gh-146525: Add ndim validation to PyBuffer_ToContiguous for defense-in-depth

[l3tchupkt]

Summary

Add validation of the ndim parameter in PyBuffer_ToContiguous() to prevent potential integer overflow in memory allocation calculations. This is a defense-in-depth hardening measure.

Fixes #146525

Problem

The allocation in PyBuffer_ToContiguous():

fb = PyMem_Malloc(sizeof *fb + 3 * src->ndim * (sizeof *fb->array));

Could theoretically overflow if ndim is excessively large. While Python-level code already enforces PyBUF_MAX_NDIM (64), C extensions implementing custom getbufferproc could potentially provide invalid ndim values.

Solution

Add explicit validation before allocation to ensure ndim is within the valid range (0 to PyBUF_MAX_NDIM).

Changes

Objects/memoryobject.c:

• Added runtime validation in PyBuffer_ToContiguous() to check ndim is within valid range (0-64)
• Added assertion in buffer_to_contiguous() for consistency

Lib/test/test_memoryview.py:

• Added test_ndim_limit() test case to verify:
  • ndim > 64 raises ValueError
  • Negative ndim is rejected

Misc/NEWS.d/next/C_API/2026-03-27-00-00-00.gh-146525.abc123.rst:

• Added NEWS entry documenting the change

Testing

All existing tests pass: python -m test test_memoryview (140 tests OK)
New validation test passes
Verified the fix correctly rejects invalid ndim values

Security Classification

This is a hardening improvement, not a fix for an actively exploitable vulnerability.

No CVE is warranted because:

• The overflow would require ndim > ~3.8×10^17 on 64-bit systems, which is not practically achievable
• Python-level code already enforces PyBUF_MAX_NDIM (64)
• Would require a malicious C extension providing impossible ndim values
• This adds defense-in-depth validation for code quality and robustness

Why This Matters

1. Defense-in-depth: Adds an extra layer of protection against malformed C extensions
2. Code Quality: Makes assumptions explicit through validation
3. Consistency: Aligns C-level checks with Python-level enforcement of PyBUF_MAX_NDIM
4. Documentation: Clarifies the valid range for ndim in the C API

Author: Lakshmikanthan K (badassletchu@gmail.com)

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Most changes to Python require a NEWS entry. Add one using the blurb_it web app or the blurb command-line tool.

If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.

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If this change has little impact on Python users, wait for a maintainer to apply the skip news label instead.

[picnixz]

I'm not convinced by this change. cc @vstinner

[picnixz]

I don't think it's really necessary:

• we already document that ndim must be valid: https://docs.python.org/3/c-api/buffer.html#c.Py_buffer.ndim.
• if this check is needed, we also need to update the docs.

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