[libvirt] [PATCH 4/5] qemu: Enable secure boot

Laszlo Ersek lersek at redhat.com
Thu Aug 4 13:45:39 UTC 2016

On 08/04/16 15:14, Pavel Hrdina wrote:
> On Wed, Jul 27, 2016 at 05:11:59PM +0200, Laszlo Ersek wrote:
>> On 07/27/16 10:43, Michal Privoznik wrote:
>>> In qemu, enabling this feature boils down to adding the following
>>> onto the command line:
>>>   -global driver=cfi.pflash01,property=secure,value=on
>>> However, there are some constraints resulting from the
>>> implementation. For instance, System Management Mode (SMM) is
>>> required to be enabled, the machine type must be q35-2.5 or
> s/q35-2.5/q35-2.4/
>>> later, and the guest should be x86_64. While technically it is
>>> possible to have 32 bit guests with secure boot, some non-trivial
>>> CPU flags tuning is required (for instance lm and nx flags must
>>> be prohibited). Given complexity of our CPU driver, this is not
>>> trivial. Therefore I've chosen to forbid 32 bit guests for now.
>>> If there's ever need, we can refine the check later.
>>> Signed-off-by: Michal Privoznik <mprivozn at redhat.com>
>>> ---
>>>  src/qemu/qemu_command.c                            |  7 ++++++
>>>  src/qemu/qemu_domain.c                             | 27 ++++++++++++++++++++
>>>  .../qemuxml2argv-bios-nvram-secure.args            | 29 ++++++++++++++++++++++
>>>  tests/qemuxml2argvtest.c                           |  7 ++++++
>>>  4 files changed, 70 insertions(+)
>>>  create mode 100644 tests/qemuxml2argvdata/qemuxml2argv-bios-nvram-secure.args
>> This patch looks almost complete to me (it causes all necessary QEMU
>> options to appear, directly or indirectly (= via requiring SMM)).
>> However, can you also enforce that the Q35 machtype has version 2.5 or
>> later? Technically, "pc-q35-2.4" exists too, and it's not good enough
>> (according to the instructions I wrote up in OvmfPkg/README earlier). I
>> certainly never tested it.
>> Thanks,
>> Laszlo
> I've tested it and it seems to work also with "pc-q35-2.4".  I've installed
> Fedora 24 inside a guest and I can see "Secure boot enabled" in dmesg output.
> Unless Laszlo has some more information about secure boot and why it shouldn't
> work with "pc-q35-2.4" this patch can be pushed as is.

The Secure Boot feature and the SMM driver stack are orthogonal
build-time options in OVMF. You may enable both, you may enable neither,
and you may enable only one of them as well.

However, the Secure Boot feature is not actually secure without the SMM
driver stack. Meaning, the software interfaces that relate to Secure
Boot will be available, and -- once certificates have been enrolled -- a
"well behaved" guest will see Secure Boot enabled. However, a malicious
guest can directly tamper with the pflash chip that stores the
authenticated UEFI variables. In other words, without the SMM driver
stack, a malicious guest can subvert / circumvent Secure Boot.

Therefore, for "production environments", it makes sense to refer *only*
to the combination

  Secure Boot Feature + SMM Driver Stack

as "secure". This is the meaning of "secure" that you can see in the
commit messages and the new XML tags here.

Therefore, whether or not your test results provide actual information,
depends on the following question: did your OVMF build include the SMM
Driver Stack *as well*, or only the Secure Boot feature?

Because, if solely the latter was built into your OVMF binary, that
would suffice for the

  Secure boot enabled

message to appear in the guest dmesg.

However, per se the message doesn't prove that the SMM driver stack was
built into the binary. Consequently, the message also doesn't prove that
pc-q35-2.4 provides everything that's needed for SMM.

Where did you get your OVMF binary?

... If you want to verify the presence of the SMM driver stack, please
add the following to your domain XML (note the QEMU namespace
declaration in the root element):


      <qemu:arg value='-global'/>
      <qemu:arg value='isa-debugcon.iobase=0x402'/>
      <qemu:arg value='-debugcon'/>
      <qemu:arg value='file:/tmp/GUEST_NAME.log'/>

Then look for the following string in /tmp/GUEST_NAME.log:

  SMM CPU Module exit from SMRAM with EFI_SUCCESS

If you see it, your OVMF binary contains the SMM driver stack, and then
the message in the dmesg is meaningful as evidence. If you don't see it,
then the message in the dmesg is worthless, as evidence for pc-q35-2.4.


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