Re: [PATCH v2 3/4] ima: Use tpm_chip_find() and access TPM functions using it

From: Jason Gunthorpe <jgg@ziepe.ca>
To: Stefan Berger <stefanb@linux.vnet.ibm.com>
Cc: Mimi Zohar <zohar@linux.vnet.ibm.com>,
	linux-integrity@vger.kernel.org, jarkko.sakkinen@linux.intel.com,
	linux-kernel@vger.kernel.org
Subject: Re: [PATCH v2 3/4] ima: Use tpm_chip_find() and access TPM functions using it
Date: Fri, 22 Jun 2018 08:19:51 -0600	[thread overview]
Message-ID: <20180622141951.GC19151@ziepe.ca> (raw)
In-Reply-To: <495fdcf7-b5b9-0341-796b-66fdf537811b@linux.vnet.ibm.com>

On Fri, Jun 22, 2018 at 07:40:37AM -0400, Stefan Berger wrote:
> On 06/21/2018 11:25 PM, Jason Gunthorpe wrote:
> >On Thu, Jun 21, 2018 at 04:59:55PM -0400, Stefan Berger wrote:
> >>On 06/21/2018 04:53 PM, Mimi Zohar wrote:
> >>>On Wed, 2018-06-20 at 16:42 -0400, Stefan Berger wrote:
> >>>>Rather than accessing the TPM functions using a NULL pointer, which
> >>>>causes a lookup for a suitable chip every time, get a hold of a tpm_chip
> >>>>and access the TPM functions using this chip. We call the tpm_chip
> >>>>ima_tpm_chip and protect it, once initialization is done, using a
> >>>>rw_semaphore called ima_tpm_chip_lock.
> >>>>
> >>>>Use ima_shutdown to release the tpm_chip.
> >>>>
> >>>>Signed-off-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
> >>>>  security/integrity/ima/ima.h        |  3 +++
> >>>>  security/integrity/ima/ima_crypto.c | 12 ++++++++++--
> >>>>  security/integrity/ima/ima_init.c   | 19 ++++++++++++-------
> >>>>  security/integrity/ima/ima_queue.c  |  7 +++++--
> >>>>  4 files changed, 30 insertions(+), 11 deletions(-)
> >>>>
> >>>>diff --git a/security/integrity/ima/ima.h b/security/integrity/ima/ima.h
> >>>>index 354bb5716ce3..53a88d578ca5 100644
> >>>>+++ b/security/integrity/ima/ima.h
> >>>>@@ -24,6 +24,7 @@
> >>>>  #include <linux/hash.h>
> >>>>  #include <linux/tpm.h>
> >>>>  #include <linux/audit.h>
> >>>>+#include <linux/rwsem.h>
> >>>>  #include <crypto/hash_info.h>
> >>>>
> >>>>  #include "../integrity.h"
> >>>>@@ -56,6 +57,8 @@ extern int ima_policy_flag;
> >>>>  extern int ima_used_chip;
> >>>>  extern int ima_hash_algo;
> >>>>  extern int ima_appraise;
> >>>>+extern struct rw_semaphore ima_tpm_chip_lock;
> >>>>+extern struct tpm_chip *ima_tpm_chip;
> >>>ima_add_templatE_entry() synchronizes appending a measurement to the
> >>>measurement list and extending the TPM by taking a lock.  Do we really
> >>>need to introduce another lock?
> >>This lock protects the ima_tpm_chip from going from != NULL to NULL in the
> >>ima_shutdown function. Basically, a global pointer accessed by concurrent
> >>threads should be protected if its value can change. However, in this case
> >>ima_shutdown would be called so late that there shouldn't be concurrency
> >>anymore. Though, I found it better to protect it. Maybe someone else has an
> >>opinion?
> >Why have a shutdown block? There is no harm in holding a kref if the
> >machine is shutting down.
> 
> Looking around at other drivers' usage of the reboot notifier, I find other
> drivers as well that use spinlocks or mutexes during the shutdown. Besides
> that, we do have the shutdown block already when device_shutdown calls
> tpm_class_shutdown() and we get the ops_sem.

But the shutdown handler in TPM an actual purpose, we are doing
something to the persistent state in the TPM itself during shutdown.

I can't see why IMA needs a shutdown handler. You shouldn't add one
'just because'

Jason