netdev - Re: [PATCH net] qmi_wwan: Clone the skb when in pass-through mode

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Date:   Mon, 14 Jun 2021 13:05:30 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Bjørn Mork <bjorn@...k.no>,
        Kristian Evensen <kristian.evensen@...il.com>
Cc:     netdev@...r.kernel.org, subashab@...eaurora.org
Subject: Re: [PATCH net] qmi_wwan: Clone the skb when in pass-through mode

On Mon, 14 Jun 2021 16:45:55 +0200 Bjørn Mork wrote:
> Kristian Evensen <kristian.evensen@...il.com> writes:
> 
> > The skb that we pass to the rmnet driver is owned by usbnet and is freed
> > soon after the rx_fixup() callback is called (in usbnet_bh()).  There is
> > no guarantee that rmnet is done handling the skb before it is freed. We
> > should clone the skb before we call netif_rx() to prevent use-after-free
> > and misc. kernel oops.
> >
> > Fixes: 59e139cf0b32 ("net: qmi_wwan: Add pass through mode")
> >
> > Signed-off-by: Kristian Evensen <kristian.evensen@...il.com>
> > ---
> >  drivers/net/usb/qmi_wwan.c | 4 ++++
> >  1 file changed, 4 insertions(+)
> >
> > diff --git a/drivers/net/usb/qmi_wwan.c b/drivers/net/usb/qmi_wwan.c
> > index db8d3a4f2678..5ac307eb0bfd 100644
> > --- a/drivers/net/usb/qmi_wwan.c
> > +++ b/drivers/net/usb/qmi_wwan.c
> > @@ -620,6 +620,10 @@ static int qmi_wwan_rx_fixup(struct usbnet *dev, struct sk_buff *skb)
> >  		return qmimux_rx_fixup(dev, skb);
> >  
> >  	if (info->flags & QMI_WWAN_FLAG_PASS_THROUGH) {
> > +		skb = skb_clone(skb, GFP_ATOMIC);
> > +		if (!skb)
> > +			return 0;
> > +
> >  		skb->protocol = htons(ETH_P_MAP);
> >  		return (netif_rx(skb) == NET_RX_SUCCESS);
> >  	}  
> 
> Thanks for pointing this out.  But it still looks strange to me.  Why do
> we call netif_rx(skb) here instead of just returning 1 and leave that
> for usbnet_skb_return()?  With cloning we end up doing eth_type_trans()
> on the duplicate - is that wise?

Agreed on the cloning being a strange solution. Kristian, were you able
to reproduce the problem on upstream kernels?

It does look pretty strange that qmimux_rx_fixup() copies out all
packets and receives them, and then let's usbnet to process the
multi-frame skb without even fulling off the qmimux_hdr. I'm probably
missing something.. otherwise sth like FLAG_MULTI_PACKET may be in
order?