linux-kernel - Re: [FYI PATCH 0/7] Mitigation for CVE-2018-12207

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Message-Id: <130E72B7-E7C0-4E96-A580-8F96FAF59996@gmail.com>
Date:   Wed, 13 Nov 2019 22:02:00 -0800
From:   Nadav Amit <nadav.amit@...il.com>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Jan Kiszka <jan.kiszka@...mens.com>,
        linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
        Ralf Ramsauer <ralf.ramsauer@...-regensburg.de>,
        "Gupta, Pawan Kumar" <pawan.kumar.gupta@...el.com>
Subject: Re: [FYI PATCH 0/7] Mitigation for CVE-2018-12207



> On Nov 13, 2019, at 9:26 PM, Dave Hansen <dave.hansen@...el.com> wrote:
> 
> On 11/13/19 5:17 PM, Nadav Amit wrote:
>> But is it always the case? Looking at __split_large_page(), it seems that the
>> TLB invalidation is only done after the PMD is changed. Can't this leave a
>> small time window in which a malicious actor triggers a machine-check on 
>> another core than the one that runs __split_large_page()?
> 
> It's not just a split.  It has to be a change that results in
> inconsistencies between two entries in the TLB.  A normal split doesn't
> change the resulting final translations and is never inconsistent
> between the two translations.
> 
> To have an inconsistency, you need to change the backing physical
> address (or cache attributes?).  I'd need to go double-check the erratum
> to be sure about the cache attributes.
> 
> In any case, that's why we decided that normal kernel mapping
> split/merges don't need to be mitigated.  But, we should probably
> document this somewhere if it's not clear.
> 
> Pawan, did we document the results of the audit you did anywhere?

Thank you for explaining. I now see that it is clearly written:

"Software modifies the paging structures so that the same linear address
is translated using a large page (2 MB, 4 MB, or 1 GB) with a different
physical address or memory type.” [1]

My bad.


[1] https://software.intel.com/security-software-guidance/insights/deep-dive-machine-check-error-avoidance-page-size-change-0