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

Jarkko Sakkinen
2017-01-31T16:27:31Z

On Tue, Jan 31, 2017 at 06:21:15PM +0200, Jarkko Sakkinen wrote:
> Now that I understand what is happening I'll give some code level
> feedback. Overally I think this is in really good shape!
>
> 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@HansenPartnership.com>
> >
> > ---
> >
> > 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);
>
> Move the declaration here so that you don't have to jump back and forth
> in the code in order to understand what is happening.
>
> > +
> > 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);
>
> Maybe a comment here would make sense (for maitenance purposes) as it
> isn't inherently obvious why and when these error codes are used.
>
> > } else if (rc > 0) {
> > dev_warn(&chip->dev, "%s: failed with a TPM error 0x%04X\n",
> > __func__, rc);
> > @@ -121,12 +135,30 @@ static int tpm2_save_context(struct tpm_chip *chip, u32 handle, u8 *buf,
> > }
> >
> > memcpy(&buf[*offset], &tbuf.data[TPM_HEADER_SIZE], body_size);
> > - tpm2_flush_context_cmd(chip, handle, TPM_TRANSMIT_UNLOCKED);
> > *offset += body_size;
> > tpm_buf_destroy(&tbuf);
> > return 0;
> > }
> >
> > +static int tpm2_session_add(struct tpm_chip *chip, u32 handle)
> > +{
> > + struct tpm_space *space = &chip->work_space;
> > + int i;
> > +
> > + for (i = 0; i < ARRAY_SIZE(space->session_tbl); i++)
> > + if (space->session_tbl[i] == 0)
> > + break;
> > + if (i == ARRAY_SIZE(space->session_tbl)) {
> > + dev_err(&chip->dev, "out of session slots\n");
>
> I think using dev_err is not correct here as this is a legit situation.
> I would lower this down to dev_dbg.
>
> > + tpm2_flush_context_cmd(chip, handle, TPM_TRANSMIT_UNLOCKED);
> > + return -ENOMEM;
> > + }
> > +
> > + space->session_tbl[i] = handle;
> > +
> > + return 0;
> > +}
> > +
> > static void tpm2_flush_space(struct tpm_chip *chip)
> > {
> > struct tpm_space *space = &chip->work_space;
> > @@ -136,6 +168,25 @@ static void tpm2_flush_space(struct tpm_chip *chip)
> > if (space->context_tbl[i] && ~space->context_tbl[i])
> > tpm2_flush_context_cmd(chip, space->context_tbl[i],
> > TPM_TRANSMIT_UNLOCKED);
> > +
> > + for (i = 0; i < ARRAY_SIZE(space->session_tbl); i++) {
> > + if (space->session_tbl[i])
> > + tpm2_flush_context_cmd(chip, space->session_tbl[i],
> > + TPM_TRANSMIT_UNLOCKED);
> > + }
> > +}
> > +
> > +static void tpm2_kill_sessions(struct tpm_chip *chip, struct tpm_space *space)
> > +{
> > + int i;
> > +
> > + mutex_lock(&chip->tpm_mutex);
> > + for (i = 0; i < ARRAY_SIZE(space->session_tbl); i++) {
> > + if (space->session_tbl[i])
> > + tpm2_flush_context_cmd(chip, space->session_tbl[i],
> > + TPM_TRANSMIT_UNLOCKED);
> > + }
> > + mutex_unlock(&chip->tpm_mutex);
> > }
>
> Please use this in tpm2_flush_space() and take the mutex in those call
> sites where you need it.

... and maybe for consistency this should be tpm2_flush_sessions?

/Jarkko