linux-kernel - Re: [RFC PATCH v7 00/16] Add support for eXclusive Page Frame Ownership

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Date:   Fri, 11 Jan 2019 10:59:32 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Khalid Aziz <khalid.aziz@...cle.com>, juergh@...il.com,
        tycho@...ho.ws, jsteckli@...zon.de, ak@...ux.intel.com,
        torvalds@...ux-foundation.org, liran.alon@...cle.com,
        keescook@...gle.com, konrad.wilk@...cle.com,
        deepa.srinivasan@...cle.com, chris.hyser@...cle.com,
        tyhicks@...onical.com, dwmw@...zon.co.uk,
        andrew.cooper3@...rix.com, jcm@...hat.com,
        boris.ostrovsky@...cle.com, kanth.ghatraju@...cle.com,
        joao.m.martins@...cle.com, jmattson@...gle.com,
        pradeep.vincent@...cle.com, john.haxby@...cle.com,
        tglx@...utronix.de, kirill.shutemov@...ux.intel.com, hch@....de,
        steven.sistare@...cle.com, kernel-hardening@...ts.openwall.com,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [RFC PATCH v7 00/16] Add support for eXclusive Page Frame
 Ownership

On Thu, Jan 10, 2019 at 03:40:04PM -0800, Dave Hansen wrote:
> Anything else you do will have *some* reduced mitigation value, which
> isn't a deal-breaker (to me at least).  Some ideas:
> 
> Take a look at the SWITCH_TO_KERNEL_CR3 in head_64.S.  Every time that
> gets called, we've (potentially) just done a user->kernel transition and
> might benefit from flushing the TLB.  We're always doing a CR3 write (on
> Meltdown-vulnerable hardware) and it can do a full TLB flush based on if
> X86_CR3_PCID_NOFLUSH_BIT is set.  So, when you need a TLB flush, you
> would set a bit that ADJUST_KERNEL_CR3 would see on the next
> user->kernel transition on *each* CPU.  Potentially, multiple TLB
> flushes could be coalesced this way.  The downside of this is that
> you're exposed to the old TLB entries if a flush is needed while you are
> already *in* the kernel.

I would really prefer not to depend on the PTI crud for new stuff. We
really want to get rid of that code on unaffected CPUs.