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

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 split_page(), 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,
swap_count_continued() incorrectly assumes the continuation list is valid
and iterates over uninitialized page->lru containing LIST_POISON values,
causing a crash:

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

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

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 3b8000ae18 ("mm/vmalloc: huge vmalloc backing pages should be split rather than compound")
Signed-off-by: Mikhail Gavrilov <[email protected]>
Suggested-by: Zi Yan <[email protected]>
Acked-by: Zi Yan <[email protected]>
Acked-by: David Hildenbrand (Arm) <[email protected]>
Reviewed-by: Vlastimil Babka <[email protected]>
Cc: Brendan Jackman <[email protected]>
Cc: Chris Li <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Kairui Song <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Nicholas Piggin <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

mm/page_alloc.c

@@ -1429,6 +1429,7 @@ __always_inline bool free_pages_prepare(struct page *page,
 
 	page_cpupid_reset_last(page);
 	page->flags.f &= ~PAGE_FLAGS_CHECK_AT_PREP;
+	page->private = 0;
 	reset_page_owner(page, order);
 	page_table_check_free(page, order);
 	pgalloc_tag_sub(page, 1 << order);