Re: [PATCH 1/5] net: renesas: rswitch: fix possible early skb release

[Jacob Keller <jacob.e.keller@intel.com>]

On 12/2/2024 5:49 AM, Nikita Yushchenko wrote:
> When sending frame split into multiple descriptors, hardware processes
> descriptors one by one, including writing back DT values. The first
> descriptor could be already marked as completed when processing of
> next descriptors for the same frame is still in progress.
> 
> Although only the last descriptor is configured to generate interrupt,
> completion of the first descriptor could be noticed by the driver when
> handling interrupt for the previous frame.
> 
> Currently, driver stores skb in the entry that corresponds to the first
> descriptor. This results into skb could be unmapped and freed when
> hardware did not complete the send yet. This opens a window for
> corrupting the data being sent.
> 
> Fix this by saving skb in the entry that corresponds to the last
> descriptor used to send the frame.
> 
> Fixes: d2c96b9d5f83 ("net: rswitch: Add jumbo frames handling for TX")
> Signed-off-by: Nikita Yushchenko <nikita.yoush@cogentembedded.com>
> ---
>  drivers/net/ethernet/renesas/rswitch.c | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/drivers/net/ethernet/renesas/rswitch.c b/drivers/net/ethernet/renesas/rswitch.c
> index b80aa27a7214..32b32aa7e01f 100644
> --- a/drivers/net/ethernet/renesas/rswitch.c
> +++ b/drivers/net/ethernet/renesas/rswitch.c
> @@ -1681,8 +1681,9 @@ static netdev_tx_t rswitch_start_xmit(struct sk_buff *skb, struct net_device *nd
>  	if (dma_mapping_error(ndev->dev.parent, dma_addr_orig))
>  		goto err_kfree;
>  
> -	gq->skbs[gq->cur] = skb;
> -	gq->unmap_addrs[gq->cur] = dma_addr_orig;
> +	/* Stored the skb at the last descriptor to avoid skb free before hardware completes send */
> +	gq->skbs[(gq->cur + nr_desc - 1) % gq->ring_size] = skb;
> +	gq->unmap_addrs[(gq->cur + nr_desc - 1) % gq->ring_size] = dma_addr_orig;
>  

nr_desc is non-zero, so if nr_desc was 1, then this would point to
gq->cur + 0 mod ring_size, i.e. gq->cur.

I might have possibly computed that separately as a local variable since
this expression is repeated twice, but I don't think thats going to do
too much for readability either way.

Ok

Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>

>  	/* DT_FSTART should be set at last. So, this is reverse order. */
>  	for (i = nr_desc; i-- > 0; ) {