[PATCH v8 14/22] tpm: Add HMAC session start and end functions

James Bottomley
2024-04-29T20:30:28Z

Add session based HMAC authentication plus parameter decryption and
response encryption using AES. The basic design is to segregate all
the nasty crypto, hash and hmac code into tpm2-sessions.c and export a
usable API. The API first of all starts off by gaining a session with
tpm2_start_auth_session() which initiates a session with the TPM and
allocates an opaque tpm2_auth structure to handle the session
parameters. The design is that session use will be single threaded
from start to finish under the ops lock, so the tpm2_auth structure is
stored in struct tpm2_chip to simpify the externally visible API.

The session can be ended with tpm2_end_auth_session() which is
designed only to be used in error legs. Ordinarily the further
session API (future patches) will end or continue the session
appropriately without having to call this.

Signed-off-by: James Bottomley <James.Bottomley@HansenPartnership.com>
Reviewed-by: Ard Biesheuvel <ard.biesheuvel@linaro.org> # crypto API parts

---

v6: split into new patch, update config variable
v7: add tpm2_ prefix and memzero_explicit
v8: cosmetic changes, split out KDFe and KDFa
---
drivers/char/tpm/Kconfig | 2 +
drivers/char/tpm/tpm-buf.c | 1 +
drivers/char/tpm/tpm-chip.c | 3 +
drivers/char/tpm/tpm2-sessions.c | 285 +++++++++++++++++++++++++++++++
include/linux/tpm.h | 34 ++++
5 files changed, 325 insertions(+)

diff --git a/drivers/char/tpm/Kconfig b/drivers/char/tpm/Kconfig
index 4873e6eae255..cecf617c9c90 100644
--- a/drivers/char/tpm/Kconfig
+++ b/drivers/char/tpm/Kconfig
@@ -30,6 +30,8 @@ if TCG_TPM
config TCG_TPM2_HMAC
bool "Use HMAC and encrypted transactions on the TPM bus"
default y
+ select CRYPTO_ECDH
+ select CRYPTO_LIB_AESCFB
select CRYPTO_LIB_SHA256
help
Setting this causes us to deploy a scheme which uses request
diff --git a/drivers/char/tpm/tpm-buf.c b/drivers/char/tpm/tpm-buf.c
index bb81180495d1..274130398569 100644
--- a/drivers/char/tpm/tpm-buf.c
+++ b/drivers/char/tpm/tpm-buf.c
@@ -44,6 +44,7 @@ void tpm_buf_reset(struct tpm_buf *buf, u16 tag, u32 ordinal)
head->tag = cpu_to_be16(tag);
head->length = cpu_to_be32(sizeof(*head));
head->ordinal = cpu_to_be32(ordinal);
+ buf->handles = 0;
}
EXPORT_SYMBOL_GPL(tpm_buf_reset);

diff --git a/drivers/char/tpm/tpm-chip.c b/drivers/char/tpm/tpm-chip.c
index 42b1062e33cd..d93937326b2e 100644
--- a/drivers/char/tpm/tpm-chip.c
+++ b/drivers/char/tpm/tpm-chip.c
@@ -275,6 +275,9 @@ static void tpm_dev_release(struct device *dev)
kfree(chip->work_space.context_buf);
kfree(chip->work_space.session_buf);
kfree(chip->allocated_banks);
+#ifdef CONFIG_TCG_TPM2_HMAC
+ kfree(chip->auth);
+#endif
kfree(chip);
}

