linux-kernel - Re: [RFC v2 00/27] Kernel Address Space Isolation

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Date:   Wed, 31 Jul 2019 18:31:26 +0200
From:   Dario Faggioli <raistlin.df@...il.com>
To:     Alexandre Chartre <alexandre.chartre@...cle.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     pbonzini@...hat.com, rkrcmar@...hat.com, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        dave.hansen@...ux.intel.com, luto@...nel.org, kvm@...r.kernel.org,
        x86@...nel.org, linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        konrad.wilk@...cle.com, jan.setjeeilers@...cle.com,
        liran.alon@...cle.com, jwadams@...gle.com, graf@...zon.de,
        rppt@...ux.vnet.ibm.com, Paul Turner <pjt@...gle.com>
Subject: Re: [RFC v2 00/27] Kernel Address Space Isolation

Hello all,

I know this is a bit of an old thread, so apologies for being late to
the party. :-)

I would have a question about this:

> > > On 7/12/19 2:36 PM, Peter Zijlstra wrote:
> > > > On Fri, Jul 12, 2019 at 02:17:20PM +0200, Alexandre Chartre
> > > > wrote:
> > > > > On 7/12/19 1:44 PM, Peter Zijlstra wrote:
> > > > > > AFAIK3 this wants/needs to be combined with core-scheduling 
> > > > > > to be
> > > > > > useful, but not a single mention of that is anywhere.
> > > > > 
> > > > > No. This is actually an alternative to core-scheduling.
> > > > > Eventually, ASI
> > > > > will kick all sibling hyperthreads when exiting isolation and
> > > > > it needs to
> > > > > run with the full kernel page-table (note that's currently
> > > > > not in these
> > > > > patches).
> 
I.e., about the fact that ASI is presented as an alternative to
core-scheduling or, at least, as it will only need integrate a small
subset of the logic (and of the code) from core-scheduling, as said
here:

> I haven't looked at details about what has been done so far.
> Hopefully, we
> can do something not too complex, or reuse a (small) part of co-
> scheduling.
> 
Now, sticking to virtualization examples, if you don't have core-
scheduling, it means that you can have two vcpus, one from VM A and the
other from VM B, running on the same core, one on thread 0 and the
other one on thread 1, at the same time.

And if VM A's vcpu, running on thread 0, exits, then VM B's vcpu
running in guest more on thread 1 can read host memory, as it is
speculatively accessed (either "normally" or because of cache load
gadgets) and brought in L1D cache by thread 0. And Indeed I do see how
ASI protects us from this attack scenario.

However, when the two VMs' vcpus are both running in guest mode, each
one on a thread of the same core, VM B's vcpu running on thread 1 can
exploit L1TF to peek at and steal secrets that VM A's vcpu, running on
thread 0, is accessing, as they're brought into L1D cache... can't it? 

How can, ASI *without* core-scheduling, prevent this other attack
scenario?

Because I may very well be missing something, but it looks to me that
it can't. In which case, I'm not sure we can call it "alternative" to
core-scheduling.... Or is the second attack scenario that I tried to
describe above, not considered interesting?

Thanks and Regards
-- 
Dario Faggioli, Ph.D
http://about.me/dario.faggioli
Virtualization Software Engineer
SUSE Labs, SUSE https://www.suse.com/
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