Re: [PATCH v3] mm/codetag: move tag retrieval back upfront in __free_pages()

[Vlastimil Babka <vbabka@suse.cz>]

On 5/5/25 21:30, David Wang wrote:
> Commit 51ff4d7486f0 ("mm: avoid extra mem_alloc_profiling_enabled()
>  checks") introduces a possible use-after-free scenario, when page
> is non-compound, page[0] could be released by other thread right
> after put_page_testzero failed in current thread, pgalloc_tag_sub_pages
> afterwards would manipulate an invalid page for accounting remaining
> pages:
> 
> [timeline]   [thread1]                     [thread2]
>   |          alloc_page non-compound
>   V
>   |                                        get_page, rf counter inc
>   V
>   |          in ___free_pages
>   |          put_page_testzero fails
>   V
>   |                                        put_page, page released
>   V
>   |          in ___free_pages,
>   |          pgalloc_tag_sub_pages
>   |          manipulate an invalid page
>   V
> 
> Restore __free_pages() to its state before, retrieve alloc tag
> beforehand.
> 
> Fixes: 51ff4d7486f0 ("mm: avoid extra mem_alloc_profiling_enabled() checks")

That's a 6.15-rc1 commit, so no cc: stable needed, but should go to mm-hotfixes.

> Signed-off-by: David Wang <00107082@163.com>

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

Thanks!

> ---
>  include/linux/pgalloc_tag.h |  8 ++++++++
>  mm/page_alloc.c             | 15 ++++++---------
>  2 files changed, 14 insertions(+), 9 deletions(-)
> 
> diff --git a/include/linux/pgalloc_tag.h b/include/linux/pgalloc_tag.h
> index c74077977830..8a7f4f802c57 100644
> --- a/include/linux/pgalloc_tag.h
> +++ b/include/linux/pgalloc_tag.h
> @@ -188,6 +188,13 @@ static inline struct alloc_tag *__pgalloc_tag_get(struct page *page)
>  	return tag;
>  }
>  
> +static inline struct alloc_tag *pgalloc_tag_get(struct page *page)
> +{
> +	if (mem_alloc_profiling_enabled())
> +		return __pgalloc_tag_get(page);
> +	return NULL;
> +}
> +
>  void pgalloc_tag_split(struct folio *folio, int old_order, int new_order);
>  void pgalloc_tag_swap(struct folio *new, struct folio *old);
>  
> @@ -199,6 +206,7 @@ static inline void clear_page_tag_ref(struct page *page) {}
>  static inline void alloc_tag_sec_init(void) {}
>  static inline void pgalloc_tag_split(struct folio *folio, int old_order, int new_order) {}
>  static inline void pgalloc_tag_swap(struct folio *new, struct folio *old) {}
> +static inline struct alloc_tag *pgalloc_tag_get(struct page *page) { return NULL; }
>  
>  #endif /* CONFIG_MEM_ALLOC_PROFILING */
>  
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index 5669baf2a6fe..1b00e14a9780 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -1151,14 +1151,9 @@ static inline void pgalloc_tag_sub(struct page *page, unsigned int nr)
>  		__pgalloc_tag_sub(page, nr);
>  }
>  
> -static inline void pgalloc_tag_sub_pages(struct page *page, unsigned int nr)
> +/* When tag is not NULL, assuming mem_alloc_profiling_enabled */
> +static inline void pgalloc_tag_sub_pages(struct alloc_tag *tag, unsigned int nr)
>  {
> -	struct alloc_tag *tag;
> -
> -	if (!mem_alloc_profiling_enabled())
> -		return;
> -
> -	tag = __pgalloc_tag_get(page);
>  	if (tag)
>  		this_cpu_sub(tag->counters->bytes, PAGE_SIZE * nr);
>  }
> @@ -1168,7 +1163,7 @@ static inline void pgalloc_tag_sub_pages(struct page *page, unsigned int nr)
>  static inline void pgalloc_tag_add(struct page *page, struct task_struct *task,
>  				   unsigned int nr) {}
>  static inline void pgalloc_tag_sub(struct page *page, unsigned int nr) {}
> -static inline void pgalloc_tag_sub_pages(struct page *page, unsigned int nr) {}
> +static inline void pgalloc_tag_sub_pages(struct alloc_tag *tag, unsigned int nr) {}
>  
>  #endif /* CONFIG_MEM_ALLOC_PROFILING */
>  
> @@ -5065,11 +5060,13 @@ static void ___free_pages(struct page *page, unsigned int order,
>  {
>  	/* get PageHead before we drop reference */
>  	int head = PageHead(page);
> +	/* get alloc tag in case the page is released by others */
> +	struct alloc_tag *tag = pgalloc_tag_get(page);
>  
>  	if (put_page_testzero(page))
>  		__free_frozen_pages(page, order, fpi_flags);
>  	else if (!head) {
> -		pgalloc_tag_sub_pages(page, (1 << order) - 1);
> +		pgalloc_tag_sub_pages(tag, (1 << order) - 1);
>  		while (order-- > 0)
>  			__free_frozen_pages(page + (1 << order), order,
>  					    fpi_flags);