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 19:31:47 +0200
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Ben Hutchings <bhutchings@...arflare.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 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.
> 
> Signed-off-by: Ben Hutchings <bhutchings@...arflare.com>
> ---


Hmm, isnt GRO path also vulnerable ?

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...)

Or you could introduce a new wk->sk_gso_max_segments, that your sfc
driver sets to whatever limit ?


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