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

From: Nadav Amit <nadav.amit@gmail.com>
To: Dave Hansen <dave.hansen@intel.com>
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>
Subject: Re: [FYI PATCH 0/7] Mitigation for CVE-2018-12207
Date: Wed, 13 Nov 2019 17:17:39 -0800	[thread overview]
Message-ID: <6C0513A5-6C73-4F17-B73B-6F19E7D9EAF0@gmail.com> (raw)
In-Reply-To: <dffb19ab-daa2-a513-531e-c43279d8a4bf@intel.com>

> On Nov 13, 2019, at 1:24 PM, Dave Hansen <dave.hansen@intel.com> wrote:
> 
> 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?
> 
> It's "something worse".
> 
> 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.
> 
> 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 
another core than the one that runs __split_large_page()?