linux-kernel - Re: [tpmdd-devel] [PATCH] TPM: Let the tpm char device be openable multiple times

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Message-ID: <20091104042717.GS1966@obsidianresearch.com>
Date:	Tue, 3 Nov 2009 21:27:17 -0700
From:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
To:	Valdis.Kletnieks@...edu
Cc:	Hal Finney <hal.finney@...il.com>,
	tpmdd-devel@...ts.sourceforge.net, linux-kernel@...r.kernel.org,
	srajiv@...ux.vnet.ibm.com
Subject: Re: [tpmdd-devel] [PATCH] TPM: Let the tpm char device be openable multiple times

On Tue, Nov 03, 2009 at 10:24:29PM -0500, Valdis.Kletnieks@...edu wrote:

> A number of other things under drivers/ implement "only one open" semantics,
> but those are hard-coded into the driver. But for the TPM, it's unclear if
> exclusive or non-exclusive is the right model. 

The underlying hardware already supports multiplexing multiple clients
in the same command stream - I'm not sure why this shouldn't be
exported to user space as-is. The kernel already accesses the TPM
without going through the middleware for in kernel features..

> Maybe the right answer is to default to multiple opens, but have an
> ioctl() that turns on exclusive mode.  If you have a 'tcsd' daemon,
> it will need to get launched early enough to do the open/ioctl

Why is this an issue? /dev/tpm is root only accessible. There are a lot
of things that can go horribly wrong if root does improper things, and
you can create quite reasonable multi-process tpm using applications
without the middleware.

Even if another root process does open /dev/tpm - what is the worst it
can do?

Jason
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