Re: [PATCH V4 05/18] iommu/ioasid: Redefine IOASID set and allocation APIs

David Gibson
2021-06-01T06:36:18Z

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On Thu, May 27, 2021 at 04:06:20PM -0300, Jason Gunthorpe wrote:
> On Thu, May 27, 2021 at 02:53:42PM +1000, David Gibson wrote:
>
> > > > If the physical device had a bug which meant the mdevs *weren't*
> > > > properly isolated from each other, then those mdevs would share a
> > > > group, and you *would* care about it. Depending on how the isolation
> > > > failed the mdevs might or might not also share a group with the parent
> > > > physical device.
> > >
> > > That isn't a real scenario.. mdevs that can't be isolated just
> > > wouldn't be useful to exist
> >
> > Really? So what do you do when you discover some mdevs you thought
> > were isolated actually aren't due to a hardware bug? Drop support
> > from the driver entirely? In which case what do you say to the people
> > who understandably complain "but... we had all the mdevs in one guest
> > anyway, we don't care if they're not isolated"?
>
> I've never said to eliminate groups entirely.
>
> What I'm saying is that all the cases we have for mdev today do not
> require groups, but are forced to create a fake group anyhow just to
> satisfy the odd VFIO requirement to have a group FD.
>
> If some future mdev needs groups then sure, add the appropriate group
> stuff.
>
> But that doesn't effect the decision to have a VFIO group FD, or not.
>
> > > > It ensures that they're parked at the moment the group moves from
> > > > kernel to userspace ownership, but it can't prevent dpdk from
> > > > accessing and unparking those devices via peer to peer DMA.
> > >
> > > Right, and adding all this group stuff did nothing to alert the poor
> > > admin that is running DPDK to this risk.
> >
> > Didn't it? Seems to me the admin that in order to give the group to
> > DPDK, the admin had to find and unbind all the things in it... so is
> > therefore aware that they're giving everything in it to DPDK.
>
> Again, I've never said the *group* should be removed. I'm only
> concerned about the *group FD*

Ok, that wasn't really clear to me.

I still wouldn't say the group for mdevs is a fiction though.. rather
that the group device used for (no internal IOMMU case) mdevs is just
plain wrong.

> When the admin found and unbound they didn't use the *group FD* in any
> way.

No, they are likely to have changed permissions on the group device
node as part of the process, though.

> > > You put the same security labels you'd put on the group to the devices
> > > that consitute the group. It is only more tricky in the sense that the
> > > script that would have to do this will need to do more than ID the
> > > group to label but also ID the device members of the group and label
> > > their char nodes.
> >
> > Well, I guess, if you take the view that root is allowed to break the
> > kernel. I tend to prefer that although root can obviously break the
> > kernel if they intend do, we should make it hard to do by accident -
> > which in this case would mean the kernel *enforcing* that the devices
> > in the group have the same security labels, which I can't really see
> > how to do without an exposed group.
>
> How is this "break the kernel"? It has nothing to do with the
> kernel. Security labels are a user space concern.

*thinks*... yeah, ok, that was much too strong an assertion. What I
was thinking of is the fact that this means that guarantees you'd
normally expect the kernel to enforce can be obviated by bad
configuration: chown-ing a device to root doesn't actually protect it
if there's another device in the same group exposed to other users.

But I guess you could say the same about, say, an unauthenticated nbd
export of a root-owned block device, so I guess that's not something
the kernel can reasonably enforce.

Ok.. you might be finally convincing me, somewhat.

--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson

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