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

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Date:   Tue, 14 Sep 2021 00:15:15 +0300
From:   Jarkko Sakkinen <jarkko@...nel.org>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Dave Hansen <dave.hansen@...el.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:     x86@...nel.org, linux-sgx@...r.kernel.org,
        dave.hansen@...ux.intel.com, yang.zhong@...el.com
Subject: Re: [PATCH 1/2] x86: sgx_vepc: extract sgx_vepc_remove_page

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