Add session handling to tpm spaces

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.

Signed-off-by James Bottomley <[email protected]>

---
 drivers/char/tpm/tpm-chip.c   |  6 +++
 drivers/char/tpm/tpm.h        |  3 ++
 drivers/char/tpm/tpm2-space.c | 99 +++++++++++++++++++++++++++++++++++++++----
 drivers/char/tpm/tpms-dev.c   |  8 ++++
 4 files changed, 107 insertions(+), 9 deletions(-)

drivers/char/tpm/tpm-chip.c +6 -0

index ed4f887..6282ad0 100644
@@ -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;
 

drivers/char/tpm/tpm.h +2 -0

index c48255e..b77fc60 100644
@@ -159,6 +159,8 @@ enum tpm2_cc_attrs {
 struct tpm_space {
 	u32 context_tbl[3];
 	u8 *context_buf;
+	u32 session_tbl[6];
+	u8 *session_buf;
 };
 
 enum tpm_chip_flags {

drivers/char/tpm/tpm2-space.c +52 -3

 	if (rc < 0) {
 		dev_warn(&chip->dev, "%s: failed with a system error %d\n",
 			 __func__, rc);
 		tpm_buf_destroy(&tbuf);
 		return -EFAULT;
+	} else if ((rc & TPM2_RC_HANDLE) == TPM2_RC_HANDLE ||
+		   rc == TPM2_RC_REFERENCE_H0) {
+		rc = -ENOENT;
+		tpm_buf_destroy(&tbuf);
 	} else if (rc > 0) {
 		dev_warn(&chip->dev, "%s: failed with a TPM error 0x%04X\n",
 			 __func__, rc);

@@ -116,21 +121,44 @@ 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 void tpm2_flush_space(struct tpm_chip *chip)
+static int tpm2_session_add(struct tpm_chip *chip,
+			    struct tpm_space *space, u32 handle)
+{
+	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");
+		tpm2_flush_context_cmd(chip, handle, TPM_TRANSMIT_UNLOCKED);
+		return -ENOMEM;
+	}
+
+	space->session_tbl[i] = handle;
+
+	return 0;
+}
+
+void tpm2_flush_space(struct tpm_chip *chip, struct tpm_space *space)
 {
-	struct tpm_space *space = &chip->work_space;
 	int i;
 
 	for (i = 0; i < ARRAY_SIZE(space->context_tbl); i++)
 		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 int tpm2_load_space(struct tpm_chip *chip)

@@ -156,6 +184,28 @@ 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, space);
+			return rc;
+		}
+		if (handle != space->session_tbl[i]) {
+			dev_warn(&chip->dev, "session restored to wrong handle\n");
+			tpm2_flush_space(chip, space);
+			return -EFAULT;
+		}
+	}
+
 	return 0;
 }
 

Quoted 19 lines · James Bottomley

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.

Quoted 2 lines

Signed-off-by: James Bottomley < James.Bottomley-d9PhHud1JfjCXq6kfMZ53/[email protected]>

I'm wondering if you ever need more than two sessions at once? If we would limit the number of sessions to that you could probably simplify a lot.

Quoted 20 lines · Jarkko Sakkinen

Quoted 19 lines · James Bottomley

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.

Quoted 6 lines

Quoted 2 lines

Signed-off-by: James Bottomley < James.Bottomley-d9PhHud1JfjCXq6kfMZ53/[email protected]>

I'm wondering if you ever need more than two sessions at once? If we would limit the number of sessions to that you could probably simplify a lot.

Three seems to be the agreed maximum: hmac authority, parameter encryption and command audit.

I'll fix up the rest

James

Quoted 4 lines

I'm wondering if you ever need more than two sessions at once? If we would limit the number of sessions to that you could probably simplify a lot.

Three seems to be the agreed maximum: hmac authority, parameter encryption and command audit.

