Re: [PATCH v2] mm/page_alloc: clear page->private in free_pages_prepare()

[Vlastimil Babka <vbabka@suse.cz>]

On 2/7/26 16:37, Mikhail Gavrilov wrote:
> 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@vger.kernel.org
> Suggested-by: Zi Yan <ziy@nvidia.com>
> Signed-off-by: Mikhail Gavrilov <mikhail.v.gavrilov@gmail.com>

Seems cheap enough and robust to just do it. Could be different for tail
pages (although we do modify them too anyway).

Reviewed-by: Vlastimil Babka <vbabka@suse.cz>

> ---
>  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);