Re: [E1000-devel] [PATCH 1/2] if_link: Add VF multicast promiscuous mode control

[Jeff Kirsher]

On Thu, 2015-01-22 at 09:50 +0000, David Laight wrote:
> From: Skidmore, Donald C 
> > > > From: Hiroshi Shimamoto
> > > > > My concern is what is the real issue that VF multicast
> promiscuous mode
> > > can cause.
> > > > > I think there is the 4k entries to filter multicast address,
> and the
> > > > > current ixgbe/ixgbevf can turn all bits on from VM. That is
> almost same as
> > > enabling multicast promiscuous mode.
> > > > > I mean that we can receive all multicast addresses by an
> onerous
> > > operation in untrusted VM.
> > > > > I think we should clarify what is real security issue in this
> context.
> > > >
> > > > If you are worried about passing un-enabled multicasts to users
> then
> > > > what about doing a software hash of received multicasts and
> checking
> > > > against an actual list of multicasts enabled for that hash
> entry.
> > > > Under normal conditions there is likely to be only a single
> address to check.
> > > >
> > > > It may (or may not) be best to use the same hash as any hashing
> > > > hardware filter uses.
> > >
> > > thanks for the comment. But I don't think that is the point.
> > >
> > > I guess, introducing VF multicast promiscuous mode seems to add
> new
> > > privilege to peek every multicast packet in VM and that doesn't
> look good.
> > > On the other hand, I think that there has been the same privilege
> in the
> > > current ixgbe/ixgbevf implementation already. Or I'm reading the
> code
> > > wrongly.
> > > I'd like to clarify what is the issue of allowing to receive all
> multicast packets.
> > 
> > Allowing a VM to give itself the privilege of seeing every multicast
> packet
> > could be seen as a hole in VM isolation.
> > Now if the host system allows this policy I don't see this as an
> issue as
> > someone specifically allowed this to happen and then must not be
> concerned.
> > We could even log that it has occurred, which I believe your patch
> did do.
> > The issue is also further muddied, as you mentioned above, since
> some of
> > these multicast packets are leaking anyway (the HW currently uses a
> 12 bit mask).
> > It's just that this change would greatly enlarge that hole from a
> fraction to
> > all multicast packets.
> 
> Why does it have anything to do with VM isolation?
> Isn't is just the same as if the VM were connected directly to the
> ethernet cable?

So give an example of when the VF driver is connected directly to the
ethernet cable and a PF driver (ixgbe) does not exist, at least that is
what you are suggesting.

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