linux-kernel - Re: [PATCH 1/2] x86: sgx_vepc: extract sgx_vepc_remove

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Date:   Mon, 13 Sep 2021 20:35:53 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Dave Hansen <dave.hansen@...el.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     x86@...nel.org, linux-sgx@...r.kernel.org, jarkko@...nel.org,
        dave.hansen@...ux.intel.com, yang.zhong@...el.com
Subject: Re: [PATCH 1/2] x86: sgx_vepc: extract sgx_vepc_remove_page

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