Quoted 12 lines

drivers/char/tpm/tpm.h +2 -0

index c48255e..b77fc60 100644
@@ -159,6 +159,8 @@ enum tpm2_cc_attrs {
 struct tpm_space {
 	u32 context_tbl[3];
 	u8 *context_buf;
+	u32 session_tbl[6];
+	u8 *session_buf;
 };
 

Quoted 9 lines

drivers/char/tpm/tpm.h +1 -0

 enum tpm_chip_flags {

@@ -588,4 +590,5 @@ 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,
 		      u32 cc, u8 *buf, size_t bufsiz);
+void tpm2_flush_space(struct tpm_chip *chip, struct tpm_space

*space);

Why the extra parameter?

Quoted 9 lines

#endif

drivers/char/tpm/tpm2 renamed from drivers/char/tpm/tpm2-space.c +0 -1

-space.c

index 7fd2fc5..ba4310a 100644

drivers/char/tpm/tpm2-space.c +0 -0

@@ -59,11 +59,16 @@ static int tpm2_load_context(struct tpm_chip
 *chip, u8 *buf,
 

drivers/char/tpm/tpm2-space.c static int tpm2_load_context(struct tpm_chip *chip, u8 *buf, · line 62

Quoted 3 lines

drivers/char/tpm/tpm2-space.c +1 -0

 	rc = tpm_transmit_cmd(chip, NULL, tbuf.data, PAGE_SIZE, 4,
 			      TPM_TRANSMIT_UNLOCKED, NULL);
+

cruft

drivers/char/tpm/tpm2-space.c int tpm2_prepare_space(struct tpm_chip *chip, · line 274

Quoted 11 lines

drivers/char/tpm/tpm2-space.c +3 -0

@@ -215,17 +265,20 @@ 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);

For transient objects the rollback is straight forward and totally predictable. Given that with sessions you always keep some information in the TPM the rollback would be a bit more complicated.

Now your code seems to just keep the previous session_buf, doesn't it? Does that always work or not?

PS. I have a high-level idea of attack vectors that are prevented by having meta-data for session inside the TPM but can you point me to the correct place in the TPM 2.0 specification that discusses about this?

/Jarkko

On Thu, 2017-01-26 at 14:51 +0200, Jarkko Sakkinen wrote:
[...]

Quoted 12 lines

Quoted 12 lines

drivers/char/tpm/tpm.h +2 -0

index c48255e..b77fc60 100644
@@ -159,6 +159,8 @@ enum tpm2_cc_attrs {
 struct tpm_space {
 	u32 context_tbl[3];
 	u8 *context_buf;
+	u32 session_tbl[6];
+	u8 *session_buf;
 };
 

Quoted 11 lines

Quoted 9 lines

drivers/char/tpm/tpm.h +1 -0

 enum tpm_chip_flags {

@@ -588,4 +590,5 @@ 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,
 		      u32 cc, u8 *buf, size_t bufsiz);
+void tpm2_flush_space(struct tpm_chip *chip, struct tpm_space

*space);

Why the extra parameter?

Because it was called from tpms-dev in release to flush all the sessions still active. At that point the work space is gone, so we have to call it with the real space. However, I realised that callsite didn't hold the tpm_mutex like it should and that we don't need to flush the contexts because they'll be guaranteed empty, so I added a new function tpm2_kill_space() that does this.

Quoted 9 lines

Quoted 9 lines

#endif

drivers/char/tpm/tpm2 renamed from drivers/char/tpm/tpm2-space.c +0 -1

-space.c

index 7fd2fc5..ba4310a 100644

drivers/char/tpm/tpm2-space.c +0 -0

@@ -59,11 +59,16 @@ static int tpm2_load_context(struct tpm_chip
 *chip, u8 *buf,
 

Quoted 5 lines

drivers/char/tpm/tpm2-space.c static int tpm2_load_context(struct tpm_chip *chip, u8 *buf, · line 62

Quoted 3 lines

drivers/char/tpm/tpm2-space.c +1 -0

 	rc = tpm_transmit_cmd(chip, NULL, tbuf.data, PAGE_SIZE, 4,
 			      TPM_TRANSMIT_UNLOCKED, NULL);
+

cruft

Removed.

