Re: Xen Security Advisory 155 (CVE-2015-8550) - paravirtualized drivers incautious about shared memory

[David Vrabel <david.vrabel@citrix.com>]

On 04/01/16 16:56, Marek Marczykowski-Górecki wrote:
> On Mon, Jan 04, 2016 at 04:22:32PM +0000, David Vrabel wrote:
>> On 04/01/16 13:06, Marek Marczykowski-Górecki wrote:
>>> On Tue, Dec 22, 2015 at 10:06:25AM -0500, Eric Shelton wrote:
>>>> The XSA mentions that "PV frontend patches will be developed and
>>>> released (publicly) after the embargo date."  Has anything been done
>>>> towards this that should also be incorporated into MiniOS?  On a
>>>> system utilizing a "driver domain," where a backend is running on a
>>>> domain that is considered unprivileged and untrusted (such as the
>>>> example described in http://wiki.xenproject.org/wiki/Driver_Domain),
>>>> it seems XSA-155-style double fetch vulnerabilities in the frontends
>>>> are also a potential security concern, and should be eliminated.
>>>> However, perhaps that does not include pcifront, since pciback would
>>>> always be running in dom0.
>>>
>>> And BTW the same applies to Linux frontends, for which also I haven't seen
>>> any public development. In attachment my email to
>>> xen-security-issues-discuss list (sent during embargo), with patches
>>> attached there. I haven't got any response.
>>
>> There are no similar security concerns with frontends since they trust
>> the backend.
>>
>> I note that you say:
>>
>>   "But in some cases (namely: if driver domains are in use), frontends
>>    may be more trusted/privileged than backends."
>>
>> But this cannot be the case since the backend can always trivially DoS
>> the frontend by (for example) not unmapping grant references when
>> required by the protocol.
> 
> DoS is one thing, code execution is another.

The DoS is a trivial and obvious example to illustrate that your
suggestion that:

"...frontends may be more trusted/privileged than backends."

is ill-advised.

Anyway, none of this means we won't consider your netfront patches.  But
you do need to post them to the correct lists (netdev and xen-devel).

David

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