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gh-157335: Fix out-of-bounds write in mmap.mmap.__setitem__ #157438

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gh-157335: Fix out-of-bounds write in mmap.mmap.__setitem__ #157438

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gh-157335: Fixed out of bounds write in mmap.mmap.__setitem__. Assign…
Joekrry Sep 13, 2026
884eb9d
Merge branch 'main' into fix-mmap-setitem-resize-reentrancy
Joekrry Sep 13, 2026
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18 changes: 18 additions & 0 deletions Lib/test/test_mmap.py
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Original file line number Diff line number Diff line change
Expand Up @@ -953,6 +953,24 @@ def test_resize_down_anonymous_mapping(self):
with self.assertRaises(ValueError):
m.resize(start_size)

def test_setitem_resize_reentrancy(self):
"""Resizing the mmap from inside __index__ while assigning to a
single item must not access memory past the new bounds (gh-157335)."""
size = 2 * PAGESIZE
new_size = PAGESIZE

class ResizeOnIndex:
def __init__(self, m):
self.m = m
def __index__(self):
self.m.resize(new_size)
return 0

with mmap.mmap(-1, size) as m:
with self.assertRaises(IndexError):
m[size - 1] = ResizeOnIndex(m)
self.assertEqual(len(m), new_size)

@unittest.skipUnless(os.name == 'nt', 'requires Windows')
def test_resize_fails_if_mapping_held_elsewhere(self):
"""If more than one mapping is held against a named file on Windows, neither
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3 changes: 3 additions & 0 deletions Misc/NEWS.d/next/Library/2026-09-13-15-58-28.gh-issue-157335.efaMah.rst
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Original file line number Diff line number Diff line change
@@ -0,0 +1,3 @@
Fix out-of-bounds write in :meth:`mmap.mmap.__setitem__` that could occur
when the assigned value's :meth:`~object.__index__` method resized the
mmap object during the assignment.
7 changes: 7 additions & 0 deletions Modules/mmapmodule.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1687,6 +1687,13 @@ mmap_ass_subscript_lock_held(PyObject *op, PyObject *item, PyObject *value)
return -1;
}
CHECK_VALID(-1);
/* value's __index__ may have resized the mmap, invalidating
* the earlier bounds check on i. */
if (i >= self->size) {

@vstinner vstinner Sep 13, 2026

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I would prefer to not check the bounds twice. You can reorganize the code instead:

  • call PyNumber_AsSsize_t(item) + error check
  • call PyNumber_AsSsize_t(value) + error check
  • adjust i and check bounds
  • check v bounds
  • call safe_byte_copy()

Something like that.

PyErr_SetString(PyExc_IndexError,
"mmap index out of range");
return -1;
}
Comment on lines +1690 to +1696

@picnixz picnixz Sep 13, 2026

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I think we can have the same issue in the slice-branch when doing PyObject_GetBuffer(). So instead, we could make the checks inside the safe_byte_copy and safe_memcpy functions. Though I don't know if it's an overkill. Can you verify that the slice pah is also not affected by adding tests.


char v_char = (char) v;
if (safe_byte_copy(self->data + i, &v_char) < 0) {
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