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

[Paolo Bonzini]

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