[...]

Quoted 15 lines

drivers/char/tpm/tpm2-space.c int tpm2_prepare_space(struct tpm_chip *chip, · line 274

Quoted 11 lines

drivers/char/tpm/tpm2-space.c +3 -0

@@ -215,17 +265,20 @@ 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);

For transient objects the rollback is straight forward and totally predictable. Given that with sessions you always keep some information in the TPM the rollback would be a bit more complicated.

There is basically no rollback unless you really want to understand what the commands did. If we get a fault on the context save or load that isn't one we understand, like TPM_RC_HANDLE meaning the session won't load or TPM_RC_REFERENCE_H0 meaning the session no longer exists, then I think the only option is flushing everything.

I'd like us to agree on a hard failure model: if we get some unexplained error during our context loads or saves, we should clear out the entire space (which would allow us to use pointers instead of copyring) but we don't. So we follow the soft failure. However, sessions will mostly fail to load after this with RPM_RC_HANDLE, so we get the equivalent of soft failure for transients and hard failure for sessions.

Quoted 2 lines

Now your code seems to just keep the previous session_buf, doesn't it? Does that always work or not?

Yes, after tpm_flush_space, the session memory is gone and all the sessions will refuse to load with RC_HANDLE, so we end up effectively clearing them out. It seemed better to do it this way than to try to special case all the session stuff.

Quoted 4 lines

PS. I have a high-level idea of attack vectors that are prevented by having meta-data for session inside the TPM but can you point me to the correct place in the TPM 2.0 specification that discusses about this?

The problem is replay. If I'm snooping your TPM commands and I capture your session context, if we don't have replay detection, I can re-load your session HMAC and replay your command because the session has the authorization or the encryption. The TPM designers thought the only way to avoid replay was to use a counter which was part of the session context which increments every time a session is saved. On loading you check that the counters match and fail if they don't. The only way to implement this is to keep a memory of the counter in the TPM, hence the annoying vestigial sessions.

It's note 2 of the architecture Part 1 guide, chapter "15.4 Session Handles (MSO=02_16 and 03_16 )"

James

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-d9PhHud1JfjCXq6kfMZ53/[email protected]>

---
 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 | 213 +++++++++++++++++++++++++++++++++++++++---
 drivers/char/tpm/tpms-dev.c   |  17 +++-
 5 files changed, 271 insertions(+), 14 deletions(-)

drivers/char/tpm/tpm-chip.c +1 -0

index 6282ad0..150c6b8 100644
@@ -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;
 

drivers/char/tpm/tpm.h +37 -2

index b77fc60..9fd6101 100644
@@ -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,
 };
 

@@ -137,7 +138,8 @@ 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 {

@@ -161,8 +163,24 @@ struct tpm_space {
 	u8 *context_buf;
 	u32 session_tbl[6];
 	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),

@@ -175,6 +193,12 @@ struct tpm_chip_seqops {
 	const struct seq_operations *seqops;
 };
 
+struct tpm_sessions {
+	struct tpm_space *space;
+	u32 handle;
+	unsigned long created;
+};
+
 struct tpm_chip {
 	struct device dev;
 	struct device devs;

@@ -217,8 +241,12 @@ struct tpm_chip {
 #endif /* CONFIG_ACPI */
 
 	struct tpm_space work_space;
+	struct tpm_space *space;
 	u32 nr_commands;
 	u32 *cc_attrs_tbl;
+	struct tpm_sessions *sessions;
+	int max_sessions;
+	wait_queue_head_t session_wait;
 };
 
 #define to_tpm_chip(d) container_of(d, struct tpm_chip, dev)

