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

Jarkko Sakkinen
2017-01-31T16:21:38Z

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.

>
> static int tpm2_load_space(struct tpm_chip *chip)
> @@ -161,6 +212,28 @@ static int tpm2_load_space(struct tpm_chip *chip)
> return rc;
> }
>
> + for (i = 0, offset = 0; i < ARRAY_SIZE(space->session_tbl); i++) {
> + u32 handle;
> +
> + if (!space->session_tbl[i])
> + continue;
> +
> + rc = tpm2_load_context(chip, space->session_buf,
> + &offset, &handle);
> + if (rc == -ENOENT) {
> + /* load failed, just forget session */
> + space->session_tbl[i] = 0;
> + } else if (rc) {
> + tpm2_flush_space(chip);
> + return rc;
> + }
> + if (handle != space->session_tbl[i]) {
> + dev_warn(&chip->dev, "session restored to wrong handle\n");
> + tpm2_flush_space(chip);
> + return -EFAULT;

I forgot to ask about this corner case. When can it happen?

> + }
> + }
> +
> return 0;
> }
>
> @@ -220,7 +293,10 @@ int tpm2_prepare_space(struct tpm_chip *chip, struct tpm_space *space, u32 cc,
>
> memcpy(&chip->work_space.context_tbl, &space->context_tbl,
> sizeof(space->context_tbl));
> + memcpy(&chip->work_space.session_tbl, &space->session_tbl,
> + sizeof(space->session_tbl));
> memcpy(chip->work_space.context_buf, space->context_buf, PAGE_SIZE);
> + memcpy(chip->work_space.session_buf, space->session_buf, PAGE_SIZE);
>
> rc = tpm2_load_space(chip);
> if (rc) {
> @@ -240,7 +316,7 @@ int tpm2_prepare_space(struct tpm_chip *chip, struct tpm_space *space, u32 cc,
> static int tpm2_map_response(struct tpm_chip *chip, u32 cc, u8 *rsp, size_t len)
> {
> struct tpm_space *space = &chip->work_space;
> - u32 phandle;
> + u32 phandle, phandle_type;

Extremely cosmetic comment but I prefer one line per declaration.

> u32 vhandle;
> u32 attrs;
> u32 return_code = get_unaligned_be32((__be32 *)&rsp[6]);
> @@ -259,9 +335,15 @@ static int tpm2_map_response(struct tpm_chip *chip, u32 cc, u8 *rsp, size_t len)
> return 0;
>
> phandle = be32_to_cpup((__be32 *)&rsp[TPM_HEADER_SIZE]);
> - if ((phandle & 0xFF000000) != TPM2_HT_TRANSIENT)
> + phandle_type = (phandle & 0xFF000000);
> + if (phandle_type != TPM2_HT_TRANSIENT &&
> + phandle_type != TPM2_HT_HMAC_SESSION &&
> + phandle_type != TPM2_HT_POLICY_SESSION)
> return 0;
>
> + if (phandle_type != TPM2_HT_TRANSIENT)
> + return tpm2_session_add(chip, phandle);
> +
> /* Garbage collect a dead context. */
> for (i = 0; i < ARRAY_SIZE(space->context_tbl); i++) {
> if (space->context_tbl[i] == phandle) {
> @@ -307,9 +389,28 @@ static int tpm2_save_space(struct tpm_chip *chip)
> } else if (rc)
> return rc;
>
> + tpm2_flush_context_cmd(chip, space->context_tbl[i],
> + TPM_TRANSMIT_UNLOCKED);
> space->context_tbl[i] = ~0;
> }
>
> + for (i = 0, offset = 0; i < ARRAY_SIZE(space->session_tbl); i++) {
> + if (!space->session_tbl[i])
> + continue;
> +
> + rc = tpm2_save_context(chip, space->session_tbl[i],
> + space->session_buf, PAGE_SIZE,
> + &offset);
> +
> + if (rc == -ENOENT) {
> + /* handle error saving session, just forget it */
> + space->session_tbl[i] = 0;
> + } else if (rc < 0) {
> + tpm2_flush_space(chip);
> + return rc;
> + }
> + }
> +
> return 0;
> }
>
> @@ -335,7 +436,10 @@ int tpm2_commit_space(struct tpm_chip *chip, struct tpm_space *space,
>
> memcpy(&space->context_tbl, &chip->work_space.context_tbl,
> sizeof(space->context_tbl));
> + memcpy(&space->session_tbl, &chip->work_space.session_tbl,
> + sizeof(space->session_tbl));
> memcpy(space->context_buf, chip->work_space.context_buf, PAGE_SIZE);
> + memcpy(space->session_buf, chip->work_space.session_buf, PAGE_SIZE);
>
> return 0;
> }
> diff --git a/drivers/char/tpm/tpms-dev.c b/drivers/char/tpm/tpms-dev.c
> index 5720885..4c98db8 100644
> --- a/drivers/char/tpm/tpms-dev.c
> +++ b/drivers/char/tpm/tpms-dev.c
> @@ -39,7 +39,7 @@ static int tpms_release(struct inode *inode, struct file *file)
> struct tpms_priv *priv = container_of(fpriv, struct tpms_priv, priv);
>
> tpm_common_release(file, fpriv);
> - tpm2_del_space(&priv->space);
> + tpm2_del_space(fpriv->chip, &priv->space);
> kfree(priv);
>
> return 0;
> --
> 2.6.6
>

/Jarkko