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Date:   Mon, 30 Jan 2017 00:03:00 +0200
From:   Jarkko Sakkinen <jarkko.sakkinen@...ux.intel.com>
To:     James Bottomley <James.Bottomley@...senPartnership.com>
Cc:     tpmdd-devel@...ts.sourceforge.net,
        linux-security-module@...r.kernel.org,
        open list <linux-kernel@...r.kernel.org>
Subject: Re: [tpmdd-devel] [PATCH 2/2] tpm2-space: add handling for global
 session exhaustion

On Mon, Jan 30, 2017 at 12:02:19AM +0200, Jarkko Sakkinen wrote:
> On Fri, Jan 27, 2017 at 04:33:54PM -0800, James Bottomley wrote:
> > In a TPM2, sessions can be globally exhausted once there are
> > TPM_PT_ACTIVE_SESSION_MAX of them (even if they're all context saved).
> > The Strategy for handling this is to keep a global count of all the
> > sessions along with their creation time.  Then if we see the TPM run
> > out of sessions (via the TPM_RC_SESSION_HANDLES) we first wait for one
> > to become free, but if it doesn't, we forcibly evict an existing one.
> > The eviction strategy waits until the current command is repeated to
> > evict the session which should guarantee there is an available slot.
> > 
> > On the force eviction case, we make sure that the victim session is at
> > least SESSION_TIMEOUT old (currently 2 seconds).  The wait queue for
> > session slots is a FIFO one, ensuring that once we run out of
> > sessions, everyone will get a session in a bounded time and once they
> > get one, they'll have SESSION_TIMEOUT to use it before it may be
> > subject to eviction.
> > 
> > Signed-off-by: James Bottomley <James.Bottomley@...senPartnership.com>
> > ---
> >  drivers/char/tpm/tpm-chip.c   |   1 +
> >  drivers/char/tpm/tpm.h        |  39 +++++++-
> >  drivers/char/tpm/tpm2-cmd.c   |  15 +++
> >  drivers/char/tpm/tpm2-space.c | 209 ++++++++++++++++++++++++++++++++++++++++--
> >  drivers/char/tpm/tpms-dev.c   |  17 +++-
> >  5 files changed, 271 insertions(+), 10 deletions(-)
> > 
> > diff --git a/drivers/char/tpm/tpm-chip.c b/drivers/char/tpm/tpm-chip.c
> > index 6282ad0..150c6b8 100644
> > --- a/drivers/char/tpm/tpm-chip.c
> > +++ b/drivers/char/tpm/tpm-chip.c
> > @@ -164,6 +164,7 @@ struct tpm_chip *tpm_chip_alloc(struct device *pdev,
> >  
> >  	mutex_init(&chip->tpm_mutex);
> >  	init_rwsem(&chip->ops_sem);
> > +	init_waitqueue_head(&chip->session_wait);
> >  
> >  	chip->ops = ops;
> >  
> > diff --git a/drivers/char/tpm/tpm.h b/drivers/char/tpm/tpm.h
> > index 10c57b9..658e5e2 100644
> > --- a/drivers/char/tpm/tpm.h
> > +++ b/drivers/char/tpm/tpm.h
> > @@ -95,7 +95,8 @@ enum tpm2_return_codes {
> >  	TPM2_RC_HANDLE		= 0x008B,
> >  	TPM2_RC_INITIALIZE	= 0x0100, /* RC_VER1 */
> >  	TPM2_RC_DISABLED	= 0x0120,
> > -	TPM2_RC_TESTING		= 0x090A, /* RC_WARN */
> > +	TPM2_RC_SESSION_HANDLES	= 0x0905, /* RC_WARN */
> > +	TPM2_RC_TESTING		= 0x090A,
> >  	TPM2_RC_REFERENCE_H0	= 0x0910,
> >  };
> >  
> > @@ -139,7 +140,8 @@ enum tpm2_capabilities {
> >  };
> >  
> >  enum tpm2_properties {
> > -	TPM_PT_TOTAL_COMMANDS	= 0x0129,
> > +	TPM_PT_TOTAL_COMMANDS		= 0x0129,
> > +	TPM_PT_ACTIVE_SESSIONS_MAX	= 0x0111,
> >  };
> >  
> >  enum tpm2_startup_types {
> > @@ -163,8 +165,24 @@ struct tpm_space {
> >  	u8 *context_buf;
> >  	u32 session_tbl[3];
> >  	u8 *session_buf;
> > +	u32 reserved_handle;
> >  };
> >  
> > +#define TPM2_HANDLE_FORCE_EVICT 0xFFFFFFFF
> > +
> > +static inline void tpm2_session_force_evict(struct tpm_space *space)
> > +{
> > +	/* if reserved handle is not empty, we already have a
> > +	 * session for eviction, so no need to force one
> > +	 */
> > +	if (space->reserved_handle == 0)
> > +		space->reserved_handle = TPM2_HANDLE_FORCE_EVICT;
> > +}
> > +static inline bool tpm2_is_session_force_evict(struct tpm_space *space)
> > +{
> > +	return space->reserved_handle == TPM2_HANDLE_FORCE_EVICT;
> > +}
> > +
> >  enum tpm_chip_flags {
> >  	TPM_CHIP_FLAG_TPM2		= BIT(1),
> >  	TPM_CHIP_FLAG_IRQ		= BIT(2),
> > @@ -177,6 +195,12 @@ struct tpm_chip_seqops {
> >  	const struct seq_operations *seqops;
> >  };
> >  
> > +struct tpm_sessions {
> > +	struct tpm_space *space;
> > +	u32 handle;
> > +	unsigned long created;
> > +};
> 
> I would rethink this a bit. I kind of dislike this structure as it
adds extra layer of complexity.

/Jarkko

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