Palladium: technical limits and implications

Adam Back adam at cypherspace.org
Mon Aug 12 09:14:42 PDT 2002


On Mon, Aug 12, 2002 at 01:52:39PM +0100, Ben Laurie wrote:
> AARG!Anonymous wrote:
> > [...]
> > What Palladium can do, though, is arrange that the app can't get at
> > previously sealed data if the OS has meddled with it.  The sealing
> > is done by hardware based on the app's hash.  So if the OS has changed
> > the app per the above, it won't be able to get at old sealed data.
> 
> I don't buy this: how does Palladium know what an app is without the OS' 
> help?

Here's a slightly updated version of the diagram I posted earlier:

+---------------+------------+  
| trusted-agent | user mode  |  
|    space      | app space  |  
|    (code      +------------+  
| compartment)  | supervisor |  
|               | mode / OS  |  
+---------------+------------+
| ring -1 / TOR              |
+----------------------------+  
| hardware / SCP key manager |
+----------------------------+  

Integrity Metrics in a given level are computed by the level below.

The TOR starts Trusted Agents, the Trusted Agents are outside the OS
control.  Therefore a remote application based on remote attestation
can know about the integrity of the trusted-agent, and TOR.

ring -1/TOR is computed by SCP/hardware; Trusted Agent is computed by
TOR;

The parallel stack to the right: OS is computed by TOR; Application is
computed OS.

So for general applications you still have to trust the OS, but the OS
could itself have it's integrity measured by the TOR.  Of course given
the rate of OS exploits especially in Microsoft products, it seems
likley that the aspect of the OS that checks integrity of loaded
applications could itself be tampered with using a remote exploit.

Probably the latter problem is the reason Microsoft introduced ring -1
in palladium (it seems to be missing in TCPA).

Adam
--
http://www.cypherspace.org/adam/

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