Skip to content

Potential NULL Pointer Dereference in RefdbBackend_init #1443

@mpap10

Description

@mpap10

We are researchers from the University of Athens, working on cross-language analysis of Python packages with C/C++ native extensions.

Problem

We found an issue in the pygit2 package: the RefdbBackend_init() function [1] does not check the return value of its internal memory allocation call (calloc() [2]). Immediately after allocating memory for the backend structure, it calls git_refdb_init_backend() [3] and initializes several fields, which dereference the pointer without verifying that it is non-NULL.

If the allocation fails, RefdbBackend_init() continues execution with a NULL pointer. This can lead to undefined behavior, including potential segmentation faults under memory-constrained conditions.

Observed Behavior in Python

To explore the bug, we ran the following script, which internally trigger the RefdbBackend_init() funtion:

import pygit2
import tempfile
import os

ITERS = 100000
MB = 1024 ** 2
ORDER = 512 * MB

y = []

for i in range(ITERS):
    print(i)

    x = b"a" * ORDER
    y.append(x)

    path = tempfile.mkdtemp()

    repo = pygit2.init_repository(path)
    y.append(repo)

In practice, Python often raises MemoryError first when the interpreter itself cannot allocate memory. As a result, the NULL pointer dereference is rarely observed as a segfault in the Python environment, but the underlying bug could trigger undefined behavior under constrained memory conditions.

Potential Fix

  • Ensure that the return value of calloc() is checked before using the pointer, so the function does not proceed if allocation fails.
  • Ensure the allocated backend structure is freed if any initialization step fails.

References

[1]

RefdbBackend_init(RefdbBackend *self, PyObject *args, PyObject *kwds)

[2]
struct pygit2_refdb_backend *be = calloc(1, sizeof(struct pygit2_refdb_backend));

[3]
git_refdb_init_backend(&be->backend, GIT_REFDB_BACKEND_VERSION);

Metadata

Metadata

Assignees

No one assigned

    Labels

    No labels
    No labels

    Type

    No type

    Projects

    No projects

    Milestone

    No milestone

    Relationships

    None yet

    Development

    No branches or pull requests

    Issue actions