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 <[email protected]> Reviewed-by: Ard Biesheuvel <[email protected]> # crypto API parts --- v6: split into new patch, update config variable v7: add tpm2_ prefix and memzero_explicit --- drivers/char/tpm/Kconfig | 3 + drivers/char/tpm/tpm-buf.c | 1 + drivers/char/tpm/tpm-chip.c | 3 + drivers/char/tpm/tpm2-sessions.c | 385 +++++++++++++++++++++++++++++++ include/linux/tpm.h | 34 +++ 5 files changed, 426 insertions(+) diff --git a/drivers/char/tpm/Kconfig b/drivers/char/tpm/Kconfig index e3c39a83171b..086cb8588493 100644 --- a/drivers/char/tpm/Kconfig +++ b/drivers/char/tpm/Kconfig @@ -30,6 +30,9 @@ if TCG_TPM config TCG_TPM2_HMAC bool "Use encrypted and HMACd 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 and response HMACs in addition to encryption for 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 9fc263ee02c2..c8792a6b8bd4 100644 --- a/drivers/char/tpm/tpm2-sessions.c +++ b/drivers/char/tpm/tpm2-sessions.c @@ -3,18 +3,399 @@ /* * Copyright (C) 2018 [email protected] * + * 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/aes.h> +#include <crypto/kpp.h> +#include <crypto/ecdh.h> +#include <crypto/hash.h> +#include <crypto/hmac.h> /* if you change to AES256, you only need change this */ #define AES_KEYBYTES AES_KEYSIZE_128 #define AES_KEYBITS (AES_KEYBYTES*8) +#define AUTH_MAX_NAMES 3 + +/* + * 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_KEYBYTES + 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 + * on every operation, so we weld the hmac init and final functions in + * here to give it the same usage characteristics as a regular hash + */ +static void hmac_init(struct sha256_state *sctx, u8 *key, int keylen) +{ + u8 pad[SHA256_BLOCK_SIZE]; + int i; + + sha256_init(sctx); + for (i = 0; i < sizeof(pad); i++) { + if (i < keylen) + pad[i] = key[i]; + else + pad[i] = 0; + pad[i] ^= HMAC_IPAD_VALUE; + } + sha256_update(sctx, pad, sizeof(pad)); +} + +static void hmac_final(struct sha256_state *sctx, u8 *key, int keylen, u8 *out) +{ + u8 pad[SHA256_BLOCK_SIZE]; + int i; + + for (i = 0; i < sizeof(pad); i++) { + if (i < keylen) + pad[i] = key[i]; + else + pad[i] = 0; + pad[i] ^= HMAC_OPAD_VALUE; + } + + /* collect the final hash; use out as temporary storage */ + sha256_final(sctx, out); + + sha256_init(sctx); + sha256_update(sctx, pad, sizeof(pad)); + sha256_update(sctx, out, SHA256_DIGEST_SIZE); + sha256_final(sctx, out); +} + +/* + * assume hash sha256 and nonces u, v of size SHA256_DIGEST_SIZE but + * otherwise standard KDFa. Note output is in bytes not bits. + */ +static void KDFa(u8 *key, int keylen, const char *label, u8 *u, + u8 *v, int bytes, u8 *out) +{ + u32 counter; + const __be32 bits = cpu_to_be32(bytes * 8); + + for (counter = 1; bytes > 0; bytes -= SHA256_DIGEST_SIZE, counter++, + out += SHA256_DIGEST_SIZE) { + struct sha256_state sctx; + __be32 c = cpu_to_be32(counter); + + hmac_init(&sctx, key, keylen); + sha256_update(&sctx, (u8 *)&c, sizeof(c)); + sha256_update(&sctx, label, strlen(label)+1); + sha256_update(&sctx, u, SHA256_DIGEST_SIZE); + sha256_update(&sctx, v, SHA256_DIGEST_SIZE); + sha256_update(&sctx, (u8 *)&bits, sizeof(bits)); + hmac_final(&sctx, key, keylen, out); + } +} + +/* + * Somewhat of a bastardization of the real KDFe. We're assuming + * we're working with known point sizes for the input parameters and + * the hash algorithm is fixed at sha256. Because we know that the + * point size is 32 bytes like the hash size, there's no need to loop + * in this KDF. + */ +static void KDFe(u8 z[EC_PT_SZ], const char *str, u8 *pt_u, u8 *pt_v, + u8 *keyout) +{ + struct sha256_state sctx; + /* + * this should be an iterative counter, but because we know + * we're only taking 32 bytes for the point using a sha256 + * hash which is also 32 bytes, there's only one loop + */ + __be32 c = cpu_to_be32(1); + + sha256_init(&sctx); + /* counter (BE) */ + sha256_update(&sctx, (u8 *)&c, sizeof(c)); + /* secret value */ + sha256_update(&sctx, z, EC_PT_SZ); + /* string including trailing zero */ + sha256_update(&sctx, str, strlen(str)+1); + sha256_update(&sctx, pt_u, EC_PT_SZ); + sha256_update(&sctx, pt_v, EC_PT_SZ); + sha256_final(&sctx, keyout); +} + +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 + */ + 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 */ + 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; + unsigned int offset = 0; /* dummy offset for null seed context */ + 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_KEYBITS); + /* 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); static int tpm2_parse_create_primary(struct tpm_chip *chip, struct tpm_buf *buf, u32 *nullkey) @@ -271,6 +652,10 @@ 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; } EXPORT_SYMBOL(tpm2_sessions_init); diff --git a/include/linux/tpm.h b/include/linux/tpm.h index 3060ab794afb..444f0a83682a 100644 --- a/include/linux/tpm.h +++ b/include/linux/tpm.h @@ -30,6 +30,14 @@ 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 { @@ -199,6 +207,7 @@ struct tpm_chip { u8 null_key_name[TPM2_NAME_SIZE]; /* name of NULL seed */ u8 null_ec_key_x[EC_PT_SZ]; u8 null_ec_key_y[EC_PT_SZ]; + struct tpm2_auth *auth; #endif }; @@ -262,6 +271,7 @@ 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, @@ -345,20 +355,30 @@ 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), }; 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 { @@ -449,4 +469,18 @@ 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
