Re: [PATCH] mm: page_frag: Warn_on when frag_alloc size is bigger than PAGE_SIZE

[Andrew Morton <akpm@linux-foundation.org>]

On Sat, 28 May 2022 23:39:33 +0800 Chen Lin <chen45464546@163.com> wrote:

> netdev_alloc_frag->page_frag_alloc may cause memory corruption in 
> the following process:
> 
> 1. A netdev_alloc_frag function call need alloc 200 Bytes to build a skb.
> 
> 2. Insufficient memory to alloc PAGE_FRAG_CACHE_MAX_ORDER(32K) in 
> __page_frag_cache_refill to fill frag cache, then one page(eg:4K) 
> is allocated, now current frag cache is 4K, alloc is success, 
> nc->pagecnt_bias--.
> 
> 3. Then this 200 bytes skb in step 1 is freed, page->_refcount--.
> 
> 4. Another netdev_alloc_frag function call need alloc 5k, page->_refcount 
> is equal to nc->pagecnt_bias, reset page count bias and offset to 
> start of new frag. page_frag_alloc will return the 4K memory for a 
> 5K memory request.
> 
> 5. The caller write on the extra 1k memory which is not actual allocated 
> will cause memory corruption.
> 
> page_frag_alloc is for fragmented allocation. We should warn the caller 
> to avoid memory corruption.
> 

Let's cc Alexander and the networking developers.

> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5574,6 +5574,11 @@ void *page_frag_alloc_align(struct page_frag_cache *nc,
>  	struct page *page;
>  	int offset;
>  
> +	/* frag_alloc is not suitable for memory alloc which fragsz
> +	 * is bigger than PAGE_SIZE, use kmalloc or alloc_pages instead.
> +	 */
> +	WARN_ON(fragsz > PAGE_SIZE);
> +
>  	if (unlikely(!nc->va)) {
>  refill:
>  		page = __page_frag_cache_refill(nc, gfp_mask);

Odd.  All this does is generate a warning.  If the kernel is corrupting
memory, that's a bug which needs fixing?