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

[Skidmore, Donald C]

> -----Original Message-----
> From: Hiroshi Shimamoto [mailto:h-shimamoto@xxxxxxxxxxxxx]
> Sent: Wednesday, January 21, 2015 4:18 AM
> To: David Laight; Skidmore, Donald C; BjÃrn Mork
> Cc: e1000-devel@xxxxxxxxxxxxxxxxxxxxx; netdev@xxxxxxxxxxxxxxx; Choi, Sy
> Jong; linux-kernel@xxxxxxxxxxxxxxx; Hayato Momma
> Subject: RE: [E1000-devel] [PATCH 1/2] if_link: Add VF multicast promiscuous
> mode control
> 
> > Subject: RE: [E1000-devel] [PATCH 1/2] if_link: Add VF multicast
> > promiscuous mode control
> >
> > 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.    

> 
> thanks,
> Hiroshi