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

[Stefan Wahren <wahrenst@xxxxxxx>]

Hi Jakub,

Am 29.10.24 um 20:10 schrieb Jakub Kicinski:
> 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.
You mean something like this?

diff --git a/drivers/net/ethernet/vertexcom/mse102x.c
b/drivers/net/ethernet/vertexcom/mse102x.c
index a04d4073def9..2c37957478fb 100644
--- a/drivers/net/ethernet/vertexcom/mse102x.c
+++ b/drivers/net/ethernet/vertexcom/mse102x.c
@@ -222,7 +222,7 @@ static int mse102x_tx_frame_spi(struct mse102x_net
*mse, struct sk_buff *txp,
     struct mse102x_net_spi *mses = to_mse102x_spi(mse);
     struct spi_transfer *xfer = &mses->spi_xfer;
     struct spi_message *msg = &mses->spi_msg;
-    struct sk_buff *tskb;
+    struct sk_buff *tskb = NULL;
     int ret;

     netif_dbg(mse, tx_queued, mse->ndev, "%s: skb %p, %d@%p\n",
@@ -235,7 +235,6 @@ static int mse102x_tx_frame_spi(struct mse102x_net
*mse, struct sk_buff *txp,
         if (!tskb)
             return -ENOMEM;

-        dev_kfree_skb(txp);
         txp = tskb;
     }

@@ -257,6 +256,8 @@ static int mse102x_tx_frame_spi(struct mse102x_net
*mse, struct sk_buff *txp,
         mse->stats.xfer_err++;
     }

+    dev_kfree_skb(tskb);
+
     return ret;
 }