Re: [tpmdd-devel] [PATCH 2/2] tpm2-space: add handling for global session exhaustion

Jarkko Sakkinen
2017-01-29T22:02:51Z

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@HansenPartnership.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

I would rather have

struct tpm_session {
u32 handle;
unsigned long created;
};

and in struct tpm_space:

struct tpm_session session_tbl[3];
struct list_head session_list;

and keep those instances that have sessions in that linked list.

What do you think?

I'll study the actual functionality in this patch properly later.

/Jarkko