Subject: Re: [PATCH net] net: vertexcom: mse102x: Fix possible double free of TX skb

From: Jakub Kicinski <[email-masked]>
Date: Tue, 29 Oct 2024 12:10:28 -0700

On Tue, 22 Oct 2024 17:52:42 +0200 Stefan Wahren wrote:
> -static int mse102x_tx_frame_spi(struct mse102x_net *mse, struct sk_buff *txp,
> +static int mse102x_tx_frame_spi(struct mse102x_net *mse, struct sk_buff **txp,
>  				unsigned int pad)
>  {
>  	struct mse102x_net_spi *mses = to_mse102x_spi(mse);
> @@ -226,29 +226,29 @@ static int mse102x_tx_frame_spi(struct mse102x_net *mse, struct sk_buff *txp,
>  	int ret;
> 
>  	netif_dbg(mse, tx_queued, mse->ndev, "%s: skb %p, %d@%p\n",
> -		  __func__, txp, txp->len, txp->data);
> +		  __func__, *txp, (*txp)->len, (*txp)->data);
> 
> -	if ((skb_headroom(txp) < DET_SOF_LEN) ||
> -	    (skb_tailroom(txp) < DET_DFT_LEN + pad)) {
> -		tskb = skb_copy_expand(txp, DET_SOF_LEN, DET_DFT_LEN + pad,
> +	if ((skb_headroom(*txp) < DET_SOF_LEN) ||
> +	    (skb_tailroom(*txp) < DET_DFT_LEN + pad)) {
> +		tskb = skb_copy_expand(*txp, DET_SOF_LEN, DET_DFT_LEN + pad,
>  				       GFP_KERNEL);
>  		if (!tskb)
>  			return -ENOMEM;
> 
> -		dev_kfree_skb(txp);
> -		txp = tskb;
> +		dev_kfree_skb(*txp);
> +		*txp = tskb;
>  	}
> 
> -	mse102x_push_header(txp);
> +	mse102x_push_header(*txp);
> 
>  	if (pad)
> -		skb_put_zero(txp, pad);
> +		skb_put_zero(*txp, pad);
> 
> -	mse102x_put_footer(txp);
> +	mse102x_put_footer(*txp);
> 
> -	xfer->tx_buf = txp->data;
> +	xfer->tx_buf = (*txp)->data;
>  	xfer->rx_buf = NULL;
> -	xfer->len = txp->len;
> +	xfer->len = (*txp)->len;
> 
>  	ret = spi_sync(mses->spidev, msg);
>  	if (ret < 0) {
> @@ -368,7 +368,7 @@ static void mse102x_rx_pkt_spi(struct mse102x_net *mse)
>  	mse->ndev->stats.rx_bytes += rxlen;

Isn't it easier to change this function to free the copy rather than
the original? That way the original will remain valid for the callers.