Re: [PATCH for-4.5 v6 00/16] Xen VMware tools support

[George Dunlap <george.dunlap@eu.citrix.com>]

On 09/30/2014 08:05 AM, Jan Beulich wrote:
>>>> On 30.09.14 at 01:13, <dslutz@verizon.com> wrote:
>> On 09/29/14 09:27, George Dunlap wrote:
>>> On 09/29/2014 07:50 AM, Jan Beulich wrote:
>>>> a) I don't think it is okay to base our emulation layer entirely
>>>> on observed behavior. At least some form of specification should
>>>> be there to follow. This is both for reviewing the code you want
>>>> committed and maintainability.
>>> While that would be nice, I think that's unlikely; and overall I think
>>> it would be better to have a reverse-engineered implementation than no
>>> implementation at all.  Having a reverse-engineered spec might be a
>>> good idea though.
>>>
>> I could work on a reverse-engineered spec.  Is having this on the wiki
>> good enough or does it need to be in the code?
> I don't think the place it's at matters that much. All that does matter
> is if it's something outside of our control, it should be a place that
> reasonably certainly won't go away any time soon, so that a link
> placed somewhere in our tree won't become stale.

I think long term it would make sense to have a document in-tree that 
describes what the code is trying to do.

>
>>>> b) I don't think it is okay to introduce security issues into a guest
>>>> even if that is something that isn't enabled by default.
>>> I agree with this; in particular, it's quite possible that someone
>>> will decide to enable VMWare functionality by default, "just in case",
>>> and then forget that they've done so.
>>>
>> I am assuming that the phrase "security issues" is used as a
>> reference to things like http://xenbits.xen.org/xsa/ or
>> http://wiki.xen.org/wiki/Securing_Xen.
>>
>> Or as it might be stated -- A way to cause a guest to crash or have
>> a DoS (/Denial of Service) or a way in from outside (like "/SMASH the
>> Bash bug".
>>
>>
>> But not the area of
>> http://en.wikipedia.org/wiki/Rainbow_Series or
>> http://en.wikipedia.org/wiki/Trusted_Computer_System_Evaluation_Criteria
>>
>> Which talks about "Covert Channel Analysis" and other complex
>> security issues. (like *"Evaluation Assurance Level", **"Trusted
>> Computer System Evaluation Criteria", etc.)*
> Covert channels are consider security issues too when applying
> strict criteria. But the main concern here are indeed ways for guest
> user mode to badly affect the guest as a whole (or the host, but I
> think that should really go without saying).

Just to bring home the point -- this code makes it so that some 
instructions, namely IO instructions, running with no privilege checks 
in ring 3, can access certain extra bits of potentially arbitrarily 
complicated "virtual hardware" functionality which the OS doesn't know 
anything about and has no way to contain or prevent.  This opens up the 
possibility that there's a bug in the functionality somehow (either in 
how VMWare implements it, or how we implement it) which an attacker can 
leverage to gain privileges within the guest.

I think Jan's point is that *we* need to be thinking carefully about the 
functionality itself, and how we implement it, to make sure (as far as 
we are able) that we don't introduce such a vulnerability. Saying "this 
is the observed functionality of VMWare" isn't enough, because, well, 
they're not perfect. :-)

>> I feel it is "safe" to run all guests with vmware_port=1 and
>> vmware_hw=7.  However I am not stating that all guests function
>> the same with just this.  I do know that xen_platform_pci=0
>> may also need to be specified to get expected results.
>>
>> I also do not understand the statement "enable VMWare functionality by
>> default".  I must be missing something because as far as I know each
>> guest (domU) has it's own config.  Is this a xl tool stack feature (some
>> common config for guests)? Or is it some other tool stack feature?
> Higher layer management tools may choose to create guest configs
> that have certain settings always enabled (like at least used to be
> the case in XenServer for the Viridian flag - not sure if that got
> changed -, i.e. enabling this even for non-Windows guests, which
> caused issues with Linux).

Or "vmware_hw=7" gets into a "howto" on the internet and mindlessly 
copied.  Or a template which is then cloned over and over again without 
checking.  Don't vmdk's include some guest configuration as well?  Or as 
Jan said, XenServer or OpenStack or CloudStack or XenOrchestra or oVirt 
set it as a default, because it can't hurt, right?

  -George