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Re: Xen Security Advisory 378 v3 (CVE-2021-28694,CVE-2021-28695,CVE-2021-28696) - IOMMU page mapping issues on x86

  • To: Jason Andryuk <jandryuk@xxxxxxxxx>, Xen.org security team <security@xxxxxxx>
  • From: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
  • Date: Wed, 1 Sep 2021 15:15:42 +0100
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On 01/09/2021 14:22, Jason Andryuk wrote:
> On Wed, Sep 1, 2021 at 5:34 AM Xen.org security team <security@xxxxxxx> wrote:
>> Hash: SHA256
>>  Xen Security Advisory CVE-2021-28694,CVE-2021-28695,CVE-2021-28696 / XSA-378
>>                                    version 3
>>                    IOMMU page mapping issues on x86
>> ====================
>> Warn about dom0=pvh breakage in Resolution section.
>> =================
>> Both AMD and Intel allow ACPI tables to specify regions of memory
>> which should be left untranslated, which typically means these
>> addresses should pass the translation phase unaltered.  While these
>> are typically device specific ACPI properties, they can also be
>> specified to apply to a range of devices, or even all devices.
>> On all systems with such regions Xen failed to prevent guests from
>> undoing/replacing such mappings (CVE-2021-28694).
> Hi,
> Is there a way to identify if a system's ACPI tables have untranslated
> regions?  Does it show up in xen or linux dmesg or can it be
> identified in sysfs?

It's possible, but a little convoluted to do.  In dom0 (and in an empty
directory) you want:

acpidump > acpi.dmp
acpixtract -a acpi.dmp

On Intel, open up rmad.dat and hexedit the first 4 bytes from RMAD to
DMAR (yes - really - this is how we stop the dom0 kernel from trying to
poke the IOMMU directly.)

Then disassemble (iasl -d) either rmad.dat or ivrs.dat depending on
whether you're on Intel or AMD.

On Intel, you're looking for Reserved Memory Regions, while on AMD
you're looking for IVMD ranges (specifically, types 20 thru 22)

These, if present, describe a range of memory needing identity mapping,
and a scope of the PCI device(s) the range applies to.




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