KVM hook for code integrity checking

KVM hook for code integrity checking

[Suen Chun Hui]

Dear KVM developers,

I'm currently working on an open source security patch to use KVM to
implement code verification on a guest VM in runtime. Thus, it would be
very helpful if someone can point to me the right function or place to
look at for adding 2 hooks into the KVM paging code to:

1. Detect a new guest page (which I assume will imply a new pte and
imply a new spte).
Currently, I'm considering putting a hook in the function
mmu_set_spte(), but may there is a better place.
This hook will be used as the main entry point into the code
verification function

2. Detect a write fault to a read-only spte (eg. for the case of
updating the dirty bit back to the guest pte)
Unfortunately, I'm unable to find an appropriate place where this
actually takes place after reading the code many times.
This hook will be used to prevent a secondary "peek" page from modifying
an existing verified code page.

I will gladly contribute my work(my phd thesis) to the list once it is
workable, if anyone find it useful.
Thank you in advance.

Regards,
Chun Hui

Re: KVM hook for code integrity checking

[Avi Kivity]

On 04/30/2010 05:53 PM, Suen Chun Hui wrote:
> Dear KVM developers,
>
> I'm currently working on an open source security patch to use KVM to
> implement code verification on a guest VM in runtime. Thus, it would be
> very helpful if someone can point to me the right function or place to
> look at for adding 2 hooks into the KVM paging code to:
>
> 1. Detect a new guest page (which I assume will imply a new pte and
> imply a new spte).
> Currently, I'm considering putting a hook in the function
> mmu_set_spte(), but may there is a better place.
> This hook will be used as the main entry point into the code
> verification function
>    

This is in general not possible.  Hosts with npt or ept will not see new 
guest ptes.

It could be done with physical pages, but you'll have no way of knowing 
if the pages are used in userspace, the kernel, or both.

> 2. Detect a write fault to a read-only spte (eg. for the case of
> updating the dirty bit back to the guest pte)
> Unfortunately, I'm unable to find an appropriate place where this
> actually takes place after reading the code many times.
> This hook will be used to prevent a secondary "peek" page from modifying
> an existing verified code page.
>    

set_spte() or mmu_set_spte() may work.

-- 
error compiling committee.c: too many arguments to function

Re: KVM hook for code integrity checking

[Suen Chun Hui]

Hi,

Thanks for the reply.

On 05/05/2010 11:05 AM, Avi Kivity wrote:
> On 04/30/2010 05:53 PM, Suen Chun Hui wrote:
>> Dear KVM developers,
>>
>> I'm currently working on an open source security patch to use KVM to
>> implement code verification on a guest VM in runtime. Thus, it would be
>> very helpful if someone can point to me the right function or place to
>> look at for adding 2 hooks into the KVM paging code to:
>>
>> 1. Detect a new guest page (which I assume will imply a new pte and
>> imply a new spte).
>> Currently, I'm considering putting a hook in the function
>> mmu_set_spte(), but may there is a better place.
>> This hook will be used as the main entry point into the code
>> verification function
>>    
>
> This is in general not possible.  Hosts with npt or ept will not see
> new guest ptes.
>

Yes, I was only considering the case of using shadow paging. Would this
be possible then, since the walker would have to parse gpte anyway?

> It could be done with physical pages, but you'll have no way of
> knowing if the pages are used in userspace, the kernel, or both.
>
>> 2. Detect a write fault to a read-only spte (eg. for the case of
>> updating the dirty bit back to the guest pte)
>> Unfortunately, I'm unable to find an appropriate place where this
>> actually takes place after reading the code many times.
>> This hook will be used to prevent a secondary "peek" page from modifying
>> an existing verified code page.
>>    
>
> set_spte() or mmu_set_spte() may work.
>

Re: KVM hook for code integrity checking

[Avi Kivity]

On 05/06/2010 07:51 AM, Suen Chun Hui wrote:
> Hi,
>
> Thanks for the reply.
>
> On 05/05/2010 11:05 AM, Avi Kivity wrote:
>    
>> On 04/30/2010 05:53 PM, Suen Chun Hui wrote:
>>      
>>> Dear KVM developers,
>>>
>>> I'm currently working on an open source security patch to use KVM to
>>> implement code verification on a guest VM in runtime. Thus, it would be
>>> very helpful if someone can point to me the right function or place to
>>> look at for adding 2 hooks into the KVM paging code to:
>>>
>>> 1. Detect a new guest page (which I assume will imply a new pte and
>>> imply a new spte).
>>> Currently, I'm considering putting a hook in the function
>>> mmu_set_spte(), but may there is a better place.
>>> This hook will be used as the main entry point into the code
>>> verification function
>>>
>>>        
>> This is in general not possible.  Hosts with npt or ept will not see
>> new guest ptes.
>>
>>      
> Yes, I was only considering the case of using shadow paging. Would this
> be possible then, since the walker would have to parse gpte anyway?
>    

It's possible, but it's not a good idea to require shadow paging.  It's 
slow and doesn't scale well.

-- 
error compiling committee.c: too many arguments to function