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

Jarkko Sakkinen
2021-09-13T21:15:18Z

On Mon, 2021-09-13 at 20:35 +0200, Paolo Bonzini wrote:
> 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.

AFAIK, as long you have open files for a device, they work
as expected.

/Jarkko