Subject: Re: [PATCH v2] mm: page_frag: Warn_on when frag_alloc size is bigger than PAGE_SIZE

From: Jakub Kicinski <[email-masked]>
Date: Mon, 30 May 2022 12:27:05 -0700

On Mon, 30 May 2022 21:39:02 +0800 Chen Lin 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.
> 
> When fragsz is larger than one page, we report the failure and return.
> I don't think it is a good idea to make efforts to support the
> allocation of more than one page in this function because the total
> frag cache size(PAGE_FRAG_CACHE_MAX_SIZE 32768) is relatively small.
> When the request is larger than one page, the caller should switch to
> use other kernel interfaces, such as kmalloc and alloc_Pages.
> 
> This bug is mainly caused by the reuse of the previously allocated
> frag cache memory by the following LARGER allocations. This bug existed
> before page_frag_alloc was ported from __netdev_alloc_frag in 
> net/core/skbuff.c, so most Linux versions have this problem.
> 
> Signed-off-by: Chen Lin <[email-masked]>
> ---
>  mm/page_alloc.c |   10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index e008a3d..1e9e2c4 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5574,6 +5574,16 @@ 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.
> +	 */
> +	if (unlikely(fragsz > PAGE_SIZE)) {
> +		WARN(1, "alloc fragsz(%d) > PAGE_SIZE(%ld) not supported,
> +			alloc fail\n", fragsz, PAGE_SIZE);
> +
> +		return NULL;
> +	}
> +
>  	if (unlikely(!nc->va)) {
>  refill:
>  		page = __page_frag_cache_refill(nc, gfp_mask);

Let's see what Alex says (fixing his email now). It seems a little too
drastic to me. I'd go with something like:

diff --git a/mm/page_alloc.c b/mm/page_alloc.c
index e008a3df0485..360a545ee5e8 100644
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -5537,6 +5537,7 @@ EXPORT_SYMBOL(free_pages);
  * sk_buff->head, or to be used in the "frags" portion of skb_shared_info.
  */
 static struct page *__page_frag_cache_refill(struct page_frag_cache *nc,
+					     unsigned int fragsz,
 					     gfp_t gfp_mask)
 {
 	struct page *page = NULL;
@@ -5549,7 +5550,7 @@ static struct page *__page_frag_cache_refill(struct page_frag_cache *nc,
 				PAGE_FRAG_CACHE_MAX_ORDER);
 	nc->size = page ? PAGE_FRAG_CACHE_MAX_SIZE : PAGE_SIZE;
 #endif
-	if (unlikely(!page))
+	if (unlikely(!page && fragsz <= PAGE_SIZE))
 		page = alloc_pages_node(NUMA_NO_NODE, gfp, 0);
 
 	nc->va = page ? page_address(page) : NULL;
@@ -5576,7 +5577,7 @@ void *page_frag_alloc_align(struct page_frag_cache *nc,
 
 	if (unlikely(!nc->va)) {
 refill:
-		page = __page_frag_cache_refill(nc, gfp_mask);
+		page = __page_frag_cache_refill(nc, fragsz, gfp_mask);
 		if (!page)
 			return NULL;