netdev - RE: [PATCH net-next 2/2] r6040: use ETH_ZLEN instead of MISR for SKB length checking

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Date:	Thu, 16 Jan 2014 09:10:19 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Ben Hutchings' <ben@...adent.org.uk>,
	Florian Fainelli <florian@...nwrt.org>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"davem@...emloft.net" <davem@...emloft.net>
Subject: RE: [PATCH net-next 2/2] r6040: use ETH_ZLEN instead of MISR for
 SKB length checking

From: Ben Hutchings
> On Wed, 2014-01-15 at 13:04 -0800, Florian Fainelli wrote:
> > Ever since this driver was merged the following code was included:
...
> > -	if (skb->len < MISR)
> > -		descptr->len = MISR;
> > +	if (skb->len < ETH_ZLEN)
> > +		descptr->len = ETH_ZLEN;
> 
> It looks like this is just increasing the TX descriptor length so the
> packet is tail-padded with whatever happens to come next in the skb.
> This is absolutely incorrect behaviour and may leak sensitive
> information.

And possibly page fault if the data is right at the end of a page.

> Presumably it is necessary to pad the frame because the MAC is too lame
> to do it, and the correct way to do that is using skb_padto(skb,
> ETH_ZLEN).  But this may fail as it might have to allocate memory

Alternatively use two ring entries with the 'more' bit set on
the first one and transmit the padding from a permanently allocated
and dma-mapped block of zeros.
Assuming the hardware support SG transmit and doesn't have a
constraint on the length of the first fragment.

Or have a pre-allocated an dma-mapped array of short buffers (one
for each ring slot) and copy short packets into the array instead
of dma-mapping the skb data.

	David