diff --git a/drivers/char/tpm/tpm2-sessions.c b/drivers/char/tpm/tpm2-sessions.c
index 8429e596f1eb..71b3c0e75760 100644
--- a/drivers/char/tpm/tpm2-sessions.c
+++ b/drivers/char/tpm/tpm2-sessions.c
@@ -3,13 +3,101 @@
/*
* Copyright (C) 2018 James.Bottomley@HansenPartnership.com
*
+ * Cryptographic helper routines for handling TPM2 sessions for
+ * authorization HMAC and request response encryption.
+ *
+ * The idea is to ensure that every TPM command is HMAC protected by a
+ * session, meaning in-flight tampering would be detected and in
+ * addition all sensitive inputs and responses should be encrypted.
+ *
+ * The basic way this works is to use a TPM feature called salted
+ * sessions where a random secret used in session construction is
+ * encrypted to the public part of a known TPM key. The problem is we
+ * have no known keys, so initially a primary Elliptic Curve key is
+ * derived from the NULL seed (we use EC because most TPMs generate
+ * these keys much faster than RSA ones). The curve used is NIST_P256
+ * because that's now mandated to be present in 'TCG TPM v2.0
+ * Provisioning Guidance'
+ *
+ * Threat problems: the initial TPM2_CreatePrimary is not (and cannot
+ * be) session protected, so a clever Man in the Middle could return a
+ * public key they control to this command and from there intercept
+ * and decode all subsequent session based transactions. The kernel
+ * cannot mitigate this threat but, after boot, userspace can get
+ * proof this has not happened by asking the TPM to certify the NULL
+ * key. This certification would chain back to the TPM Endorsement
+ * Certificate and prove the NULL seed primary had not been tampered
+ * with and thus all sessions must have been cryptographically secure.
+ * To assist with this, the initial NULL seed public key name is made
+ * available in a sysfs file.
+ *
+ * Use of these functions:
+ *
+ * The design is all the crypto, hash and hmac gunk is confined in this
+ * file and never needs to be seen even by the kernel internal user. To
+ * the user there's an init function tpm2_sessions_init() that needs to
+ * be called once per TPM which generates the NULL seed primary key.
+ *
+ * These are the usage functions:
+ *
+ * tpm2_start_auth_session() which allocates the opaque auth structure
+ * and gets a session from the TPM. This must be called before
+ * any of the following functions. The session is protected by a
+ * session_key which is derived from a random salt value
+ * encrypted to the NULL seed.
+ * tpm2_end_auth_session() kills the session and frees the resources.
+ * Under normal operation this function is done by
+ * tpm_buf_check_hmac_response(), so this is only to be used on
+ * error legs where the latter is not executed.
*/

#include "tpm.h"
+#include <linux/random.h>
+#include <linux/scatterlist.h>
#include <asm/unaligned.h>
+#include <crypto/kpp.h>
+#include <crypto/ecdh.h>
#include <crypto/hash.h>
#include <crypto/hmac.h>

+/*
+ * This is the structure that carries all the auth information (like
+ * session handle, nonces, session key and auth) from use to use it is
+ * designed to be opaque to anything outside.
+ */
+struct tpm2_auth {
+ u32 handle;
+ /*
+ * This has two meanings: before tpm_buf_fill_hmac_session()
+ * it marks the offset in the buffer of the start of the
+ * sessions (i.e. after all the handles). Once the buffer has
+ * been filled it markes the session number of our auth
+ * session so we can find it again in the response buffer.
+ *
+ * The two cases are distinguished because the first offset
+ * must always be greater than TPM_HEADER_SIZE and the second
+ * must be less than or equal to 5.
+ */
+ u32 session;
+ /*
+ * the size here is variable and set by the size of our_nonce
+ * which must be between 16 and the name hash length. we set
+ * the maximum sha256 size for the greatest protection
+ */
+ u8 our_nonce[SHA256_DIGEST_SIZE];
+ u8 tpm_nonce[SHA256_DIGEST_SIZE];
+ /*
+ * the salt is only used across the session command/response
+ * after that it can be used as a scratch area
+ */
+ union {
+ u8 salt[EC_PT_SZ];
+ /* scratch for key + IV */
+ u8 scratch[AES_KEY_BYTES + AES_BLOCK_SIZE];
+ };
+ u8 session_key[SHA256_DIGEST_SIZE];
+};
+
/*
* It turns out the crypto hmac(sha256) is hard for us to consume
* because it assumes a fixed key and the TPM seems to change the key
@@ -113,6 +201,199 @@ static void tpm2_KDFe(u8 z[EC_PT_SZ], const char *str, u8 *pt_u, u8 *pt_v,
sha256_final(&sctx, out);
}

