netdev - Re: [PATCH v3] mm: page_frag: Warn_on when frag_alloc size is bigger than PAGE

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Message-ID: <b711ca5f-4180-d658-a330-89bd5dcb0acb@gmail.com>
Date:   Tue, 31 May 2022 16:45:31 -0700
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Chen Lin <chen45464546@....com>, akpm@...ux-foundation.org
Cc:     kuba@...nel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        alexander.duyck@...il.com, netdev@...r.kernel.org
Subject: Re: [PATCH v3] mm: page_frag: Warn_on when frag_alloc size is bigger
 than PAGE_SIZE


On 5/31/22 07:43, 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 <chen45464546@....com>
> ---
>   mm/page_alloc.c |    9 +++++++++
>   1 file changed, 9 insertions(+)
>
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index e008a3d..ffc42b5 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5574,6 +5574,15 @@ 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 (WARN_ONCE(fragz > PAGE_SIZE,
> +		      "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);


I do not think this patch is needed, nor correct. (panic_on_warn=1 will 
panic the box)

Or provide a stack trace ?

Please fix the caller (presumably a network driver ?), and provide an 
appropriate Fixes: tag.

Thanks.