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Date:   Fri, 12 Jul 2019 17:20:52 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Dave Hansen <dave.hansen@intel.com>
Cc:     Alexandre Chartre <alexandre.chartre@oracle.com>,
        Thomas Gleixner <tglx@linutronix.de>, pbonzini@redhat.com,
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        x86@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org,
        konrad.wilk@oracle.com, jan.setjeeilers@oracle.com,
        liran.alon@oracle.com, jwadams@google.com, graf@amazon.de,
        rppt@linux.vnet.ibm.com, Paul Turner <pjt@google.com>
Subject: Re: [RFC v2 00/27] Kernel Address Space Isolation
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On Fri, Jul 12, 2019 at 06:54:22AM -0700, Dave Hansen wrote:
> On 7/12/19 5:50 AM, Peter Zijlstra wrote:
> > PTI is not mapping         kernel space to avoid             speculation crap (meltdown).
> > ASI is not mapping part of kernel space to avoid (different) speculation crap (MDS).
> > 
> > See how very similar they are?
> 
> That's an interesting point.
> 
> I'd add that PTI maps a part of kernel space that partially overlaps
> with what ASI wants.

Right, wherever we put the boundary, we need whatever is required to
cross it.

> > But looking at it that way, it makes no sense to retain 3 address
> > spaces, namely:
> > 
> >   user / kernel exposed / kernel private.
> > 
> > Specifically, it makes no sense to expose part of the kernel through MDS
> > but not through Meltdown. Therefore we can merge the user and kernel
> > exposed address spaces.
> > 
> > And then we've fully replaced PTI.
> 
> So, in one address space (PTI/user or ASI), we say, "screw it" and all
> the data mapped is exposed to speculation attacks.  We have to be very
> careful about what we map and expose here.

Yes, which is why, in an earlier email, I've asked for a clear
definition of 'sensitive" :-)

> So, maybe we're not replacing PTI as much as we're growing PTI so that
> we can run more kernel code with the (now inappropriately named) user
> page tables.

Right.