+static void tpm_buf_append_salt(struct tpm_buf *buf, struct tpm_chip *chip)
+{
+ struct crypto_kpp *kpp;
+ struct kpp_request *req;
+ struct scatterlist s[2], d[1];
+ struct ecdh p = {0};
+ u8 encoded_key[EC_PT_SZ], *x, *y;
+ unsigned int buf_len;
+
+ /* secret is two sized points */
+ tpm_buf_append_u16(buf, (EC_PT_SZ + 2)*2);
+ /*
+ * we cheat here and append uninitialized data to form
+ * the points. All we care about is getting the two
+ * co-ordinate pointers, which will be used to overwrite
+ * the uninitialized data
+ */
+ tpm_buf_append_u16(buf, EC_PT_SZ);
+ x = &buf->data[tpm_buf_length(buf)];
+ tpm_buf_append(buf, encoded_key, EC_PT_SZ);
+ tpm_buf_append_u16(buf, EC_PT_SZ);
+ y = &buf->data[tpm_buf_length(buf)];
+ tpm_buf_append(buf, encoded_key, EC_PT_SZ);
+ sg_init_table(s, 2);
+ sg_set_buf(&s[0], x, EC_PT_SZ);
+ sg_set_buf(&s[1], y, EC_PT_SZ);
+
+ kpp = crypto_alloc_kpp("ecdh-nist-p256", CRYPTO_ALG_INTERNAL, 0);
+ if (IS_ERR(kpp)) {
+ dev_err(&chip->dev, "crypto ecdh allocation failed\n");
+ return;
+ }
+
+ buf_len = crypto_ecdh_key_len(&p);
+ if (sizeof(encoded_key) < buf_len) {
+ dev_err(&chip->dev, "salt buffer too small needs %d\n",
+ buf_len);
+ goto out;
+ }
+ crypto_ecdh_encode_key(encoded_key, buf_len, &p);
+ /* this generates a random private key */
+ crypto_kpp_set_secret(kpp, encoded_key, buf_len);
+
+ /* salt is now the public point of this private key */
+ req = kpp_request_alloc(kpp, GFP_KERNEL);
+ if (!req)
+ goto out;
+ kpp_request_set_input(req, NULL, 0);
+ kpp_request_set_output(req, s, EC_PT_SZ*2);
+ crypto_kpp_generate_public_key(req);
+ /*
+ * we're not done: now we have to compute the shared secret
+ * which is our private key multiplied by the tpm_key public
+ * point, we actually only take the x point and discard the y
+ * point and feed it through KDFe to get the final secret salt
+ */
+ sg_set_buf(&s[0], chip->null_ec_key_x, EC_PT_SZ);
+ sg_set_buf(&s[1], chip->null_ec_key_y, EC_PT_SZ);
+ kpp_request_set_input(req, s, EC_PT_SZ*2);
+ sg_init_one(d, chip->auth->salt, EC_PT_SZ);
+ kpp_request_set_output(req, d, EC_PT_SZ);
+ crypto_kpp_compute_shared_secret(req);
+ kpp_request_free(req);
+
+ /*
+ * pass the shared secret through KDFe for salt. Note salt
+ * area is used both for input shared secret and output salt.
+ * This works because KDFe fully consumes the secret before it
+ * writes the salt
+ */
+ tpm2_KDFe(chip->auth->salt, "SECRET", x, chip->null_ec_key_x,
+ chip->auth->salt);
+
+ out:
+ crypto_free_kpp(kpp);
+}
+/**
+ * tpm2_end_auth_session() - kill the allocated auth session
+ * @chip: the TPM chip structure
+ *
+ * ends the session started by tpm2_start_auth_session and frees all
+ * the resources. Under normal conditions,
+ * tpm_buf_check_hmac_response() will correctly end the session if
+ * required, so this function is only for use in error legs that will
+ * bypass the normal invocation of tpm_buf_check_hmac_response().
+ */
+void tpm2_end_auth_session(struct tpm_chip *chip)
+{
+ tpm2_flush_context(chip, chip->auth->handle);
+ memzero_explicit(chip->auth, sizeof(*chip->auth));
+}
+EXPORT_SYMBOL(tpm2_end_auth_session);
+
+static int tpm2_parse_start_auth_session(struct tpm2_auth *auth,
+ struct tpm_buf *buf)
