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

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Date:	Tue, 9 Aug 2016 13:36:29 +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 Thu, Jul 21, 2016 at 10:25:36AM -0600, Jason Gunthorpe wrote:
> On Thu, Jul 21, 2016 at 12:02:45PM +0300, Jarkko Sakkinen wrote:
> > On Wed, Jul 20, 2016 at 03:13:32PM -0600, Jason Gunthorpe wrote:
> > > On Wed, Jul 20, 2016 at 11:53:14PM +0300, Jarkko Sakkinen wrote:
> > > 
> > > > 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.
> > > 
> > > Right, but that is just a reflection of what the in kernel users are
> > > doing today, not necessarily what they should be doing.
> > > 
> > > We should not break the put/get semantics..
> > > 
> > > > 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?
> > > 
> > > The put/get is intended to allow a kapi user to hold a ref to tpm
> > > without it geting destroyed. It is not intended to be an exclusive lock.
> > 
> > These operations *are not* exposed to kapi. They are interal to the
> > driver. That's why it does not make sense speak about kapi user.
> 
> Right now yes, but look at other subsystems and you will see
> operations like that, because that is typical design pattern. When I
> wrote them I made sure they could be used in that typical way.
> 
> We have issues in our kapi users with regards to hot plug and multiple
> tpms. Fortunately that basically never happens, but it does indicate
> the API is not sufficient..

Functionally my patch should not break anything. I understand the need
for clean up of locking but why doing this now to make the driver
non-racy would make clean up later on any harder?

I would rather think of clean up after the code is non-racy than doing a
huge clean up for racy code. Correct functionality is more important
than clean code because it has direct effect to users.

/Jarkko