Re: [Xen-devel] [PATCH v3 1/2] x86/mm: Change default value for suppress #VE in set_mem_access()

[Razvan Cojocaru <rcojocaru@xxxxxxxxxxxxxxx>]

On 07/20/2017 07:46 PM, Tamas K Lengyel wrote:
> On Thu, Jul 20, 2017 at 10:43 AM, George Dunlap
> <George.Dunlap@xxxxxxxxxxxxx> wrote:
>> On Wed, Jul 19, 2017 at 7:24 PM, Tamas K Lengyel <tamas@xxxxxxxxxxxxx> wrote:
>>>> I think the issue would be whether to allow a domain to set/clear the
>>>> suppress #VE bit for its pages by calling the new HVMOP on itself.
>>>
>>> This problem is not limited to setting the SVE bit. It also applies to
>>> swapping altp2m views. Pretty much all altp2m HVMOPs can be issued
>>> from a user-space program without any way to check whether that
>>> process is allowed to do that or not. If you don't think it is safe
>>> for a domain to set SVE, the none of the altp2m ops are safe for the
>>> domain to issue on itself. If we could say ensure only the kernel can
>>> issue the hvmops, that would be OK. But that's not possible at the
>>> moment AFAICT.
>>
>> Wait, is that right?  I think we normally restrict hypercalls to only
>> being made from the guest kernel, don't we?
>>
> 
> If that's the case then it's good to know (can you point me where that
> restriction is done?) I was just referring to the fact that
> technically a userspace program can issue VMCALL.

It is the case AFAIK. We had to do this trick to allow guest request
hypercalls coming from guest userspace:

https://github.com/xenserver/xen-4.7.pg/blob/master/master/xen-x86-hvm-Allow-guest_request-vm_events-coming-from-us.patch

By default they can only come from the guest kernel.


Thanks,
Razvan

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