CVE-2026-43303: mm/page_alloc: clear page->private in free_pages_prepare()

In the Linux kernel, the following vulnerability has been resolved:

mm/pagealloc: clear page->private in freepagesprepare()

Several subsystems (slub, shmem, ttm, etc.) use page->private but don't clear it before freeing pages. When these pages are later allocated as high-order pages and split via splitpage(), tail pages retain stale page->private values.

This causes a use-after-free in the swap subsystem. The swap code uses page->private to track swap count continuations, assuming freshly allocated pages have page->private == 0. When stale values are present, swapcountcontinued() incorrectly assumes the continuation list is valid and iterates over uninitialized page->lru containing LISTPOISON values, causing a crash:

KASAN: maybe wild-memory-access in range [0xdead000000000100-0xdead000000000107] RIP: 0010:dosysswapoff+0x1151/0x1860

Fix this by clearing page->private in freepagesprepare(), ensuring all freed pages have clean state regardless of previous use.