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

[Stefan Berger]

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@xxxxxxxxxxxxxxxxxx>
> > > >   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. ÂDo 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.

ÂÂÂ Stefan
> Jason