Re: [tpmdd-devel] [PATCH v2 1/2] tpm2: add session handle context saving and restoring to the space code

[James Bottomley]

On Sun, 2017-01-29 at 23:39 +0200, Jarkko Sakkinen wrote:
> On Fri, Jan 27, 2017 at 04:32:38PM -0800, James Bottomley wrote:
> > sessions are different from transient objects in that their handles
> > may not be virtualized (because they're used for some hmac
> > calculations).  Additionally when a session is context saved, a
> > vestigial memory remains in the TPM and if it is also flushed, that
> > will be lost and the session context will refuse to load next time,
> > so
> > the code is updated to flush only transient objects after a context
> > save.  Add a separate array (chip->session_tbl) to save and restore
> > sessions by handle.  Use the failure of a context save or load to
> > signal that the session has been flushed from the TPM and we can
> > remove its memory from chip->session_tbl.
> > 
> > Sessions are also isolated during each instance of a tpm space. 
> >  This
> > means that spaces shouldn't be able to see each other's sessions
> > and
> > is enforced by ensuring that a space user may only refer to
> > sessions
> > handles that are present in their own chip->session_tbl.  Finally
> > when
> > a space is closed, all the sessions belonging to it should be
> > flushed
> > so the handles may be re-used by other spaces.
> > 
> > Note that if we get a session save or load error, all sessions are
> > effectively flushed.  Even though we restore the session buffer,
> > all
> > the old sessions will refuse to load after the flush and they'll be
> > purged from our session memory.  This means that while transient
> > context handling is still soft in the face of errors, session
> > handling
> > is hard (any failure of the model means all sessions are lost).
> > 
> > Signed-off-by: James Bottomley <
> > James.Bottomley@xxxxxxxxxxxxxxxxxxxxx>
> > 
> > ---
> > 
> > v2: - add kill space to remove sessions on close
> >     - update changelog
> >     - reduce session table to 3 entries
> > ---
> >  drivers/char/tpm/tpm-chip.c   |   6 +++
> >  drivers/char/tpm/tpm.h        |   4 +-
> >  drivers/char/tpm/tpm2-space.c | 112
> > ++++++++++++++++++++++++++++++++++++++++--
> >  drivers/char/tpm/tpms-dev.c   |   2 +-
> >  4 files changed, 118 insertions(+), 6 deletions(-)
> > 
> > diff --git a/drivers/char/tpm/tpm-chip.c b/drivers/char/tpm/tpm
> > -chip.c
> > index ed4f887..6282ad0 100644
> > --- a/drivers/char/tpm/tpm-chip.c
> > +++ b/drivers/char/tpm/tpm-chip.c
> > @@ -130,6 +130,7 @@ static void tpm_dev_release(struct device *dev)
> >  
> >  	kfree(chip->log.bios_event_log);
> >  	kfree(chip->work_space.context_buf);
> > +	kfree(chip->work_space.session_buf);
> >  	kfree(chip);
> >  }
> >  
> > @@ -223,6 +224,11 @@ struct tpm_chip *tpm_chip_alloc(struct device
> > *pdev,
> >  		rc = -ENOMEM;
> >  		goto out;
> >  	}
> > +	chip->work_space.session_buf = kzalloc(PAGE_SIZE,
> > GFP_KERNEL);
> > +	if (!chip->work_space.session_buf) {
> > +		rc = -ENOMEM;
> > +		goto out;
> > +	}
> >  
> >  	return chip;
> >  
> > diff --git a/drivers/char/tpm/tpm.h b/drivers/char/tpm/tpm.h
> > index c4b8ec9..10c57b9 100644
> > --- a/drivers/char/tpm/tpm.h
> > +++ b/drivers/char/tpm/tpm.h
> > @@ -161,6 +161,8 @@ enum tpm2_cc_attrs {
> >  struct tpm_space {
> >  	u32 context_tbl[3];
> >  	u8 *context_buf;
> > +	u32 session_tbl[3];
> > +	u8 *session_buf;
> >  };
> >  
> >  enum tpm_chip_flags {
> > @@ -583,7 +585,7 @@ unsigned long tpm2_calc_ordinal_duration(struct
> > tpm_chip *chip, u32 ordinal);
> >  int tpm2_probe(struct tpm_chip *chip);
> >  int tpm2_find_cc(struct tpm_chip *chip, u32 cc);
> >  int tpm2_init_space(struct tpm_space *space);
> > -void tpm2_del_space(struct tpm_space *space);
> > +void tpm2_del_space(struct tpm_chip *chip, struct tpm_space
> > *space);
> >  int tpm2_prepare_space(struct tpm_chip *chip, struct tpm_space
> > *space, u32 cc,
> >  		       u8 *cmd);
> >  int tpm2_commit_space(struct tpm_chip *chip, struct tpm_space
> > *space,
> > diff --git a/drivers/char/tpm/tpm2-space.c b/drivers/char/tpm/tpm2
> > -space.c
> > index d241c2a..2b3d1ad 100644
> > --- a/drivers/char/tpm/tpm2-space.c
> > +++ b/drivers/char/tpm/tpm2-space.c
> > @@ -32,18 +32,28 @@ struct tpm2_context {
> >  	__be16 blob_size;
> >  } __packed;
> >  
> > +static void tpm2_kill_sessions(struct tpm_chip *chip, struct
> > tpm_space *space);
> > +
> >  int tpm2_init_space(struct tpm_space *space)
> >  {
> >  	space->context_buf = kzalloc(PAGE_SIZE, GFP_KERNEL);
> >  	if (!space->context_buf)
> >  		return -ENOMEM;
> >  
> > +	space->session_buf = kzalloc(PAGE_SIZE, GFP_KERNEL);
> > +	if (space->session_buf == NULL) {
> > +		kfree(space->context_buf);
> > +		return -ENOMEM;
> > +	}
> > +
> >  	return 0;
> >  }
> >  
> > -void tpm2_del_space(struct tpm_space *space)
> > +void tpm2_del_space(struct tpm_chip *chip, struct tpm_space
> > *space)
> >  {
> > +	tpm2_kill_sessions(chip, space);
> >  	kfree(space->context_buf);
> > +	kfree(space->session_buf);
> >  }
> >  
> >  static int tpm2_load_context(struct tpm_chip *chip, u8 *buf,
> > @@ -69,6 +79,10 @@ static int tpm2_load_context(struct tpm_chip
> > *chip, u8 *buf,
> >  			 __func__, rc);
> >  		tpm_buf_destroy(&tbuf);
> >  		return -EFAULT;
> > +	} else if (tpm2_rc_value(rc) == TPM2_RC_HANDLE ||
> > +		   rc == TPM2_RC_REFERENCE_H0) {
> > +		rc = -ENOENT;
> > +		tpm_buf_destroy(&tbuf);
> 
> 1. Why isn't the check in tpm2_save_context sufficient to know that
>    session was flushed?

Because sessions are global.  If something flushes the session not via
our space (like /dev/tpm0) then our only indication will be a load
failure.

> 2. Can it really return both TPM_RC_HANDLE and TPM_RC_REFERENCE_H0? 

Yes, it seems that a session that doesn't exist (because it's been
flushed) then it returns TPM_RC_REFERNCE_H0, but if the context has a
sequence mismatch (because it's been flushed and reloaded) then we get
TPM_RC_HANDLE.

James

> /Jarkko
>