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Date:   Tue, 25 Aug 2020 13:13:36 +0200
From:   Maciej Fijalkowski <maciej.fijalkowski@...el.com>
To:     Björn Töpel <bjorn.topel@...il.com>
Cc:     jeffrey.t.kirsher@...el.com, intel-wired-lan@...ts.osuosl.org,
        Björn Töpel <bjorn.topel@...el.com>,
        magnus.karlsson@...el.com, magnus.karlsson@...il.com,
        netdev@...r.kernel.org, piotr.raczynski@...el.com,
        maciej.machnikowski@...el.com, lirongqing@...du.com
Subject: Re: [PATCH net 1/3] i40e: avoid premature Rx buffer reuse

On Tue, Aug 25, 2020 at 11:16:27AM +0200, Björn Töpel wrote:
> From: Björn Töpel <bjorn.topel@...el.com>
> 
> The page recycle code, incorrectly, relied on that a page fragment
> could not be freed inside xdp_do_redirect(). This assumption leads to
> that page fragments that are used by the stack/XDP redirect can be
> reused and overwritten.
> 
> To avoid this, store the page count prior invoking xdp_do_redirect().
> 
> Longer explanation:
> 
> Intel NICs have a recycle mechanism. The main idea is that a page is
> split into two parts. One part is owned by the driver, one part might
> be owned by someone else, such as the stack.
> 
> t0: Page is allocated, and put on the Rx ring
>               +---------------
> used by NIC ->| upper buffer
> (rx_buffer)   +---------------
>               | lower buffer
>               +---------------
>   page count  == USHRT_MAX
>   rx_buffer->pagecnt_bias == USHRT_MAX
> 
> t1: Buffer is received, and passed to the stack (e.g.)
>               +---------------
>               | upper buff (skb)
>               +---------------
> used by NIC ->| lower buffer
> (rx_buffer)   +---------------
>   page count  == USHRT_MAX
>   rx_buffer->pagecnt_bias == USHRT_MAX - 1
> 
> t2: Buffer is received, and redirected
>               +---------------
>               | upper buff (skb)
>               +---------------
> used by NIC ->| lower buffer
> (rx_buffer)   +---------------
> 
> Now, prior calling xdp_do_redirect():
>   page count  == USHRT_MAX
>   rx_buffer->pagecnt_bias == USHRT_MAX - 2
> 
> This means that buffer *cannot* be flipped/reused, because the skb is
> still using it.
> 
> The problem arises when xdp_do_redirect() actually frees the
> segment. Then we get:
>   page count  == USHRT_MAX - 1
>   rx_buffer->pagecnt_bias == USHRT_MAX - 2
> 
> From a recycle perspective, the buffer can be flipped and reused,
> which means that the skb data area is passed to the Rx HW ring!
> 
> To work around this, the page count is stored prior calling
> xdp_do_redirect().
> 
> Note that this is not optimal, since the NIC could actually reuse the
> "lower buffer" again. However, then we need to track whether
> XDP_REDIRECT consumed the buffer or not.
> 
> Fixes: d9314c474d4f ("i40e: add support for XDP_REDIRECT")
> Reported-by: Li RongQing <lirongqing@...du.com>
> Signed-off-by: Björn Töpel <bjorn.topel@...el.com>
> ---
>  drivers/net/ethernet/intel/i40e/i40e_txrx.c | 28 +++++++++++++++------
>  1 file changed, 21 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/net/ethernet/intel/i40e/i40e_txrx.c b/drivers/net/ethernet/intel/i40e/i40e_txrx.c
> index 3e5c566ceb01..5e446dc39190 100644
> --- a/drivers/net/ethernet/intel/i40e/i40e_txrx.c
> +++ b/drivers/net/ethernet/intel/i40e/i40e_txrx.c
> @@ -1873,7 +1873,8 @@ static inline bool i40e_page_is_reusable(struct page *page)
>   *
>   * In either case, if the page is reusable its refcount is increased.
>   **/
> -static bool i40e_can_reuse_rx_page(struct i40e_rx_buffer *rx_buffer)
> +static bool i40e_can_reuse_rx_page(struct i40e_rx_buffer *rx_buffer,
> +				   int rx_buffer_pgcnt)
>  {
>  	unsigned int pagecnt_bias = rx_buffer->pagecnt_bias;
>  	struct page *page = rx_buffer->page;
> @@ -1884,7 +1885,7 @@ static bool i40e_can_reuse_rx_page(struct i40e_rx_buffer *rx_buffer)
>  
>  #if (PAGE_SIZE < 8192)
>  	/* if we are only owner of page we can reuse it */
> -	if (unlikely((page_count(page) - pagecnt_bias) > 1))
> +	if (unlikely((rx_buffer_pgcnt - pagecnt_bias) > 1))
>  		return false;
>  #else
>  #define I40E_LAST_OFFSET \
> @@ -1939,6 +1940,15 @@ static void i40e_add_rx_frag(struct i40e_ring *rx_ring,
>  #endif
>  }
>  
> +static int i40e_rx_buffer_page_count(struct i40e_rx_buffer *rx_buffer)
> +{
> +#if (PAGE_SIZE < 8192)
> +	return page_count(rx_buffer->page);
> +#else
> +	return 0;
> +#endif
> +}
> +
>  /**
>   * i40e_get_rx_buffer - Fetch Rx buffer and synchronize data for use
>   * @rx_ring: rx descriptor ring to transact packets on
> @@ -1948,11 +1958,13 @@ static void i40e_add_rx_frag(struct i40e_ring *rx_ring,
>   * for use by the CPU.
>   */
>  static struct i40e_rx_buffer *i40e_get_rx_buffer(struct i40e_ring *rx_ring,
> -						 const unsigned int size)
> +						 const unsigned int size,
> +						 int *rx_buffer_pgcnt)
>  {
>  	struct i40e_rx_buffer *rx_buffer;
>  
>  	rx_buffer = i40e_rx_bi(rx_ring, rx_ring->next_to_clean);
> +	*rx_buffer_pgcnt = i40e_rx_buffer_page_count(rx_buffer);

