Python upb: empty extended slice in del repeated_field[start:stop:step] performs an out-of-bounds memmove

Summary

Deleting an empty extended slice from a repeated field performs an out-of-bounds memmove
and crashes the process, instead of being a no-op as it is for a Python list.

from google.protobuf import descriptor_pb2
f = descriptor_pb2.FileDescriptorProto()
del f.dependency[::2]        # empty repeated field; a no-op in Python

Observed on protobuf==7.36.1 (upb runtime, google._upb._message):

Windows fatal exception: access violation
exit code 3221225477 (0xC0000005)

The pure-Python runtime and CPython list both treat this as a no-op.

Root cause

PyUpb_RepeatedContainer_DeleteSubscript() (python/repeated.c) assumes at least one
element is selected in the step > 1 branch:

if (step > 1) {
  src = start + 1;                 // assumes count > 0
  for (Py_ssize_t i = 1; i < count; i++, dst += step - 1, src += step) {
    upb_Array_Move(arr, dst, src, step);
  }
} else {
  src = start + count;
}

size_t tail = upb_Array_Size(arr) - src;   // underflows when src > size
size_t new_size = dst + tail;
assert(new_size == upb_Array_Size(arr) - count);   // compiled out in release builds
upb_Array_Move(arr, dst, src, tail);               // memmove(..., SIZE_MAX << lg2)

PySlice_AdjustIndices() returns count == 0 with start == len(array) for del x[len:len:2],
del x[::2] on an empty array, and del x[k:k:2] when k == len. The step > 1 branch then
sets src = start + 1 > size, so tail underflows to SIZE_MAX, and upb_Array_Move()
(upb/message/array.c) issues a memmove of ~2^64 bytes.

When start < len(array) the same branch instead silently drops one element, e.g.
del x[0:0:2] on 5 elements leaves 4, rather than being a no-op.

Reproduction matrix (protobuf 7.36.1, upb runtime)

Expected behavior

Deleting an empty slice should be a no-op, matching list and the pure-Python runtime.
python/minimal_test.py:test_repeated_field_slice_delete asserts exactly this list-parity
contract, but only for non-empty ranges.

Proposed fix

 static int PyUpb_RepeatedContainer_DeleteSubscript(upb_Array* arr,
                                                    Py_ssize_t idx,
                                                    Py_ssize_t count,
                                                    Py_ssize_t step) {
+  // An empty slice deletes nothing.  Without this, the step>1 branch below
+  // sets src = start + 1, which can exceed the array size and make
+  // `tail = upb_Array_Size(arr) - src` underflow size_t, turning the
+  // subsequent upb_Array_Move() into an out-of-bounds memmove.
+  if (count == 0) return 0;
+
   // Normalize direction: deletion is order-independent.
   Py_ssize_t start = idx;

and add empty-range cases to test_repeated_field_slice_delete:

    test_slice(5, 5, 2)
    test_slice(0, 0, 2)
    test_slice(2, 2, 2)
    test_slice(20, 20, 2)

Notes

This is reachable through the in-memory container API, which SECURITY.md lists outside the
CVE threat model, so I am filing it here as a public issue rather than a private security
report, per the policy's guidance for out-of-threat-model issues. The consequence is a heap
out-of-bounds write, and the trigger is a valid list operation rather than an invalid
argument.

Start in python/repeated.c at PyUpb_RepeatedContainer_DeleteSubscript(), then read python/minimal_test.py:test_repeated_field_slice_delete. Verify empty extended-slice deletions match CPython lists and the pure-Python runtime without crashing or changing the container, and run the focused test.