netdev - Re: [PATCH net 1/2] tcp: Limit number of segments generated by GSO per skb

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Date:	Mon, 30 Jul 2012 20:56:20 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
CC:	David Miller <davem@...emloft.net>, <netdev@...r.kernel.org>,
	<linux-net-drivers@...arflare.com>
Subject: Re: [PATCH net 1/2] tcp: Limit number of segments generated by GSO
 per skb

On Mon, 2012-07-30 at 20:35 +0100, Ben Hutchings wrote:
> On Mon, 2012-07-30 at 19:31 +0200, Eric Dumazet wrote:
> > On Mon, 2012-07-30 at 18:16 +0100, Ben Hutchings wrote:
> > > A peer (or local user) may cause TCP to use a nominal MSS of as little
> > > as 88 (actual MSS of 76 with timestamps).  Given that we have a
> > > sufficiently prodigious local sender and the peer ACKs quickly enough,
> > > it is nevertheless possible to grow the window for such a connection
> > > to the point that we will try to send just under 64K at once.  This
> > > results in a single skb that expands to 861 segments.
> > > 
> > > In some drivers with TSO support, such an skb will require hundreds of
> > > DMA descriptors; a substantial fraction of a TX ring or even more than
> > > a full ring.  The TX queue selected for the skb may stall and trigger
> > > the TX watchdog repeatedly (since the problem skb will be retried
> > > after the TX reset).  This particularly affects sfc, for which the
> > > issue is designated as CVE-2012-3412.  However it may be that some
> > > hardware or firmware also fails to handle such an extreme TSO request
> > > correctly.
> > > 
> > > Therefore, limit the number of segments per skb to 100.  This should
> > > make no difference to behaviour unless the actual MSS is less than
> > > about 700.
[...]
> > An alternative would be to drop such frames in the ndo_start_xmit(), and
> > cap sk->sk_gso_max_size (since skb are no longer orphaned...)
> 
> I have implemented that workaround for the out-of-tree version of sfc.
> For the in-tree driver, I thought it would be better to limit the number
> of segments at source, which will avoid penalising any cases where the
> window can grow so much larger than MSS.
[...]

I mean any *legitimate* cases where this can happen.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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