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Subject: Re: [FYI PATCH 0/7] Mitigation for CVE-2018-12207
From:   Nadav Amit <nadav.amit@gmail.com>
In-Reply-To: <dffb19ab-daa2-a513-531e-c43279d8a4bf@intel.com>
Date:   Wed, 13 Nov 2019 17:17:39 -0800
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Jan Kiszka <jan.kiszka@siemens.com>,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        Ralf Ramsauer <ralf.ramsauer@oth-regensburg.de>,
        "Gupta, Pawan Kumar" <pawan.kumar.gupta@intel.com>
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 <dffb19ab-daa2-a513-531e-c43279d8a4bf@intel.com>
To:     Dave Hansen <dave.hansen@intel.com>
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> On Nov 13, 2019, at 1:24 PM, Dave Hansen <dave.hansen@intel.com> =
wrote:
>=20
> On 11/13/19 12:23 AM, Paolo Bonzini wrote:
>> On 13/11/19 07:38, Jan Kiszka wrote:
>>> When reading MCE, error code 0150h, ie. SRAR, I was wondering if =
that
>>> couldn't simply be handled by the host. But I suppose the symptom of
>>> that erratum is not "just" regular recoverable MCE, rather
>>> sometimes/always an unrecoverable CPU state, despite the error code, =
right?
>> The erratum documentation talks explicitly about hanging the system, =
but
>> it's not clear if it's just a result of the OS mishandling the MCE, =
or
>> something worse.  So I don't know. :(  Pawan, do you?
>=20
> It's "something worse".
>=20
> I built a kernel module reproducer for this a long time ago.  The
> symptom I observed was the whole system hanging hard, requiring me to =
go
> hit the power button.  The MCE software machinery was not involved at
> all from what I could tell.
>=20
> About creating a unit test, I'd be personally happy to share my
> reproducer, but I built it before this issue was root-caused.  There =
are
> actually quite a few underlying variants and a good unit test would =
make
> sure to exercise all of them.  My reproducer probably only exercised a
> single case.

So please correct me if I am wrong. My understanding is that the reason =
that
only KVM needs to be fixed is that there is a strong assumption that the
kernel does not hold both 4k and 2M mappings at the same time. There is =
indeed
documentation that this is the intention in __split_huge_pmd_locked(), =
for
instance, due to other AMD issues with such setup.

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=20=

another core than the one that runs __split_large_page()?