+{
+ struct tpm_header *head = (struct tpm_header *)buf->data;
+ u32 tot_len = be32_to_cpu(head->length);
+ off_t offset = TPM_HEADER_SIZE;
+ u32 val;
+
+ /* we're starting after the header so adjust the length */
+ tot_len -= TPM_HEADER_SIZE;
+
+ /* should have handle plus nonce */
+ if (tot_len != 4 + 2 + sizeof(auth->tpm_nonce))
+ return -EINVAL;
+
+ auth->handle = tpm_buf_read_u32(buf, &offset);
+ val = tpm_buf_read_u16(buf, &offset);
+ if (val != sizeof(auth->tpm_nonce))
+ return -EINVAL;
+ memcpy(auth->tpm_nonce, &buf->data[offset], sizeof(auth->tpm_nonce));
+ /* now compute the session key from the nonces */
+ tpm2_KDFa(auth->salt, sizeof(auth->salt), "ATH", auth->tpm_nonce,
+ auth->our_nonce, sizeof(auth->session_key),
+ auth->session_key);
+
+ return 0;
+}
+
+/**
+ * tpm2_start_auth_session() - create a HMAC authentication session with the TPM
+ * @chip: the TPM chip structure to create the session with
+ *
+ * This function loads the NULL seed from its saved context and starts
+ * an authentication session on the null seed, fills in the
+ * @chip->auth structure to contain all the session details necessary
+ * for performing the HMAC, encrypt and decrypt operations and
+ * returns. The NULL seed is flushed before this function returns.
+ *
+ * Return: zero on success or actual error encountered.
+ */
+int tpm2_start_auth_session(struct tpm_chip *chip)
+{
+ struct tpm_buf buf;
+ struct tpm2_auth *auth = chip->auth;
+ int rc;
+ /* null seed context has no offset, but we must provide one */
+ unsigned int offset = 0;
+ u32 nullkey;
+
+ rc = tpm2_load_context(chip, chip->null_key_context, &offset,
+ &nullkey);
+ if (rc)
+ goto out;
+
+ auth->session = TPM_HEADER_SIZE;
+
+ rc = tpm_buf_init(&buf, TPM2_ST_NO_SESSIONS, TPM2_CC_START_AUTH_SESS);
+ if (rc)
+ goto out;
+
+ /* salt key handle */
+ tpm_buf_append_u32(&buf, nullkey);
+ /* bind key handle */
+ tpm_buf_append_u32(&buf, TPM2_RH_NULL);
+ /* nonce caller */
+ get_random_bytes(auth->our_nonce, sizeof(auth->our_nonce));
+ tpm_buf_append_u16(&buf, sizeof(auth->our_nonce));
+ tpm_buf_append(&buf, auth->our_nonce, sizeof(auth->our_nonce));
+
+ /* append encrypted salt and squirrel away unencrypted in auth */
+ tpm_buf_append_salt(&buf, chip);
+ /* session type (HMAC, audit or policy) */
+ tpm_buf_append_u8(&buf, TPM2_SE_HMAC);
+
+ /* symmetric encryption parameters */
+ /* symmetric algorithm */
+ tpm_buf_append_u16(&buf, TPM_ALG_AES);
+ /* bits for symmetric algorithm */
+ tpm_buf_append_u16(&buf, AES_KEY_BITS);
+ /* symmetric algorithm mode (must be CFB) */
+ tpm_buf_append_u16(&buf, TPM_ALG_CFB);
+ /* hash algorithm for session */
+ tpm_buf_append_u16(&buf, TPM_ALG_SHA256);
+
+ rc = tpm_transmit_cmd(chip, &buf, 0, "start auth session");
+ tpm2_flush_context(chip, nullkey);
+
+ if (rc == TPM2_RC_SUCCESS)
+ rc = tpm2_parse_start_auth_session(auth, &buf);
+
+ tpm_buf_destroy(&buf);
+
+ if (rc)
+ goto out;
+
+ out:
+ return rc;
+}
+EXPORT_SYMBOL(tpm2_start_auth_session);
+
/**
* tpm2_parse_create_primary() - parse the data returned from TPM_CC_CREATE_PRIMARY
*
@@ -417,5 +698,9 @@ int tpm2_sessions_init(struct tpm_chip *chip)
if (rc)
dev_err(&chip->dev, "TPM: security failed (NULL seed derivation): %d\n", rc);