@@ -571,6 +599,13 @@ int tpm2_pcr_extend(struct tpm_chip *chip, int pcr_idx, const u8 *hash);
 int tpm2_get_random(struct tpm_chip *chip, u8 *out, size_t max);
 void tpm2_flush_context_cmd(struct tpm_chip *chip, u32 handle,
 			    unsigned int flags);
+static inline void tpm2_session_clear_reserved(struct tpm_chip *chip,
+					       struct tpm_space *space)
+{
+	if (space->reserved_handle && !tpm2_is_session_force_evict(space))
+		tpm2_flush_context_cmd(chip, space->reserved_handle, 0);
+	space->reserved_handle = 0;
+}
 int tpm2_seal_trusted(struct tpm_chip *chip,
 		      struct trusted_key_payload *payload,
 		      struct trusted_key_options *options);

drivers/char/tpm/tpm2-cmd.c +15 -0

index 0178816..3a54479 100644
@@ -1000,6 +1000,7 @@ int tpm2_auto_startup(struct tpm_chip *chip)
 	struct tpm_buf buf;
 	u32 nr_commands;
 	u32 *attrs;
+	u32 nr_sessions;
 	int rc;
 	int i;
 

@@ -1075,6 +1076,20 @@ int tpm2_auto_startup(struct tpm_chip *chip)
 
 	tpm_buf_destroy(&buf);
 
+	rc = tpm2_get_tpm_pt(chip, TPM_PT_ACTIVE_SESSIONS_MAX,
+			     &nr_sessions, NULL);
+	if (rc)
+		goto out;
+
+	if (nr_sessions > 256)
+		nr_sessions = 256;
+
+	chip->max_sessions = nr_sessions;
+	chip->sessions = devm_kzalloc(&chip->dev,
+				      nr_sessions * sizeof(*chip->sessions),
+				      GFP_KERNEL);
+	if (!chip->sessions)
+		rc = -ENOMEM;
 out:
 	if (rc > 0)
 		rc = -ENODEV;

drivers/char/tpm/tpm2-space.c +202 -11

index ba4310a..7304280 100644
@@ -32,6 +32,166 @@ struct tpm2_context {
 	__be16 blob_size;
 } __packed;
 
