linux-kernel - Re: [PATCH] tpm: fix a race condition tpm2_unseal

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Date:	Wed, 20 Jul 2016 23:53:14 +0300
From:	Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:	Jason Gunthorpe <jgunthorpe@...idianresearch.com>
Cc:	Peter Huewe <peterhuewe@....de>,
	linux-security-module@...r.kernel.org,
	Marcel Selhorst <tpmdd@...horst.net>,
	"moderated list:TPM DEVICE DRIVER" 
	<tpmdd-devel@...ts.sourceforge.net>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tpm: fix a race condition tpm2_unseal_trusted()

On Wed, Jul 20, 2016 at 10:48:18AM -0600, Jason Gunthorpe wrote:
> On Wed, Jul 20, 2016 at 03:16:32AM +0300, Jarkko Sakkinen wrote:
> > Unseal and load operations should be done as an atomic unit. This
> > commit fixes the issue by moving TPM mutex handling to tpm_try_get_ops()
> > and tpm_put_ops(), which is probably more logical place for it anyway.
> 
> No..
> 
> 'get_ops' is to be used to hold a persisent kref to a single tpm. It
> cannot block other tpm access.
> 
> Eg a upper protocol might get_ops to for a long period to ensure it
> consistently talks to the same TPM in a multi-tpm system.
> 
> We need something else to solve whatever you are concerned with
> here..

The only use cases I see at the moment for it work this way:

1. Call tpm_try_get_ops.
2. Send a TPM command.
3. Call tpm_put_ops.

I did not find any other form of use. The only use is to make sure that
there are no transactions running before the ops are cleared. Or did I
overlook something perhaps?

Trusted key unseal operation with TPM2 is broken into two operations:

1. Load the given key blob.
2. Unseal the data.

Without locking and unlocking mutex only once there is a race condition.

/Jarkko