[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

Re: [Xen-devel] [PATCH] docs/vtpm: explain dom0 physical TPM access caveats

On 03/12/2014 12:36 PM, Shuaijun Zhang wrote:
That explains the reason.
But If the dom0 can't access the TPM, you will not be able to verify the
dom0 system & the boot process. Is it not a security risk?
Is there any solution that allows me to use vTPM and also be able to verify
the dom0 system(host system)?


At the moment, you need to give dom0 access to the physical TPM to verify
the boot process/hypervisor.  I have an updated TPM Manager and vTPM domain
for Xen 4.5 that supports a "deep quote" operation, using the hardware TPM
to produce a quote of pTPM and vTPM PCR values; I plan to post this later

On 12 March 2014 14:37, Daniel De Graaf <dgdegra@xxxxxxxxxxxxx> wrote:

On 03/12/2014 09:51 AM, Konrad Rzeszutek Wilk wrote:

On Wed, Mar 12, 2014 at 12:32:24PM +0000, Shuaijun Zhang wrote:

Hi There,

In the document of VTPM (http://xenbits.xen.org/docs/
): The Linux dom0 kernel should not try accessing the TPM while the
vTPM Manager
domain is accessing it.

Anyone knows the reason why the dom0 should not access the TPM while vTPM
Mgr is accessing it?

Lets rope in the maintainer. Perhaps the doc should be updated to explain

Thanks & Regards,

I agree; this docs patch explains the reasoning behind the original
and addresses use cases that cannot yet be handled by a virtual TPM.


Signed-off-by: Daniel De Graaf <dgdegra@xxxxxxxxxxxxx>
  docs/misc/vtpm.txt | 22 ++++++++++++++++++----
  1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/docs/misc/vtpm.txt b/docs/misc/vtpm.txt
index df1dfae..d20b424 100644
--- a/docs/misc/vtpm.txt
+++ b/docs/misc/vtpm.txt
@@ -120,10 +120,24 @@ the stubdom tree.
  Compiling the LINUX dom0 kernel:
  -The Linux dom0 kernel should not try accessing the TPM while the vTPM
-Manager domain is accessing it; the simplest way to accomplish this is
-to ensure the kernel is compiled without a driver for the TPM, or avoid
-loading the driver by blacklisting the module.
+Because the TPM manager uses direct access to the physical TPM, it may
+with access to the TPM by dom0.  The simplest solution for this is to
+dom0 from accessing the physical TPM by compiling the kernel without a
driver or
+blacklisting the module.  If dom0 needs a TPM but does not need to use it
+the boot process (i.e. it is not using IMA), a virtual TPM can be
attached to
+dom0 after the system is booted.
+Because the TPM manager does not yet accept requests for deep quotes, if
a quote
+or other request needs to be fulfilled by the physical TPM, dom0 will
need to
+access the physical TPM.  In order to prevent interference, the TPM
Manager and
+dom0 should use different values for the TPM's locality; since Linux
always uses
+locality 0, using locality 2 for the TPM Manager is recommended.  If both
+and the TPM Manager attempt to access the TPM at the same time, the TPM
+will return a busy status; some applications will consider this a fatal
+instead of retrying the command at a later time.  If a vTPM gets an error
+loading its key, it will currently generate a fresh vTPM image (with a
new EK,
+SRK, and blank NVRAM).
   Compiling the LINUX domU kernel:

Daniel De Graaf
National Security Agency

Xen-devel mailing list



Lists.xenproject.org is hosted with RackSpace, monitoring our
servers 24x7x365 and backed by RackSpace's Fanatical Support®.