+static struct tpm_sessions *tpm2_session_chip_get(struct tpm_chip *chip)
+{
+	int i;
+
+	for (i = 0; i < chip->max_sessions; i++)
+		if (chip->sessions[i].space == NULL)
+			return &chip->sessions[i];
+
+	return NULL;
+}
+
+static struct tpm_sessions *tpm2_session_chip_find_oldest(struct tpm_chip *chip)
+{
+	struct tpm_sessions *sess = NULL;
+	int i;
+
+	for (i = 0; i < chip->max_sessions; i++) {
+		if (chip->sessions[i].space == NULL)
+			continue;
+
+		if (!sess || time_after(sess->created,
+					chip->sessions[i].created))
+			sess = &chip->sessions[i];
+	}
+
+	return sess;
+}
+
+static void tpm2_session_chip_add(struct tpm_chip *chip,
+				  struct tpm_space *space, u32 h)
+{
+	struct tpm_sessions *sess = tpm2_session_chip_get(chip);
+
+	sess->space = space;
+	sess->handle = h;
+	sess->created = jiffies;
+}
+
+static void tpm2_session_chip_remove(struct tpm_chip *chip, u32 h)
+{
+	int i;
+
+	for (i = 0; i < chip->max_sessions; i++)
+		if (chip->sessions[i].handle == h)
+			break;
+	if (i == chip->max_sessions) {
+		dev_warn(&chip->dev, "Missing session %08x", h);
+		return;
+	}
+
+	memset(&chip->sessions[i], 0, sizeof(chip->sessions[i]));
+	wake_up(&chip->session_wait);
+}
+
+static int tpm2_session_forget(struct tpm_chip *chip, struct tpm_space *space,
+			       u32 handle)
+{
+	int i, j;
+	struct tpm2_context *ctx;
+
+	for (i = 0, j = 0; i < ARRAY_SIZE(space->session_tbl); i++) {
+		if (space->session_tbl[i] == 0)
+			continue;
+
+		ctx = (struct tpm2_context *)&space->session_buf[j];
+		j += sizeof(*ctx) + get_unaligned_be16(&ctx->blob_size);
+
+		if (space->session_tbl[i] != handle)
+			continue;
+
+		/* forget the session context */
+		memcpy(ctx, &space->session_buf[j], PAGE_SIZE - j);
+		space->session_tbl[i] = 0;
+		break;
+	}
+	if (i == ARRAY_SIZE(space->session_tbl))
+		return -EINVAL;
+	return 0;
+}
+
+static int tpm2_session_wait(struct tpm_chip *chip, struct tpm_space *space)
+{
+	int rc, failed;
+	struct tpm_sessions *sess;
+	const unsigned long min_timeout = msecs_to_jiffies(2000);
+	unsigned long timeout = min_timeout;
+	DEFINE_WAIT(wait);
+
+	for (failed = 0; ; ) {
+		prepare_to_wait(&chip->session_wait, &wait, TASK_INTERRUPTIBLE);
+
+		mutex_unlock(&chip->tpm_mutex);
+		rc = schedule_timeout_interruptible(timeout);
+		mutex_lock(&chip->tpm_mutex);
+
+		finish_wait(&chip->session_wait, &wait);
+
+		if (signal_pending(current))
+			/* got interrupted */
+			return -EINTR;
+
+		if (rc > 0 && !tpm2_is_session_force_evict(space))
+			/* got woken, so slot is free.  We don't
+			 * reserve the slot here because a) we can't
+			 * (no pending session in the TPM to evict)
+			 * and b) no-one is hogging sessions, so no
+			 * evidence of need.
+			 */
+			return 0;
+
+		/* timed out or victim required; select a victim
+		 * session to kill
+		 */
+		sess = tpm2_session_chip_find_oldest(chip);
+		if (sess == NULL) {
+			/* we get here when we can't create a session
+			 * but there are no listed active sessions
+			 * meaning they're all in various space
+			 * structures as victim sessions.  The wait
+			 * queue is a fair sequence, so we need to
+			 * wait a bit harder
+			 */
+			if (failed++ > 3)
+				break;
+			timeout *= 2;
+			dev_info(&chip->dev, "failed to get session, waiting for %us\n", jiffies_to_msecs(timeout)/1000);
+			continue;
+		}
+		/* is the victim old enough? */
+		timeout = jiffies - sess->created;
+		if (timeout > min_timeout)
+			break;
+		/* otherwise wait until the victim is old enough */
+		timeout = min_timeout - timeout;
+	}
+	if (sess == NULL)
+		/* still can't get a victim, give up */
+		return -EINVAL;
+
+	/* store the physical handle */
+	space->reserved_handle = sess->handle;
+	dev_info(&chip->dev, "Selecting handle %08x for eviction\n",
+		 space->reserved_handle);
+
+	/* cause a mapping failure if this session handle is
+	 * ever used in the victim space again
+	 */
+	tpm2_session_forget(chip, sess->space, sess->handle);
+	/* clear the session, but don't wake any other waiters */
+	memset(sess, 0, sizeof(*sess));
+	/* so now we have a saved physical handle but this handle is
+	 * still in the tpm.  After this we repeat the command, but
+	 * flush the handle once we obtain the tpm_mutex on the repeat
+	 * so, in theory, we should have a free handle to
+	 * re-execute
+	 */
+
+	return 0;
+}
+
 int tpm2_init_space(struct tpm_chip *chip, struct tpm_space *space)
 {
 	space->context_buf = kzalloc(PAGE_SIZE, GFP_KERNEL);

@@ -65,8 +225,7 @@ static int tpm2_load_context(struct tpm_chip *chip, u8 *buf,
 			 __func__, rc);
 		tpm_buf_destroy(&tbuf);
 		return -EFAULT;
-	} else if ((rc & TPM2_RC_HANDLE) == TPM2_RC_HANDLE ||
-		   rc == TPM2_RC_REFERENCE_H0) {
+	} else if (rc == TPM2_RC_REFERENCE_H0) {
 		rc = -ENOENT;
 		tpm_buf_destroy(&tbuf);
 	} else if (rc > 0) {

