Re: [Xen-devel] [PATCH] xen-netfront: drop skb when skb->len > 65535

[annie li <annie.li@xxxxxxxxxx>]

On 2013-3-2 21:32, Wei Liu wrote:
> On Sat, Mar 02, 2013 at 02:54:17AM +0000, Ian Campbell wrote:
> > On Fri, 2013-03-01 at 17:00 +0000, Wei Liu wrote:
> > > On Fri, 2013-03-01 at 16:48 +0000, Jan Beulich wrote:
> > > > > > > On 01.03.13 at 17:31, Wei Liu <wei.liu2@xxxxxxxxxx> wrote:
> > > > > The `size' field of Xen network wired format is uint16_t, anything bigger
> > > > > than
> > > > > 65535 will cause overflow.
> > > > >
> > > > > The punishment introduced by XSA-39 is quite harsh - DomU is disconnected when
> > > > > it's discovered to be sending corrupted skbs. However, it looks like Linux
> > > > > kernel will generate some bad skbs sometimes, so drop those skbs before
> > > > > sending to over netback to avoid being disconnected.
> > > > While fixing the frontend is certainly desirable, we can't expect
> > > > everyone to deploy fixed netfronts in all their VMs - some OS
> > > > versions used in there may even be out of service. So we
> > > > ought to find a way to also more gracefully deal with the
> > > > situation in netback, without re-opening the security issue
> > > > that prompted those changes.
> > > Regarding the punishment bit, I think its worth discussing it a bit.
> > Yes, the trick is figuring out what to do without reintroducing the
> > softlockup which XSA-39 fixed.
> >
> > Perhaps we should allow silently consume (and drop) oversize skbs and
> > only shutdown the rings if they also consume too many (FSVO too many)
> > slots?
> >
> > > But the bug is always there, it drew no attention until revealed by
> > > XSA-39. It ought to be fixed anyway. :-)
> > I would have sworn that skb->len was also limited to 64k, but looking at
> > the header I see it is actually an int and the only limit of that sort
> > is related to MAX_SKB_FRAGS (which doesn't actually limit the total
> > size).
> I had the impression that skb->len was limited to 64k, too. But it
> turned out I was wrong.
>
> > OOI how big were the skbs you were seeing?
> As Nick (npegg@xxxxxxxxxx) pointed out in his email, he saw size 65538.
> I can reproduce this as well by setting vif's mtu to 100 then run iperf.
> 100 was just a random number I came up with when I played with
> fragmentation.
Did you notice any GSO packets during your iperf test?

Thanks
Annie

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