Re: [Xen-devel] [PATCH v2] IOMMU: make DMA containment of quarantined devices optional

[Jürgen Groß <jgross@xxxxxxxx>]

On 13.12.19 15:45, Jan Beulich wrote:
> On 13.12.2019 15:24, Jürgen Groß wrote:
> > On 13.12.19 15:11, Jan Beulich wrote:
> > > On 13.12.2019 14:46, Jürgen Groß wrote:
> > > > On 13.12.19 14:38, Jan Beulich wrote:
> > > > > On 13.12.2019 14:31, Jürgen Groß wrote:
> > > > > > Maybe I have misunderstood the current state, but I thought that it
> > > > > > would just silently hide quirky devices without imposing a security
> > > > > > risk. We would not learn which devices are quirky, but OTOH I doubt
> > > > > > we'd get many reports about those in case your patch goes in.
> > > > > We don't want or need such reports, that's not the point. The
> > > > > security risk comes from the quirkiness of the devices - admins
> > > > > may wrongly think all is well and expose quirky devices to not
> > > > > sufficiently trusted guests. (I say this fully realizing that
> > > > > exposing devices to untrusted guests is almost always a certain
> > > > > level of risk.)
> > > > Do we _know_ those devices are problematic from security standpoint?
> > > > Normally the IOMMU should do the isolation just fine. If it doesn't
> > > > then its not the quirky device which is problematic, but the IOMMU.
> > > >
> > > > I thought the problem was that the quirky devices would not stop all
> > > > (read) DMA even when being unassigned from the guest resulting in
> > > > fatal IOMMU faults. The dummy page should stop those faults to happen
> > > > resulting in a more stable system.
> > > IOMMU faults by themselves are not impacting stability (they will
> > > add processing overhead, yes). The problem, according to Paul's
> > > description, is that the occurrence of at least some forms of IOMMU
> > > faults (not present ones as it seems, as opposed to permission
> > > violation ones) is fatal to certain systems. Irrespective of the
> > > sink page used after de-assignment a guest can arrange for IOMMU
> > > faults to occur even while it still has the device assigned. Hence
> > > it is important for the admin to know that their system (not the
> > > the particular device) behaves in this undesirable way.
> > So how does the admin learn this? Its not as if your patch would result
> > in a system crash or hang all the time, right? This would be the case
> > only if there either is a malicious (on purpose or due to a bug) guest
> > which gets the device assigned, or if there happens to be a pending DMA
> > operation when the device gets unassigned.
> I didn't claim the change would cover all cases. All I am claiming
> is that it increases the chances of admins becoming aware of reasons
> not to pass through devices to certain guests.
So combined with your answer this means to me:

With your patch (or the original one reverted) a DoS will occur either
due to a malicious guest or in case a DMA is still pending. As a result
the admin will no longer pass this device to any untrusted guest.

With the current 4.13-staging a DoS will occur only due to a malicious
guest. The admin will then no longer pass this device to any untrusted
guest.

So right now without any untrusted guest no DoS, while possibly DoS with
your patch. How is that better?


Juergen

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