compile: double free of pc->fail_pop in codegen_pattern_or on allocation failure with nested or-pattern

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python
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compilers

Rechercherichtung

Start in Python/codegen.c at codegen_pattern_or and codegen_match, using the reported lines 6573 and 6775 to trace ownership of fail_pop through the allocation failure path. Reproduce with the provided _testcapi.call_with_nomemory script on an ASan build; done when injected failures raise MemoryError, the pointer is freed once, and the script prints “survived.”

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Beschreibung

interpreter-core type-crash

When an allocation fails while compiling a match statement whose or-pattern contains another or-pattern, codegen_pattern_or frees pc->fail_pop and codegen_match then frees it again, so compile() crashes instead of raising MemoryError.

The failure is injected with the test-only _testcapi.call_with_nomemory:

import ast, _testcapi
tree = ast.parse('def f(x):\n match x:\n  case [1|2] | 3: pass\n')
for off in range(1500):
    held = None
    held = [[] for _ in range(128)]   # refill/empty the list freelist so PyList_New in codegen really allocates
    try:
        _testcapi.call_with_nomemory(off, off + 1, compile, tree, 'a.py', 'exec')
    except (MemoryError, SyntaxError):
        pass
print('survived')

Saved as input.py and run on an ASan build that includes _testcapi:

python input.py

Output:

ERROR: AddressSanitizer: heap-use-after-free on address 0x503000002988
READ of size 1 at 0x503000002988 thread T0
    #0 in _PyMem_DebugCheckAddress Objects/obmalloc.c:3351
    #4 in PyMem_Free Objects/obmalloc.c:1292
    #5 in codegen_match Python/codegen.c:6775
...
freed by thread T0 here:
    #5 in PyMem_Free Objects/obmalloc.c:1292
    #6 in codegen_pattern_or Python/codegen.c:6573

Every injected failure should surface as MemoryError from compile(), with the pointer freed once, and the script should print survived.

Found at commit 3646ea571f3d8290806a3029d555b179f8be1539. It still reproduces at 3acc941f2b72 (latest upstream checked): ASan heap-use-after-free in the checking build, and the release build segfaults (exit code 139). case (1|2) | 3, where the inner or-pattern is not inside a sequence pattern, survives.

Looks like in the inner codegen_pattern_or, old_pc = *pc copies the outer fail_pop array, and if the first PyList_New(0) in the loop fails, goto error runs before pc->fail_pop = NULL. The error block then frees old_pc.fail_pop while pc->fail_pop still points to it, and codegen_match frees it again.

Found with an LLM-based testing tool; I used Claude to reduce it and look for the cause.

Linked PRs
  • gh-160875
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