Re: [Xen-devel] Should PV frontend drivers trust the backends?

[Paul Durrant <Paul.Durrant@xxxxxxxxxx>]

> -----Original Message-----
> From: Oleksandr Andrushchenko [mailto:andr2000@xxxxxxxxx]
> Sent: 26 April 2018 07:00
> To: Paul Durrant <Paul.Durrant@xxxxxxxxxx>; 'Juergen Gross'
> <jgross@xxxxxxxx>; xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
> Subject: Re: [Xen-devel] Should PV frontend drivers trust the backends?
> 
> On 04/25/2018 04:47 PM, Paul Durrant wrote:
> >> -----Original Message-----
> >> From: Xen-devel [mailto:xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx] On
> Behalf
> >> Of Juergen Gross
> >> Sent: 25 April 2018 13:43
> >> To: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxxx>
> >> Subject: [Xen-devel] Should PV frontend drivers trust the backends?
> >>
> >> This is a followup of a discussion on IRC:
> >>
> >> The main question of the discussion was: "Should frontend drivers
> >> trust their backends not doing malicious actions?"
> >>
> >> This IMO includes:
> >>
> >> 1. The data put by the backend on the ring page(s) is sane and
> >>     consistent, meaning that e.g. the response producer index is always
> >>     ahead of the consumer index.
> >>
> >> 2. Response data won't be modified by the backend after the producer
> >>     index has been incremented signaling the response is valid.
> >>
> >> 3. Response data is sane, e.g. an I/O data length is not larger than
> >>     the buffer originally was.
> >>
> >> 4. When a response has been sent all grants belonging to the request
> >>     have been unmapped again by the backend, meaning that the frontend
> >>     can assume the grants can be removed without conflict.
> >>
> >> Today most frontend drivers (at least in the Linux kernel) seem to
> >> assume all of the above is true (there are some exceptions, but never
> >> for all items):
> >>
> >> - they don't check sanity of ring index values
> >> - they don't copy response data into local memory before looking at it
> >> - they don't verify returned data length (or do so via BUG_ON())
> >> - they BUG() in case of a conflict when trying to remove a grant
> >>
> >> So the basic question is: should all Linux frontend drivers be modified
> >> in order to be able to tolerate buggy or malicious backends? Or is the
> >> list of trust above fine?
> >>
> >> IMO even in case the frontends do trust the backends to behave sane this
> >> doesn't mean driver domains don't make sense. Driver domains still make
> >> a Xen host more robust as they e.g. protect the host against driver
> >> failures normally leading to a crash of dom0.
> >>
> > I see the general question as being analogous to 'should a Linux device
> driver trust its hardware' and I think the answer for a general purpose OS 
> like
> linux is 'yes'.
> >
> > Now, having worked on fault tolerant systems in a past life, there are
> definitely cases where you want your OS not to implicitly trust its peripheral
> hardware and hence special device drivers are used.
> So what do you do if counters provided by the untrusted HW are ok
> and the payload is not?

Well, that depends on whether there is actually any way to verify the payload 
in a driver. Whatever layer in the system is responsible for the data needs to 
verify its integrity in a fault tolerant system. Generally the driver can only 
attempt to verify that it's hardware is working as expect and quiesce it if 
not. For that reason, in the systems I worked on, the driver had the ability to 
control FETs that disconnected peripheral h/w from the PCI bus.

> > I think the same would apply for virtual machines in situations where a
> driver domain is not wholly controlled by a host administrator or is not
> trusted to the same extent as dom0 for other reasons; i.e. they should have
> specialist frontends.

> I believe we might be able to express some common strategy for the
> frontends.
> I do understand though that it all needs to be decided on case by case
> basis,
> but common things could still be there, e.g. if prod/cons counters are
> not in sync
> what a frontend needs to do:
>   - should it keep trying to get in sync - might be a bad idea as the
> req/resp data
>     may already become inconsistent (net can probably survive, but not
> block)
>   - should it tear down the connection with the backend - this may
> render in the whole
>     system instability, e.g. imagine you tear down a "/" block device
>   - should it BUG_ON and die
> To me the second option (tear down the connection) seems to be
> more reasonable, although it can still render the guest unusable, but at
> least it
> gives a chance for the guest to recover in a proper way
> 

Absolutely that can be done and it's certainly a good idea to be somewhat 
defensive but, as you say, it's quite likely that the PV pair is part of a 
critical subsystem for the guest and so a BUG() may well be the best option to 
make sure that the inevitable guest crash actually contains pertinent 
information.

> And, if my assumption is correct, we still do trust the contents of the
> requests
> and responses, e.g. the payload is still trusted.

Why should the payload be any more trusted than the content of the shared ring? 
They are both shared with the backend and therefore can be corrupted to the 
same extent.

> This also questions
> the approach,
> e.g. if we don't trust backend's counters, then why do we trust the
> payload it sends?
> And there is no obvious way to check the payload integrity.

Quite, as I said above. 

> 
> So, either we trust the backend and accept the risks or we need to
> develop some
> complex approach to address the above.
> 

Indeed, hence my position that in the general case it's not a security issue 
for a frontend to trust its backend.

Cheers,

  Paul

> Thank you,
> Oleksandr
> >    Paul
> >
> >> Juergen
> >>
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