msgpack-python/venv-asan/lib/python3.16/site-packages/_pytest/runner.py", line 244 in call_and_report
  File "/Users/thomas.kowalski/Documents/msgpack-python/venv-asan/lib/python3.16/site-packages/_pytest/runner.py", line 137 in runtestprotocol
  File "/Users/thomas.kowalski/Documents/msgpack-python/venv-asan/lib/python3.16/site-packages/_pytest/runner.py", line 118 in pytest_runtest_protocol
  File "/Users/thomas.kowalski/Documents/msgpack-python/venv-asan/lib/python3.16/site-packages/pluggy/_callers.py", line 121 in _multicall
  File "/Users/thomas.kowalski/Documents/msgpack-python/venv-asan/lib/python3.16/site-packages/pluggy/_manager.py", line 120 in _hookexec
  File "/Users/thomas.kowalski/Documents/msgpack-python/venv-asan/lib/python3.16/site-packages/pluggy/_hooks.py", line 512 in __call__
  File "/Users/thomas.kowalski/Documents/msgpack-python/venv-asan/lib/python3.16/site-packages/_pytest/main.py", line 396 in pytest_runtestloop
  File "/Users/thomas.kowalski/Documents/msgpack-python/venv-asan/lib/python3.16/site-packages/pluggy/_callers.py", line 121 in _multicall
  File "/Users/thomas.kowalski/Documents/msgpack-python/venv-asan/lib/python3.16/site-packages/pluggy/_manager.py", line 120 in _hookexec
  File "/Users/thomas.kowalski/Documents/msgpack-python/venv-asan/lib/python3.16/site-packages/pluggy/_hooks.py", line 512 in __call__
  File "/Users/thomas.kowalski/Documents/msgpack-python/venv-asan/lib/python3.16/site-packages/_pytest/main.py", line 372 in _main
  File "/Users/thomas.kowalski/Documents/msgpack-python/venv-asan/lib/python3.16/site-packages/_pytest/main.py", line 318 in wrap_session
  File "/Users/thomas.kowalski/Documents/msgpack-python/venv-asan/lib/python3.16/site-packages/_pytest/main.py", line 365 in pytest_cmdline_main
  File "/Users/thomas.kowalski/Documents/msgpack-python/venv-asan/lib/python3.16/site-packages/pluggy/_callers.py", line 121 in _multicall
  File "/Users/thomas.kowalski/Documents/msgpack-python/venv-asan/lib/python3.16/site-packages/pluggy/_manager.py", line 120 in _hookexec
  File "/Users/thomas.kowalski/Documents/msgpack-python/venv-asan/lib/python3.16/site-packages/pluggy/_hooks.py", line 512 in __call__
  File "/Users/thomas.kowalski/Documents/msgpack-python/venv-asan/lib/python3.16/site-packages/_pytest/config/__init__.py", line 199 in main
  File "/Users/thomas.kowalski/Documents/msgpack-python/venv-asan/lib/python3.16/site-packages/_pytest/config/__init__.py", line 223 in console_main
  File "/Users/thomas.kowalski/Documents/msgpack-python/venv-asan/lib/python3.16/site-packages/pytest/__main__.py", line 9 in <module>
  File "<frozen runpy>", line 87 in _run_code
  File "<frozen runpy>", line 201 in _run_module_as_main

