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

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Date:   Thu, 21 Jun 2018 21:25:36 -0600
From:   Jason Gunthorpe <jgg@...pe.ca>
To:     Stefan Berger <stefanb@...ux.vnet.ibm.com>
Cc:     Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        linux-integrity@...r.kernel.org, jarkko.sakkinen@...ux.intel.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 3/4] ima: Use tpm_chip_find() and access TPM functions
 using it

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@...ux.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.

Jason