Re: Should PV frontend drivers trust the backends?

[Paul Durrant <Paul.Durrant@citrix.com>]

> -----Original Message-----
> From: Xen-devel [mailto:xen-devel-bounces@lists.xenproject.org] On Behalf
> Of Juergen Gross
> Sent: 25 April 2018 13:43
> To: xen-devel <xen-devel@lists.xenproject.org>
> 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. 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.

  Paul

> 
> Juergen
> 
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