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

On 09/30/2014 08:05 AM, Jan Beulich wrote:
On 30.09.14 at 01:13, <dslutz@xxxxxxxxxxx> 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

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

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?


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