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zend_mm_free_pages_ex() is the common sink for freeing large runs. It uses pages_count to reset bits in chunk->free_map via zend_mm_bitset_reset_range() and to adjust chunk->free_pages, without ever bounding page_num + pages_count to ZEND_MM_PAGES. That count is not trustworthy: - In zend_mm_free_large(), it is decoded from the 10-bit ZEND_MM_LRUN_PAGES field of the page map entry, so it can reach 1023 while a chunk only has 512 pages. The map lives in the chunk header, right next to free_map, and is reachable by a heap overflow. - In _efree_large(), it is computed straight from the caller-supplied size. The only cross-check against the map is a pair of ZEND_ASSERT()s, which are compiled out under NDEBUG, so release builds validate nothing beyond page alignment. The realloc growth path already enforces this very invariant via page_num + new_pages_count <= ZEND_MM_PAGES.
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zend_mm_free_pages_ex() is the common sink for freeing large runs. It uses pages_count to reset bits in chunk->free_map via zend_mm_bitset_reset_range() and to adjust chunk->free_pages, without ever bounding page_num + pages_count to ZEND_MM_PAGES. That count is not trustworthy:
In zend_mm_free_large(), it is decoded from the 10-bit ZEND_MM_LRUN_PAGES field of the page map entry, so it can reach 1023 while a chunk only has 512 pages. The map lives in the chunk header, right next to free_map, and is reachable by a heap overflow.
In _efree_large(), it is computed straight from the caller-supplied size. The only cross-check against the map is a pair of ZEND_ASSERT()s, which are compiled out under NDEBUG, so release builds validate nothing beyond page alignment.
The realloc growth path already enforces this very invariant via page_num + new_pages_count <= ZEND_MM_PAGES.