gh-157335: Fix out-of-bounds write in mmap.mmap.__setitem__ - #157438
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Joekrry wants to merge 2 commits into
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gh-157335: Fix out-of-bounds write in mmap.mmap.__setitem__#157438Joekrry wants to merge 2 commits into
Joekrry wants to merge 2 commits into
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…Assigning to a single index causes mmap.mmap.__setitem__ to validate the index against the object's size, which then converts the assigned value via PyNumber_AsSsize_t(). This invokes arbitrary python code via __index__(). mmap.resize(), which shrinks mapping could point past the end fo the new buffer causing an out of bounds error write.
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picnixz
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Sep 13, 2026
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| /* value's __index__ may have resized the mmap, invalidating | ||
| * the earlier bounds check on i. */ | ||
| if (i >= self->size) { | ||
| PyErr_SetString(PyExc_IndexError, | ||
| "mmap index out of range"); | ||
| return -1; | ||
| } |
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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.
vstinner
reviewed
Sep 13, 2026
| CHECK_VALID(-1); | ||
| /* value's __index__ may have resized the mmap, invalidating | ||
| * the earlier bounds check on i. */ | ||
| if (i >= self->size) { |
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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.
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Assigning to a single index in
mmap.mmap.__setitem__validates the index against the object's size, then converts the assigned value viaPyNumber_AsSsize_t(). That conversion can invoke arbitrary Python code through__index__(), and if that code callsmmap.resize()to shrink the mapping, the previously validated index can point past the end of the new, smaller buffer — causing an out-of-bounds write.This re-validates the index against the mmap's current size after the value conversion, right alongside the existing
CHECK_VALID()check that already accounts for reentrancy at that point.Fixes gh-157335.