+ chip->auth = kmalloc(sizeof(*chip->auth), GFP_KERNEL);
+ if (!chip->auth)
+ return -ENOMEM;
+
return rc;
}
diff --git a/include/linux/tpm.h b/include/linux/tpm.h
index bc8c9a350e23..81b5a70ff80d 100644
--- a/include/linux/tpm.h
+++ b/include/linux/tpm.h
@@ -31,6 +31,14 @@
struct tpm_chip;
struct trusted_key_payload;
struct trusted_key_options;
+/* opaque structure, holds auth session parameters like the session key */
+struct tpm2_auth;
+
+enum tpm2_session_types {
+ TPM2_SE_HMAC = 0x00,
+ TPM2_SE_POLICY = 0x01,
+ TPM2_SE_TRIAL = 0x02,
+};

/* if you add a new hash to this, increment TPM_MAX_HASHES below */
enum tpm_algorithms {
@@ -203,6 +211,7 @@ struct tpm_chip {
u8 null_key_name[TPM2_NAME_SIZE];
u8 null_ec_key_x[EC_PT_SZ];
u8 null_ec_key_y[EC_PT_SZ];
+ struct tpm2_auth *auth;
#endif
};

@@ -266,6 +275,7 @@ enum tpm2_command_codes {
TPM2_CC_CONTEXT_LOAD = 0x0161,
TPM2_CC_CONTEXT_SAVE = 0x0162,
TPM2_CC_FLUSH_CONTEXT = 0x0165,
+ TPM2_CC_START_AUTH_SESS = 0x0176,
TPM2_CC_VERIFY_SIGNATURE = 0x0177,
TPM2_CC_GET_CAPABILITY = 0x017A,
TPM2_CC_GET_RANDOM = 0x017B,
@@ -349,16 +359,21 @@ struct tpm_buf {
u32 flags;
u32 length;
u8 *data;
+ u8 handles;
};

enum tpm2_object_attributes {
TPM2_OA_FIXED_TPM = BIT(1),
+ TPM2_OA_ST_CLEAR = BIT(2),
TPM2_OA_FIXED_PARENT = BIT(4),
TPM2_OA_SENSITIVE_DATA_ORIGIN = BIT(5),
TPM2_OA_USER_WITH_AUTH = BIT(6),
+ TPM2_OA_ADMIN_WITH_POLICY = BIT(7),
TPM2_OA_NO_DA = BIT(10),
+ TPM2_OA_ENCRYPTED_DUPLICATION = BIT(11),
TPM2_OA_RESTRICTED = BIT(16),
TPM2_OA_DECRYPT = BIT(17),
+ TPM2_OA_SIGN = BIT(18),
};

/*
@@ -378,6 +393,11 @@ enum tpm2_object_attributes {

enum tpm2_session_attributes {
TPM2_SA_CONTINUE_SESSION = BIT(0),
+ TPM2_SA_AUDIT_EXCLUSIVE = BIT(1),
+ TPM2_SA_AUDIT_RESET = BIT(3),
+ TPM2_SA_DECRYPT = BIT(5),
+ TPM2_SA_ENCRYPT = BIT(6),
+ TPM2_SA_AUDIT = BIT(7),
};

struct tpm2_hash {
@@ -469,4 +489,18 @@ static inline void tpm_buf_append_empty_auth(struct tpm_buf *buf, u32 handle)
{
}
#endif
+#ifdef CONFIG_TCG_TPM2_HMAC
+
+int tpm2_start_auth_session(struct tpm_chip *chip);
+void tpm2_end_auth_session(struct tpm_chip *chip);
+#else
+static inline int tpm2_start_auth_session(struct tpm_chip *chip)
+{
+ return 0;
+}
+static inline void tpm2_end_auth_session(struct tpm_chip *chip)
+{
+}
+#endif /* CONFIG_TCG_TPM2_HMAC */
+
#endif
--
2.35.3