Current thread's C stack trace (most recent call first):
  Binary file "/Users/thomas.kowalski/Documents/cpython-asan/install/bin/python3.16", at _Py_DumpStack+0xf4 [0x102fc4bc0]
  Binary file "/Users/thomas.kowalski/Documents/cpython-asan/install/bin/python3.16", at faulthandler_fatal_error+0x470 [0x10300492c]
  Binary file "/usr/lib/system/libsystem_platform.dylib", at _sigtramp+0x38 [0x1829897a4]
  Binary file "/usr/lib/system/libsystem_pthread.dylib", at pthread_kill+0x128 [0x18297f8d8]
  Binary file "/usr/lib/system/libsystem_c.dylib", at abort+0x94 [0x182886790]
  Binary file "/opt/homebrew/Cellar/llvm/22.1.5/lib/clang/22/lib/darwin/libclang_rt.asan_osx_dynamic.dylib", at _ZN11__sanitizer6AtexitEPFvvE+0x0 [0x103efc4cc]
  Binary file "/opt/homebrew/Cellar/llvm/22.1.5/lib/clang/22/lib/darwin/libclang_rt.asan_osx_dynamic.dylib", at _ZN11__sanitizer3DieEv+0x68 [0x103efb9fc]
  Binary file "/opt/homebrew/Cellar/llvm/22.1.5/lib/clang/22/lib/darwin/libclang_rt.asan_osx_dynamic.dylib", at _ZN6__asan19ScopedInErrorReportD2Ev+0x4a8 [0x103eddebc]
  Binary file "/opt/homebrew/Cellar/llvm/22.1.5/lib/clang/22/lib/darwin/libclang_rt.asan_osx_dynamic.dylib", at _ZN6__asan18ReportGenericErrorEmmmmbmjb+0x78c [0x103edd130]
  Binary file "/opt/homebrew/Cellar/llvm/22.1.5/lib/clang/22/lib/darwin/libclang_rt.asan_osx_dynamic.dylib", at __asan_report_load1+0x3c [0x103ede3cc]
  Binary file "/Users/thomas.kowalski/Documents/msgpack-python/msgpack/_cmsgpack.cpython-316-darwin.so", at unpack_execute+0x3b0 [0x10ec58328]
  Binary file "/Users/thomas.kowalski/Documents/msgpack-python/msgpack/_cmsgpack.cpython-316-darwin.so", at unpack_construct+0x38 [0x10ec57f6c]
  Binary file "/Users/thomas.kowalski/Documents/msgpack-python/msgpack/_cmsgpack.cpython-316-darwin.so", at __pyx_pf_7msgpack_9_cmsgpack_2unpackb+0x7c0 [0x10ec70a48]
  Binary file "/Users/thomas.kowalski/Documents/msgpack-python/msgpack/_cmsgpack.cpython-316-darwin.so", at __pyx_pw_7msgpack_9_cmsgpack_3unpackb+0xd7c [0x10ec7010c]
  Binary file "/Users/thomas.kowalski/Documents/msgpack-python/msgpack/_cmsgpack.cpython-316-darwin.so", at __Pyx_CyFunction_Vectorcall_FASTCALL_KEYWORDS+0x198 [0x10ec6e430]
  Binary file "/Users/thomas.kowalski/Documents/cpython-asan/install/bin/python3.16", at PyObject_Vectorcall+0xdc [0x102ada240]
  Binary file "/Users/thomas.kowalski/Documents/cpython-asan/install/bin/python3.16", at _Py_VectorCallInstrumentation_StackRefSteal+0x2b8 [0x102e30e10]
  Binary file "/Users/thomas.kowalski/Documents/cpython-asan/install/bin/python3.16", at _PyEval_EvalFrameDefault+0x1d998 [0x102e50dc4]
  Binary file "/Users/thomas.kowalski/Documents/cpython-asan/install/bin/python3.16", at _PyEval_Vector+0x420 [0x102e30068]
  Binary file "/Users/thomas.kowalski/Documents/cpython-asan/install/bin/python3.16", at _PyObject_VectorcallDictTstate+0x1a4 [0x102ad88a0]
  Binary file "/Users/thomas.kowalski/Documents/cpython-asan/install/bin/python3.16", at _PyObject_Call_Prepend+0x134 [0x102adb134]
  Binary file "/Users/thomas.kowalski/Documents/cpython-asan/install/bin/python3.16", at slot_tp_call+0x124 [0x102c5d2f0]
  Binary file "/Users/thomas.kowalski/Documents/cpython-asan/install/bin/python3.16", at _PyObject_MakeTpCall+0x1ac [0x102ad8d94]
  Binary file "/Users/thomas.kowalski/Documents/cpython-asan/install/bin/python3.16", at _Py_VectorCallInstrumentation_StackRefSteal+0x2b8 [0x102e30e10]
  Binary file "/Users/thomas.kowalski/Documents/cpython-asan/install/bin/python3.16", at _PyEval_EvalFrameDefault+0x13f44 [0x102e47370]
  Binary file "/Users/thomas.kowalski/Documents/cpython-asan/install/bin/python3.16", at _PyEval_Vector+0x420 [0x102e30068]
  Binary file "/Users/thomas.kowalski/Documents/cpython-asan/install/bin/python3.16", at _PyObject_VectorcallDictTstate+0x1a4 [0x102ad88a0]
  Binary file "/Users/thomas.kowalski/Documents/cpython-asan/install/bin/python3.16", at _PyObject_Call_Prepend+0x134 [0x102adb134]
  Binary file "/Users/thomas.kowalski/Documents/cpython-asan/install/bin/python3.16", at slot_tp_call+0x124 [0x102c5d2f0]
  Binary file "/Users/thomas.kowalski/Documents/cpython-asan/install/bin/python3.16", at _PyObject_Call+0x13c [0x102ada544]
  Binary file "/Users/thomas.kowalski/Documents/cpython-asan/install/bin/python3.16", at _PyEval_EvalFrameDefault+0x2b40 [0x102e35f6c]
  Binary file "/Users/thomas.kowalski/Documents/cpython-asan/install/bin/python3.16", at _PyEval_Vector+0x420 [0x102e30068]
  <truncated rest of calls>

