linux-kernel - Re: [PATCH 1/3] integrity: TPM internel kernel interface

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Message-ID: <20081022144944.GC21612@us.ibm.com>
Date:	Wed, 22 Oct 2008 09:49:44 -0500
From:	"Serge E. Hallyn" <serue@...ibm.com>
To:	Rajiv Andrade <srajiv@...ux.vnet.ibm.com>
Cc:	Mimi Zohar <zohar@...ux.vnet.ibm.com>,
	linux-kernel@...r.kernel.org, James Morris <jmorris@...ei.org>,
	David Safford <safford@...son.ibm.com>,
	Serge Hallyn <serue@...ux.vnet.ibm.com>
Subject: Re: [PATCH 1/3] integrity: TPM internel kernel interface

Quoting Rajiv Andrade (srajiv@...ux.vnet.ibm.com):
> On Tue, 2008-10-14 at 17:23 -0500, Serge E. Hallyn wrote:
> > Quoting Mimi Zohar (zohar@...ux.vnet.ibm.com):
> > > The internal TPM kernel interface did not protect itself from
> > > the removal of the TPM driver, while being used.  We continue
> > > to protect the tpm_chip_list using the driver_lock as before,
> > > and are using an rcu lock to protect readers. The internal TPM
> > 
> > I still would like to see this spelled out somewhere - correct me
> > if I'm wrong but none of the patches sent so far have this spelled
> > out in in-line comments, do they?
> > 
> > It does look sane:
> > 
> > 	1. writes to tpm_chip_list are protected by driver_lock
> > 	2. readers of the list are protected by rcu
> > 	3. chips which are read from the tpm_chip_list, if they
> > 	   are used outside of the rcu_read_lock(), are pinned
> > 	   using get_device(chip->dev) before releasing the
> > 	   rcu_read_lock.
> > 
> > Like I say it looks sane, but something like the above summary
> > could stand to be in a comment on top of tpm.c or something.
> > 
> No problem, I'll submit a patch containing a proper comment section to
> be applied on top of these, maybe after they get accepted.

Great, thanks.

> > > kernel interface now protects itself from the driver being
> > > removed by incrementing the module reference count.
> > > 
> > > Resubmitting integrity-tpm-internal-kernel-interface.patch, which
> > > was previously Signed-off-by Kylene Hall.
> > > Updated per feedback:
> > > 
> > > Adds the following support:
> > >   - make internal kernel interface to transmit TPM commands global
> > >   - adds reading a pcr value
> > >   - adds extending a pcr value
> > >   - adds lookup the tpm_chip for given chip number and type
> > > 
> > > Signed-off-by: Mimi Zohar <zohar@...ux.vnet.ibm.com>
> > > Signed-off-by: Rajiv Andrade <srajiv@...ux.vnet.ibm.com>
> > 
> > Now there are other, existing callers of tpm_transmit.  Are they
> > all protected by sysfs pinning the kobject and thereby the device,
> > for the duration of the call?
> > 
> 
> They aren't called through sysfs, but are still protected. These new
> functions get chip data consistently by using rcu_read. Then, after
> computing what's intended to be written back to the chip, tpm_transmit
> sends the new data while using tpm_mutex, so both operations are
> performed without the risk of a race condition.

Can you show me where the refcount for dev is incremented (under the
rcu_read_lock), either in sysfs code or tpm code?  I'm not finding
it, but it may just be done in some subtle way that I'm glossing over.

thanks,
-serge
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