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

[Shuaijun Zhang <szhang@xxxxxxxxxxxxxxx>]

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)?

Regards,

Jason

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/unstable/misc/vtpm.txt
): 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
this.

Thanks & Regards,
Jason

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

----------------------------->8--------------------------------

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 interfere
+with access to the TPM by dom0.  The simplest solution for this is to prevent
+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 during
+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 Linux
+and the TPM Manager attempt to access the TPM at the same time, the TPM device
+will return a busy status; some applications will consider this a fatal error
+instead of retrying the command at a later time.  If a vTPM gets an error when
+loading its key, it will currently generate a fresh vTPM image (with a new EK,
+SRK, and blank NVRAM).
+
  Compiling the LINUX domU kernel:
 --------------------------------

--
Shuaijun Zhang
Research Engineer
Software Research Institute,
Athlone Institute of Technology, IE
Tel: +353 90 646 8196
http://www.ait.ie/sri/

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