Re: [PATCH 1/2] x86: sgx_vepc: extract sgx_vepc_remove_page

Paolo Bonzini
2021-09-13T18:36:00Z

On 13/09/21 17:29, Dave Hansen wrote:
> On 9/13/21 8:14 AM, Paolo Bonzini wrote:
>> On 13/09/21 16:55, Dave Hansen wrote:
>>>> By "Windows startup" I mean even after guest reboot.  Because another
>>>> process could sneak in and steal your EPC pages between a close() and an
>>>> open(), I'd like to have a way to EREMOVE the pages while keeping them
>>>> assigned to the specific vEPC instance, i.e.*without*  going through
>>>> sgx_vepc_free_page().
>>> Oh, so you want fresh EPC state for the guest, but you're concerned that
>>> the previous guest might have left them in a bad state.  The current
>>> method of getting a new vepc instance (which guarantees fresh state) has
>>> some other downsides.
>>>
>>> Can't another process steal pages via sgxd and reclaim at any time?
>>
>> vEPC pages never call sgx_mark_page_reclaimable, don't they?
>
> Oh, I was just looking that they were on the SGX LRU. You might be right.
> But, we certainly don't want the fact that they are unreclaimable today
> to be part of the ABI. It's more of a bug than a feature.

Sure, that's fine.

>>> What's the extra concern here about going through a close()/open()
>>> cycle?  Performance?
>>
>> Apart from reclaiming, /dev/sgx_vepc might disappear between the first
>> open() and subsequent ones.
>
> Aside from it being rm'd, I don't think that's possible.
>

Being rm'd would be a possibility in principle, and it would be ugly for
it to cause issues on running VMs. Also I'd like for it to be able to
pass /dev/sgx_vepc in via a file descriptor, and run QEMU in a chroot or
a mount namespace. Alternatively, with seccomp it may be possible to
sandbox a running QEMU process in such a way that open() is forbidden at
runtime (all hotplug is done via file descriptor passing); it is not yet
possible, but it is a goal.

Paolo