@@ -126,9 +285,9 @@ static int tpm2_save_context(struct tpm_chip *chip, u32 handle, u8 *buf,
 	return 0;
 }
 
-static int tpm2_session_add(struct tpm_chip *chip,
-			    struct tpm_space *space, u32 handle)
+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++)

@@ -141,6 +300,7 @@ static int tpm2_session_add(struct tpm_chip *chip,
 	}
 
 	space->session_tbl[i] = handle;
+	tpm2_session_chip_add(chip, chip->space, handle);
 
 	return 0;
 }

@@ -155,9 +315,18 @@ void tpm2_flush_space(struct tpm_chip *chip, struct tpm_space *space)
 					       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);
+		if (!space->session_tbl[i])
+			continue;
+
+		tpm2_session_chip_remove(chip, space->session_tbl[i]);
+		tpm2_flush_context_cmd(chip, space->session_tbl[i],
+				       TPM_TRANSMIT_UNLOCKED);
+	}
+	if (space->reserved_handle && !tpm2_is_session_force_evict(space)) {
+		tpm2_flush_context_cmd(chip, space->reserved_handle,
+				       TPM_TRANSMIT_UNLOCKED);
+		space->reserved_handle = 0;
+		/* subtlety here: if force evict is set, we don't clear it */
 	}
 }
 

@@ -194,6 +363,7 @@ static int tpm2_load_space(struct tpm_chip *chip)
 				       &offset, &handle);
 		if (rc == -ENOENT) {
 			/* load failed, just forget session */
+			tpm2_session_chip_remove(chip, space->session_tbl[i]);
 			space->session_tbl[i] = 0;
 		} else if (rc) {
 			tpm2_flush_space(chip, space);

@@ -269,6 +439,7 @@ int tpm2_prepare_space(struct tpm_chip *chip, struct tpm_space *space, u32 cc,
 	       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);
+	chip->space = space;
 
 	rc = tpm2_load_space(chip);
 	if (rc) {

@@ -282,16 +453,28 @@ int tpm2_prepare_space(struct tpm_chip *chip, struct tpm_space *space, u32 cc,
 		return rc;
 	}
 
+	if (space->reserved_handle && !tpm2_is_session_force_evict(space)) {
+		/* this is a trick to allow a previous command which
+		 * failed because it was out of handle space to
+		 * succeed.  The handle is still in the TPM, so now we
+		 * flush it under the tpm_mutex which should ensure we
+		 * can create a new one
+		 */
+		tpm2_flush_context_cmd(chip, space->reserved_handle,
+				       TPM_TRANSMIT_UNLOCKED);
+		space->reserved_handle = 0;
+	}
+
 	return 0;
 }
 
-static int tpm2_map_response(struct tpm_chip *chip, u32 cc, u8 *rsp, size_t len)
+static int tpm2_map_response(struct tpm_chip *chip, u32 cc, u8 *rsp, size_t len,
+			     u32 return_code)
 {
 	struct tpm_space *space = &chip->work_space;
 	u32 phandle, phandle_type;
 	u32 vhandle;
 	u32 attrs;
-	u32 return_code = get_unaligned_be32((__be32 *)&rsp[6]);
 	int i;
 
 	if (return_code != TPM2_RC_SUCCESS)

@@ -314,7 +497,7 @@ static int tpm2_map_response(struct tpm_chip *chip, u32 cc, u8 *rsp, size_t len)
 		return 0;
 
 	if (phandle_type != TPM2_HT_TRANSIENT)
-		return tpm2_session_add(chip, space, phandle);
+		return tpm2_session_add(chip, phandle);
 
 	/* Garbage collect a dead context. */
 	for (i = 0; i < ARRAY_SIZE(space->context_tbl); i++) {

