Re: [intel-sgx-kernel-dev] [PATCH 08/10] kvm: vmx: add guest's IA32_SGXLEPUBKEYHASHn runtime switch support

["Huang, Kai" <kai.huang@linux.intel.com>]

On 6/16/2017 4:33 PM, Huang, Kai wrote:
> 
> 
> On 6/16/2017 4:11 PM, Andy Lutomirski wrote:
>> On Thu, Jun 15, 2017 at 8:46 PM, Huang, Kai 
>> <kai.huang@linux.intel.com> wrote:
>>>
>>>
>>> On 6/13/2017 11:00 AM, Andy Lutomirski wrote:
>>>>
>>>> On Mon, Jun 12, 2017 at 3:08 PM, Huang, Kai <kai.huang@linux.intel.com>
>>>> wrote:
>>>>>
>>>>>
>>>>> I don't know whether SGX driver will have restrict on running
>>>>> provisioning
>>>>> enclave. In my understanding provisioning enclave is always from 
>>>>> Intel.
>>>>> However I am not expert here and probably be wrong. Can you point out
>>>>> *exactly* what restricts in host must/should be applied to guest so 
>>>>> that
>>>>> Jarkko can know whether he will support those restricts or not? 
>>>>> Otherwise
>>>>> I
>>>>> don't think we even need to talk about this topic at current stage.
>>>>>
>>>>
>>>> The whole point is that I don't know.  But here are two types of
>>>> restriction I can imagine demand for:
>>>>
>>>> 1. Only a particular approved provisioning enclave may run (be it
>>>> Intel's or otherwise -- with a non-Intel LE, I think you can launch a
>>>> non-Intel provisioning enclave).  This would be done to restrict what
>>>> types of remote attestation can be done. (Intel supplies a remote
>>>> attestation service that uses some contractual policy that I don't
>>>> know.  Maybe a system owner wants a different policy applied to ISVs.)
>>>>    Imposing this policy on guests more or less requires filtering 
>>>> EINIT.
>>>
>>>
>>> Hi Andy,
>>>
>>> Sorry for late reply.
>>>
>>> What is the issue if host and guest run provisioning enclave from 
>>> different
>>> vendor, for example, host runs intel's provisioning enclave, and 
>>> guest runs
>>> other vendor's provisioning enclave? Or different guests run 
>>> provisioning
>>> enclaves from different vendors?
>>
>> There's no issue unless someone has tried to impose a policy.  There
>> is clearly at least some interest in having policies that affect what
>> enclaves can run -- otherwise there wouldn't be LEs in the first
>> place.
>>
>>>
>>> One reason I am asking is that, on Xen (where we don't have concept of
>>> *host*), it's likely that we won't apply any policy at Xen hypervisor at
>>> all, and guests will be able to run any enclave from any signer as their
>>> wish.
>>
>> That seems entirely reasonable.  Someone may eventually ask Xen to add
>> support for SGX enclave restrictions, in which case you'll either have
>> to tell them that it won't happen or implement it.
>>
>>>
>>> Sorry that I don't understand (or kind of forgot) the issues here.
>>>
>>>>
>>>> 2. For kiosk-ish or single-purpose applications, I can imagine that
>>>> you would want to allow a specific list of enclave signers or even
>>>> enclave hashes. Maybe you would allow exactly one enclave hash.  You
>>>> could kludge this up with a restrictive LE policy, but you could also
>>>> do it for real by implementing the specific restriction in the kernel.
>>>> Then you'd want to impose it on the guest, and you'd do it by
>>>> filtering EINIT.
>>>
>>> Assuming the enclave hash means measurement of enclave, and assuming 
>>> we have
>>> a policy that we only allow enclave from one signer to run, would you 
>>> also
>>> elaborate the issue that, if host and guest run enclaves from different
>>> signer? If host has such policy, and we are allowing creating guests 
>>> on such
>>> host, I think that typically we will have the same policy in the guest
>>
>> Yes, I presume this too, but.
>>
>>> (vetted by guest's kernel). The owner of that host should be aware of 
>>> the
>>> risk (if there's any) by creating guest and run enclave inside it.
>>
>> No.  The host does not trust the guest in general.  If the host has a
> 
> I agree.
> 
>> policy that the only enclave that shall run is X, that doesn't mean
>> that the host shall reject all enclaves requested by the normal
>> userspace API except X but that, if /dev/kvm is used, then the user is
>> magically trusted to not load a guest that fails to respect the host
>> policy.  It means that the only enclave that shall run is X regardless
>> of what interface is used.  The host must only allow X to be loaded by
>> its userspace and the host must only allow X to be loaded by a guest.
>>
> 
> This is theoretical thing. I think your statement makes sense only if we 
> have specific example that can prove there's actual risk when allowing 
> guest to exceed X approved by host.
> 
> I will dig more in your previous emails to see whether you have listed 
> such real cases (I some kind forgot sorry) but if you don't mind, you 
> can list such cases here.

Hi Andy,

I can find an example you listed in your previous email but it is not 
related to host policy but related to SGX's key architecture issue. I 
quoted below:

"Concretely, imagine I write an enclave that seals my TLS client
certificate's private key and offers an API to sign TLS certificate
requests with it.  This way, if my system is compromised, an attacker
can use the certificate only so long as they have access to my
machine.  If I kick them out or if they merely get the ability to read
the sealed data but not to execute code, the private key should still
be safe.  But, if this system is a VM guest, the attacker could run
the exact same enclave on another guest on the same physical CPU and
sign using my key.  Whoops!"

I think you will have this problem even you apply the most strict policy 
at both host and guest -- only allow one enclave from one signer to run. 
This is indeed a flaw but virtualization cannot do anything to solve 
this -- unless we don't support virtualization at all :)

Sorry I am just trying to find out whether there's real case that really 
require we apply host's policy to guest and will have problem if we don't.

Thanks,
-Kai

> 
> Thanks,
> -Kai