Re: [PATCH 2/4] wan/hdlc_x25: fix skb handling

[Martin Schiller]

On 2019-04-05 21:15, David Miller wrote:
> From: Martin Schiller <ms@xxxxxxxxxx>
> Date: Fri, 05 Apr 2019 08:56:44 +0200
>
> > On 2019-04-05 02:32, David Miller wrote:
> > > From: Martin Schiller <ms@xxxxxxxxxx>
> > > Date: Wed,  3 Apr 2019 07:01:16 +0200
> > >
> > > >  	/* X.25 to LAPB */
> > > >  	switch (skb->data[0]) {
> > > >  	case X25_IFACE_DATA:	/* Data to be transmitted */
> > > > -		skb_pull(skb, 1);
> > > > -		if ((result = lapb_data_request(dev, skb)) != LAPB_OK)
> > > > -			dev_kfree_skb(skb);
> > > > -		return NETDEV_TX_OK;
> > > > +		skbn = skb_copy(skb, GFP_ATOMIC);
> > > > +		skb_pull(skbn, 1);
> > > > +		skb_reset_network_header(skbn);
> > > > +		if ((result = lapb_data_request(dev, skbn)) != LAPB_OK)
> > > > +			dev_kfree_skb(skbn);
> > > This leaks 'skb'.
> >
> > What exactly do you mean?
> > 'skb' will get freed at the end of x25_xmit() function:
> > https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/net/wan/hdlc_x25.c#n129
>
> Then why was it freed here in the original code?

In the original code, 'skb' is only freed here if lapb_data_request()
return a value != LAPB_OK, which is the case when the skb can't be
queued for transmission. Otherwise 'skb' won't be freed here in the
"X25_IFACE_DATA" case.

What my change do is, that 'skb' is copied to 'skbn' before the skb_pull
of the first byte, to fix the problem that tracing layer3 (ETH_P_X25)
packets results in a malformed first byte of the packets, because the
original "skb" will get modified before the frame reaches the tcpdump
output.

Everything else works like before.