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

["Jan Beulich" <JBeulich@xxxxxxxx>]

>>> On 26.09.14 at 22:00, <dslutz@xxxxxxxxxxx> wrote:
> On 09/25/14 06:37, Tim Deegan wrote:
>> At 17:18 +0100 on 22 Sep (1411402700), Jan Beulich wrote:
>>> That's indeed what was said so far. I wonder though whether opening
>>> this up without guest OS consent isn't gong to introduce a security
>>> issue inside the guest (depending on the exact functionality of these
>>> hypercalls).
>> Yes indeed.  VMware seems to have CPL checks on some of the commands
>> (but not all).  I guess Xen will be no worse than VMware if we do the
>> same, though I'd like to have an official spec to follow for that.
> 
> Yes, VMware has CPL checks on some of the commands.  Not at all
> clear the include file has the correct statement.  I have not do any
> checking of CPL nor does QEMU.  And the RPC (which is CPL 3) is
> one of the most likely to have a security issue.
> 
> I do not know of an official spec to follow.  The best I have the
> the provided include file and testing on VMware.
> 
> I do know that BDOOR_CMD_GETHZ is one that is not allowed in
> ring 3, but this makes no sense to me.  I do not see why tsc_freq
> and apic bus speed to be things to hide.  And VMware is not
> consistent.  On newer configs this same info is available via
> cpuid leaf in ring 3.
> 
> Also I have not idea if VMware did the CPL checking "correctly".
> I.E. is a #GP => CPL 3, or do they check CPL?
> 
> All this leads to I current do not check CPL on any VMware commands.
> 
> I could look into doing this, but with the xl.cfg flag vmware_port=0
> turns this all off, I do not see any need for CPL checking.

Hmm, I think we need to settle on certain things here:
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.
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. Or at the
very least it should in such a case be clearly documented _and_
the feature should be properly marked as experimental.
c) Apparent or real flaws with VMware's native implementation
should be brought up with VMware. While mimicking their behavior
as closely as possible is certainly a desirable goal, reproducing
flaws their code has should imo be avoided if at all possible.

Jan


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