// SPDX-License-Identifier: GPL-2.0 /* * 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 #include #include #include #include #include #include /* * 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 * 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 tpm2_hmac_init(struct sha256_state *sctx, u8 *key, u32 key_len) { u8 pad[SHA256_BLOCK_SIZE]; int i; sha256_init(sctx); for (i = 0; i < sizeof(pad); i++) { if (i < key_len) pad[i] = key[i]; else pad[i] = 0; pad[i] ^= HMAC_IPAD_VALUE; } sha256_update(sctx, pad, sizeof(pad)); } static void tpm2_hmac_final(struct sha256_state *sctx, u8 *key, u32 key_len, u8 *out) { u8 pad[SHA256_BLOCK_SIZE]; int i; for (i = 0; i < sizeof(pad); i++) { if (i < key_len) 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 tpm2_KDFa. Note output is in bytes not bits. */ static void tpm2_KDFa(u8 *key, u32 key_len, const char *label, u8 *u, u8 *v, u32 bytes, u8 *out) { u32 counter = 1; const __be32 bits = cpu_to_be32(bytes * 8); while (bytes > 0) { struct sha256_state sctx; __be32 c = cpu_to_be32(counter); tpm2_hmac_init(&sctx, key, key_len); 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)); tpm2_hmac_final(&sctx, key, key_len, out); bytes -= SHA256_DIGEST_SIZE; counter++; out += SHA256_DIGEST_SIZE; } } /* * 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 tpm2_KDFe(u8 z[EC_PT_SZ], const char *str, u8 *pt_u, u8 *pt_v, u8 *out) { 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, 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 * * @chip: The TPM the primary was created under * @buf: The response buffer from the chip * @handle: pointer to be filled in with the return handle of the primary * @hierarchy: The hierarchy the primary was created for * * Return: * * 0 - OK * * -errno - A system error * * TPM_RC - A TPM error */ static int tpm2_parse_create_primary(struct tpm_chip *chip, struct tpm_buf *buf, u32 *handle, u32 hierarchy) { struct tpm_header *head = (struct tpm_header *)buf->data; off_t offset_r = TPM_HEADER_SIZE, offset_t; u16 len = TPM_HEADER_SIZE; u32 total_len = be32_to_cpu(head->length); u32 val, param_len; *handle = tpm_buf_read_u32(buf, &offset_r); param_len = tpm_buf_read_u32(buf, &offset_r); /* * param_len doesn't include the header, but all the other * lengths and offsets do, so add it to parm len to make * the comparisons easier */ param_len += TPM_HEADER_SIZE; if (param_len + 8 > total_len) return -EINVAL; len = tpm_buf_read_u16(buf, &offset_r); offset_t = offset_r; /* now we have the public area, compute the name of the object */ put_unaligned_be16(TPM_ALG_SHA256, chip->null_key_name); sha256(&buf->data[offset_r], len, chip->null_key_name + 2); /* validate the public key */ val = tpm_buf_read_u16(buf, &offset_t); /* key type (must be what we asked for) */ if (val != TPM_ALG_ECC) return -EINVAL; val = tpm_buf_read_u16(buf, &offset_t); /* name algorithm */ if (val != TPM_ALG_SHA256) return -EINVAL; val = tpm_buf_read_u32(buf, &offset_t); /* object properties */ if (val != TPM2_OA_TMPL) return -EINVAL; /* auth policy (empty) */ val = tpm_buf_read_u16(buf, &offset_t); if (val != 0) return -EINVAL; /* symmetric key parameters */ val = tpm_buf_read_u16(buf, &offset_t); if (val != TPM_ALG_AES) return -EINVAL; /* symmetric key length */ val = tpm_buf_read_u16(buf, &offset_t); if (val != AES_KEY_BITS) return -EINVAL; /* symmetric encryption scheme */ val = tpm_buf_read_u16(buf, &offset_t); if (val != TPM_ALG_CFB) return -EINVAL; /* signing scheme */ val = tpm_buf_read_u16(buf, &offset_t); if (val != TPM_ALG_NULL) return -EINVAL; /* ECC Curve */ val = tpm_buf_read_u16(buf, &offset_t); if (val != TPM2_ECC_NIST_P256) return -EINVAL; /* KDF Scheme */ val = tpm_buf_read_u16(buf, &offset_t); if (val != TPM_ALG_NULL) return -EINVAL; /* extract public key (x and y points) */ val = tpm_buf_read_u16(buf, &offset_t); if (val != EC_PT_SZ) return -EINVAL; memcpy(chip->null_ec_key_x, &buf->data[offset_t], val); offset_t += val; val = tpm_buf_read_u16(buf, &offset_t); if (val != EC_PT_SZ) return -EINVAL; memcpy(chip->null_ec_key_y, &buf->data[offset_t], val); offset_t += val; /* original length of the whole TPM2B */ offset_r += len; /* should have exactly consumed the TPM2B public structure */ if (offset_t != offset_r) return -EINVAL; if (offset_r > param_len) return -EINVAL; /* creation data (skip) */ len = tpm_buf_read_u16(buf, &offset_r); offset_r += len; if (offset_r > param_len) return -EINVAL; /* creation digest (must be sha256) */ len = tpm_buf_read_u16(buf, &offset_r); offset_r += len; if (len != SHA256_DIGEST_SIZE || offset_r > param_len) return -EINVAL; /* TPMT_TK_CREATION follows */ /* tag, must be TPM_ST_CREATION (0x8021) */ val = tpm_buf_read_u16(buf, &offset_r); if (val != TPM2_ST_CREATION || offset_r > param_len) return -EINVAL; /* hierarchy */ val = tpm_buf_read_u32(buf, &offset_r); if (val != hierarchy || offset_r > param_len) return -EINVAL; /* the ticket digest HMAC (might not be sha256) */ len = tpm_buf_read_u16(buf, &offset_r); offset_r += len; if (offset_r > param_len) return -EINVAL; /* * finally we have the name, which is a sha256 digest plus a 2 * byte algorithm type */ len = tpm_buf_read_u16(buf, &offset_r); if (offset_r + len != param_len + 8) return -EINVAL; if (len != SHA256_DIGEST_SIZE + 2) return -EINVAL; if (memcmp(chip->null_key_name, &buf->data[offset_r], SHA256_DIGEST_SIZE + 2) != 0) { dev_err(&chip->dev, "NULL Seed name comparison failed\n"); return -EINVAL; } return 0; } /** * tpm2_create_primary() - create a primary key using a fixed P-256 template * * @chip: the TPM chip to create under * @hierarchy: The hierarchy handle to create under * @handle: The returned volatile handle on success * * For platforms that might not have a persistent primary, this can be * used to create one quickly on the fly (it uses Elliptic Curve not * RSA, so even slow TPMs can create one fast). The template uses the * TCG mandated H one for non-endorsement ECC primaries, i.e. P-256 * elliptic curve (the only current one all TPM2s are required to * have) a sha256 name hash and no policy. * * Return: * * 0 - OK * * -errno - A system error * * TPM_RC - A TPM error */ static int tpm2_create_primary(struct tpm_chip *chip, u32 hierarchy, u32 *handle) { int rc; struct tpm_buf buf; struct tpm_buf template; rc = tpm_buf_init(&buf, TPM2_ST_SESSIONS, TPM2_CC_CREATE_PRIMARY); if (rc) return rc; rc = tpm_buf_init_sized(&template); if (rc) { tpm_buf_destroy(&buf); return rc; } /* * create the template. Note: in order for userspace to * verify the security of the system, it will have to create * and certify this NULL primary, meaning all the template * parameters will have to be identical, so conform exactly to * the TCG TPM v2.0 Provisioning Guidance for the SRK ECC * key H template (H has zero size unique points) */ /* key type */ tpm_buf_append_u16(&template, TPM_ALG_ECC); /* name algorithm */ tpm_buf_append_u16(&template, TPM_ALG_SHA256); /* object properties */ tpm_buf_append_u32(&template, TPM2_OA_TMPL); /* sauth policy (empty) */ tpm_buf_append_u16(&template, 0); /* BEGIN parameters: key specific; for ECC*/ /* symmetric algorithm */ tpm_buf_append_u16(&template, TPM_ALG_AES); /* bits for symmetric algorithm */ tpm_buf_append_u16(&template, AES_KEY_BITS); /* algorithm mode (must be CFB) */ tpm_buf_append_u16(&template, TPM_ALG_CFB); /* scheme (NULL means any scheme) */ tpm_buf_append_u16(&template, TPM_ALG_NULL); /* ECC Curve ID */ tpm_buf_append_u16(&template, TPM2_ECC_NIST_P256); /* KDF Scheme */ tpm_buf_append_u16(&template, TPM_ALG_NULL); /* unique: key specific; for ECC it is two zero size points */ tpm_buf_append_u16(&template, 0); tpm_buf_append_u16(&template, 0); /* END parameters */ /* primary handle */ tpm_buf_append_u32(&buf, hierarchy); tpm_buf_append_empty_auth(&buf, TPM2_RS_PW); /* sensitive create size is 4 for two empty buffers */ tpm_buf_append_u16(&buf, 4); /* sensitive create auth data (empty) */ tpm_buf_append_u16(&buf, 0); /* sensitive create sensitive data (empty) */ tpm_buf_append_u16(&buf, 0); /* the public template */ tpm_buf_append(&buf, template.data, template.length); tpm_buf_destroy(&template); /* outside info (empty) */ tpm_buf_append_u16(&buf, 0); /* creation PCR (none) */ tpm_buf_append_u32(&buf, 0); rc = tpm_transmit_cmd(chip, &buf, 0, "attempting to create NULL primary"); if (rc == TPM2_RC_SUCCESS) rc = tpm2_parse_create_primary(chip, &buf, handle, hierarchy); tpm_buf_destroy(&buf); return rc; } static int tpm2_create_null_primary(struct tpm_chip *chip) { u32 null_key; int rc; rc = tpm2_create_primary(chip, TPM2_RH_NULL, &null_key); if (rc == TPM2_RC_SUCCESS) { unsigned int offset = 0; /* dummy offset for null key context */ rc = tpm2_save_context(chip, null_key, chip->null_key_context, sizeof(chip->null_key_context), &offset); tpm2_flush_context(chip, null_key); } return rc; } /** * tpm2_sessions_init() - start of day initialization for the sessions code * @chip: TPM chip * * Derive and context save the null primary and allocate memory in the * struct tpm_chip for the authorizations. */ int tpm2_sessions_init(struct tpm_chip *chip) { int rc; rc = tpm2_create_null_primary(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; }