What i previously meant was:

#if (PAGE_SIZE < 8192)
	*rx_buffer_pgcnt = page_count(rx_buffer->page);
#endif

and see below

>  	prefetchw(rx_buffer->page);
>  
>  	/* we are reusing so sync this buffer for CPU use */
> @@ -2112,9 +2124,10 @@ static struct sk_buff *i40e_build_skb(struct i40e_ring *rx_ring,
>   * either recycle the buffer or unmap it and free the associated resources.
>   */
>  static void i40e_put_rx_buffer(struct i40e_ring *rx_ring,
> -			       struct i40e_rx_buffer *rx_buffer)
> +			       struct i40e_rx_buffer *rx_buffer,
> +			       int rx_buffer_pgcnt)
>  {
> -	if (i40e_can_reuse_rx_page(rx_buffer)) {
> +	if (i40e_can_reuse_rx_page(rx_buffer, rx_buffer_pgcnt)) {
>  		/* hand second half of page back to the ring */
>  		i40e_reuse_rx_page(rx_ring, rx_buffer);
>  	} else {
> @@ -2319,6 +2332,7 @@ static int i40e_clean_rx_irq(struct i40e_ring *rx_ring, int budget)
>  	unsigned int xdp_xmit = 0;
>  	bool failure = false;
>  	struct xdp_buff xdp;
> +	int rx_buffer_pgcnt;

you could move scope this variable only for the

while (likely(total_rx_packets < (unsigned int)budget))

loop and init this to 0. then you could drop the helper function you've
added. and BTW the page_count is not being used for big pages but i agree
that it's better to have it set to 0.

>  
>  #if (PAGE_SIZE < 8192)
>  	xdp.frame_sz = i40e_rx_frame_truesize(rx_ring, 0);
> @@ -2370,7 +2384,7 @@ static int i40e_clean_rx_irq(struct i40e_ring *rx_ring, int budget)
>  			break;
>  
>  		i40e_trace(clean_rx_irq, rx_ring, rx_desc, skb);
> -		rx_buffer = i40e_get_rx_buffer(rx_ring, size);
> +		rx_buffer = i40e_get_rx_buffer(rx_ring, size, &rx_buffer_pgcnt);
>  
>  		/* retrieve a buffer from the ring */
>  		if (!skb) {
> @@ -2413,7 +2427,7 @@ static int i40e_clean_rx_irq(struct i40e_ring *rx_ring, int budget)
>  			break;
>  		}
>  
> -		i40e_put_rx_buffer(rx_ring, rx_buffer);
> +		i40e_put_rx_buffer(rx_ring, rx_buffer, rx_buffer_pgcnt);
>  		cleaned_count++;
>  
>  		if (i40e_is_non_eop(rx_ring, rx_desc, skb))
> -- 
> 2.25.1
> 

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