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Date:   Mon, 30 May 2022 13:07:05 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Chen Lin <chen45464546@163.com>
Cc:     linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        alexander.h.duyck@linux.intel.com, netdev@vger.kernel.org
Subject: Re: [PATCH v2] mm: page_frag: Warn_on when frag_alloc size is
 bigger than PAGE_SIZE
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On Mon, 30 May 2022 21:39:02 +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.
> 
> 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.
> 

I won't attempt to address the large issues here (like, should
networking be changed to support this).  But I can nitpick :)

> --- 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

Like this please:

	/*
	 * frag_alloc...

> +	 * 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);

It's neater to do

	if (WARN(fragsz > PAGE_SIZE, "alloc fragsz(%d...", ...))
		return NULL;

Also, you have a newline and a bunch of tabs in that string.

Also, please consider WARN_ONCE.  We don't want to provide misbehaved
or malicious userspace with the ability to flood the logs with
warnings.