Re: [PATCH V2] mm: prevent page_frag_alloc() from corrupting the memory

[Andrew Morton]

On Wed, 13 Jul 2022 17:01:43 +0200 Maurizio Lombardi <mlombard@xxxxxxxxxx> wrote:

> A number of drivers call page_frag_alloc() with a
> fragment's size > PAGE_SIZE.
> In low memory conditions, __page_frag_cache_refill() may fail the order 3
> cache allocation and fall back to order 0;
> In this case, the cache will be smaller than the fragment, causing
> memory corruptions.
> 
> Prevent this from happening by checking if the newly allocated cache
> is large enough for the fragment; if not, the allocation will fail
> and page_frag_alloc() will return NULL.
> 
> ...
>
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5617,6 +5617,8 @@ void *page_frag_alloc_align(struct page_frag_cache *nc,
>  		/* reset page count bias and offset to start of new frag */
>  		nc->pagecnt_bias = PAGE_FRAG_CACHE_MAX_SIZE + 1;
>  		offset = size - fragsz;
> +		if (unlikely(offset < 0))
> +			return NULL;
>  	}
>  
>  	nc->pagecnt_bias--;

I think we should have a comment here explaining (at least) why we'd
bale after a successful allocation and explaining why we don't call
free_the_page().