@@ -376,6 +559,7 @@ static int tpm2_save_space(struct tpm_chip *chip)
 
 		if (rc == -ENOENT) {
 			/* handle error saving session, just forget it */
+			tpm2_session_chip_remove(chip, space->session_tbl[i]);
 			space->session_tbl[i] = 0;
 		} else if (rc < 0) {
 			tpm2_flush_space(chip, space);

@@ -390,11 +574,12 @@ int tpm2_commit_space(struct tpm_chip *chip, struct tpm_space *space,
 		      u32 cc, u8 *buf, size_t bufsiz)
 {
 	int rc;
+	u32 return_code = get_unaligned_be32((__be32 *)&buf[6]);
 
 	if (!space)
 		return 0;
 
-	rc = tpm2_map_response(chip, cc, buf, bufsiz);
+	rc = tpm2_map_response(chip, cc, buf, bufsiz, return_code);
 	if (rc) {
 		tpm2_flush_space(chip, space);
 		return rc;

@@ -412,6 +597,12 @@ int tpm2_commit_space(struct tpm_chip *chip, struct tpm_space *space,
 	       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);
+	chip->space = NULL;
+
+	if (return_code == TPM2_RC_SESSION_HANDLES) {
+		tpm2_session_wait(chip, space);
+		return -EAGAIN;
+	}
 
 	return 0;
 }

drivers/char/tpm/tpms-dev.c +16 -1

index 6efead3..9766fb4 100644
@@ -58,8 +58,23 @@ ssize_t tpms_write(struct file *file, const char __user *buf,
 {
 	struct file_priv *fpriv = file->private_data;
 	struct tpms_priv *priv = container_of(fpriv, struct tpms_priv, priv);
+	int count = 0;
+	const int max_count = 3; /* number of retries */
+	int rc;
+
+	for (count = 0; count < max_count; count++) {
+		rc = tpm_common_write(file, buf, size, off, &priv->space);
+		if (rc != -EAGAIN)
+			break;
+		if (count == max_count - 2)
+			/* second to last go around, force an eviction if
+			 * this go fails, so final go should succeed
+			 */
+			tpm2_session_force_evict(&priv->space);
+	}
+	tpm2_session_clear_reserved(fpriv->chip, &priv->space);
 
-	return tpm_common_write(file, buf, size, off, &priv->space);
+	return rc;
 }
 
 const struct file_operations tpms_fops = {

Signature · 6 lines

2.6.6

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Quoted 17 lines · James Bottomley

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.

Quoted 2 lines

Signed-off-by: James Bottomley < James.Bottomley-d9PhHud1JfjCXq6kfMZ53/[email protected]>

This is not a proper review yet. Just quick question: why do you need a real time (i.e. created)? Maybe in the force eviction case it would be enough to sleep lets say 500 ms and pick the victim with smallest number? I.e. just have increasing u64 counter instead of real time.

That would simplify this patch a lot and does not prevent to refine later on if workloads show need for more complex logic.

Quoted 18 lines · Jarkko Sakkinen

Quoted 17 lines · James Bottomley

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.

Quoted 7 lines

Quoted 2 lines

Signed-off-by: James Bottomley < James.Bottomley-d9PhHud1JfjCXq6kfMZ53/[email protected]>

This is not a proper review yet. Just quick question: why do you need a real time (i.e. created)? Maybe in the force eviction case it would be enough to sleep lets say 500 ms and pick the victim with smallest number? I.e. just have increasing u64 counter instead of real time.

So that if the oldest session has already been around for > 2s there's no need to wait. In order to guarantee everyone gets a session for at least 2s without tracking the age of sessions, you'd have to sleep for 2s after you find the oldest session.

Quoted 2 lines

That would simplify this patch a lot and does not prevent to refine later on if workloads show need for more complex logic.

An increasing monotonic counter would actually not be much simpler: all you could really cut out would be the four lines and two comments at the bottom of the for loop in tpm2_session_wait() which check the age of the found session.

James