Extension modules: msgpack._cmsgpack (total: 1)
zsh: abort      ASAN_OPTIONS=detect_leaks=0 python -m pytest test/test_memoryview.py -k

Copilot

Pull request overview

Fixes a crash (use-after-free) when unpacking from non-contiguous buffer inputs (e.g., sliced memoryview), and adds a regression test to ensure the behavior stays correct across future changes.

Changes:

Add a regression test that unpacks from a non-contiguous stride-2 memoryview.
Adjust C-extension buffer handling in get_data_from_buffer to avoid premature deallocation.
Update the pure-Python fallback Unpacker.feed() to safely handle non-contiguous memoryview inputs.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
`test/test_memoryview.py`	Adds a regression test covering unpacking from non-contiguous `memoryview`.
`msgpack/fallback.py`	Ensures fallback `Unpacker.feed()` can ingest non-contiguous `memoryview` inputs.
`msgpack/_unpacker.pyx`	Updates contiguous-copy buffer acquisition logic to prevent use-after-free.

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- Check PyObject_GetBuffer return value and raise on failure - Avoid unnecessary tobytes() copy for contiguous memoryviews in fallback feed()

test: add test

9086469

KowalskiThomas force-pushed the kowalski/fix-uaf-step-1 branch from 882aa38 to 9845e8e Compare May 26, 2026 15:27

fix: use-after-free in get_data_from_buffer

eae29a9

KowalskiThomas force-pushed the kowalski/fix-uaf-step-1 branch from 2af2d1f to eae29a9 Compare May 26, 2026 15:48

KowalskiThomas marked this pull request as ready for review May 26, 2026 19:21

methane requested a review from Copilot May 27, 2026 08:11

Copilot started reviewing on behalf of methane May 27, 2026 08:11 View session

Copilot AI reviewed May 27, 2026

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Comment thread msgpack/_unpacker.pyx

Comment thread msgpack/fallback.py

fix: address PR review comments

7ee15a3

- Check PyObject_GetBuffer return value and raise on failure - Avoid unnecessary tobytes() copy for contiguous memoryviews in fallback feed()

methane merged commit e861f75 into msgpack:main Jun 2, 2026
26 checks passed

This was referenced Aug 3, 2026

fix: use-after-free in unpackb() ExtraData path for non-contiguous input #722

Merged

unpackb() may return corrupted ExtraData.extra for non-contiguous input #720

Closed

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fix: use-after-free in `get_data_from_buffer` - #677

fix: use-after-free in `get_data_from_buffer`#677
methane merged 3 commits into
msgpack:mainfrom
KowalskiThomas:kowalski/fix-uaf-step-1

KowalskiThomas commented May 26, 2026

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KowalskiThomas commented May 26, 2026

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