Subject: Re: [PATCH net v3] ice: fix packet corruption due to extraneous page flip

From: David Laight <[email-masked]>
Date: Wed, 13 May 2026 10:07:32 +0100

On Tue, 12 May 2026 11:19:53 -0700
John Ousterhout <[email-masked]> wrote:

> Consider the following sequence of events:
> * The bottom half of a buffer page is filled with data from
>   packet A. The page has a net reference count (reference count
>   - bias) of 1. The page is returned to the NIC, flipped to
>   use the top half.
> * Before the reference on the page is released, the NIC returns
>   the page with no data in it ('size' is zero in ice_clean_rx_irq).
>   In this case the bias does not get decremented. The page still
>   has a net reference count of 1, so it gets returned to the NIC.
>   However, ice_put_rx_mbuf flipped the page so that the bottom
>   half is active.
> * If the NIC stores another packet in the page before packet A
>   has released its reference, the data in packet A will be
>   overwritten with data from the new packet.
> * Unfortunately zero-length buffers occur frequently: they seem
>   to occur whenever a packet uses every available byte in a
>   buffer, ending precisely at the end of the buffer. When this
>   happens the NIC seems to generate an extra zero-length
>   buffer.
> The fix is for ice_put_rx_mbuf not to flip pages that have a
> size of 0.

How is this different from packet B (in the top half) being
freed before packet A (in the bottom half)?

> This patch applies directly to longterm stable versions 6.18.27
> and 6.12.86; it also seems relevant for 6.6.137 but would need
> modifcations for that version. I have not examined earlier
> versions.
> 
> Unfortunately there is no upstream commit id for this patch because
> the ICE driver has undergone a major revision (libeth refactor and
> pagepool conversion) that eliminated the buggy code. Thus the
> problem no longer exists in the main line.
> 
> Cc: [email-masked] # 6.12+
> Signed-off-by: John Ousterhout <[email-masked]>
> ---
>  drivers/net/ethernet/intel/ice/ice_txrx.c | 23 ++++++++++++++++++++---
>  1 file changed, 20 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/net/ethernet/intel/ice/ice_txrx.c b/drivers/net/ethernet/intel/ice/ice_txrx.c
> index 51c459a3e722..081c7a7392b7 100644
> --- a/drivers/net/ethernet/intel/ice/ice_txrx.c
> +++ b/drivers/net/ethernet/intel/ice/ice_txrx.c
> @@ -1215,6 +1215,13 @@ static void ice_put_rx_mbuf(struct ice_rx_ring *rx_ring, struct xdp_buff *xdp,
>  		xdp_frags = xdp_get_shared_info_from_buff(xdp)->nr_frags;
>  
>  	while (idx != ntc) {
> +		union ice_32b_rx_flex_desc *rx_desc;
> +		unsigned int size;
> +
> +		rx_desc = ICE_RX_DESC(rx_ring, idx);
> +		size = le16_to_cpu(rx_desc->wb.pkt_len) &
> +		       ICE_RX_FLX_DESC_PKT_LEN_M;
> +

Looks like you only need to calculate 'size' for the !ICE_XDP_CONSUMED path.
You could also use the (likely cheaper) test for zero:
		if (!(rx_desc->wb.pkt_len & cpu_to_le16(ICE_RX_FLX_DESC_PKT_LEN_M))

-- David

>  		buf = &rx_ring->rx_buf[idx];
>  		if (++idx == cnt)
>  			idx = 0;
> @@ -1224,10 +1231,20 @@ static void ice_put_rx_mbuf(struct ice_rx_ring *rx_ring, struct xdp_buff *xdp,
>  		 * To do this, only adjust pagecnt_bias for fragments up to
>  		 * the total remaining after the XDP program has run.
>  		 */
> -		if (verdict != ICE_XDP_CONSUMED)
> -			ice_rx_buf_adjust_pg_offset(buf, xdp->frame_sz);
> -		else if (i++ <= xdp_frags)
> +		if (verdict != ICE_XDP_CONSUMED) {
> +			/* Don't "flip" the page if size is 0: in this case
> +			 * the data in the current half will not be used so
> +			 * it's OK to reuse that half. And, since the bias
> +			 * didn't get decremented for this half, the page can
> +			 * be returned to the NIC even if the other half is
> +			 * still in use, so flipping the page could cause
> +			 * live packet data to be overwritten.
> +			 */
> +			if (size != 0)
> +				ice_rx_buf_adjust_pg_offset(buf, xdp->frame_sz);
> +		} else if (i++ <= xdp_frags) {
>  			buf->pagecnt_bias++;
> +		}
>  
>  		ice_put_rx_buf(rx_ring, buf);
>  	}