[PATCH v3] mm/page_alloc: clear page->private in free_pages_prepare()

[Mikhail Gavrilov]

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.

Fixes: 3b8000ae185c ("mm/vmalloc: huge vmalloc backing pages should be split rather than compound")
Cc: stable@xxxxxxxxxxxxxxx
Suggested-by: Zi Yan <ziy@xxxxxxxxxx>
Acked-by: Zi Yan <ziy@xxxxxxxxxx>
Signed-off-by: Mikhail Gavrilov <mikhail.v.gavrilov@xxxxxxxxx>
---
 mm/page_alloc.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index cbf758e27aa2..24ac34199f95 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -1430,6 +1430,7 @@ __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);
-- 
2.53.0