From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001 From: Daniel Axtens Date: Thu, 30 Jul 2020 01:35:43 +1000 Subject: [PATCH] appended signatures: support verifying appended signatures Building on the parsers and the ability to embed x509 certificates, as well as the existing gcrypt functionality, add a module for verifying appended signatures. This includes a verifier that requires that Linux kernels and grub modules have appended signatures, and commands to manage the list of trusted certificates for verification. Verification must be enabled by setting check_appended_signatures. If GRUB is locked down when the module is loaded, verification will be enabled and locked automatically. As with the PGP verifier, it is not a complete secure-boot solution: other mechanisms, such as a password or lockdown, must be used to ensure that a user cannot drop to the grub shell and disable verification. Signed-off-by: Daniel Axtens [pjones: fix missing format specifier] Signed-off-by: Robbie Harwood --- grub-core/Makefile.core.def | 12 + grub-core/commands/appendedsig/appendedsig.c | 645 +++++++++++++++++++++++++++ include/grub/file.h | 2 + 3 files changed, 659 insertions(+) create mode 100644 grub-core/commands/appendedsig/appendedsig.c diff --git a/grub-core/Makefile.core.def b/grub-core/Makefile.core.def index b4aaccf7b..77321d218 100644 --- a/grub-core/Makefile.core.def +++ b/grub-core/Makefile.core.def @@ -980,6 +980,18 @@ module = { cppflags = '-I$(srcdir)/lib/posix_wrap'; }; +module = { + name = appendedsig; + common = commands/appendedsig/appendedsig.c; + common = commands/appendedsig/x509.c; + common = commands/appendedsig/pkcs7.c; + common = commands/appendedsig/asn1util.c; + common = commands/appendedsig/gnutls_asn1_tab.c; + common = commands/appendedsig/pkix_asn1_tab.c; + cflags = '$(CFLAGS_POSIX)'; + cppflags = '-I$(srcdir)/lib/posix_wrap'; +}; + module = { name = hdparm; common = commands/hdparm.c; diff --git a/grub-core/commands/appendedsig/appendedsig.c b/grub-core/commands/appendedsig/appendedsig.c new file mode 100644 index 000000000..bf8b18b62 --- /dev/null +++ b/grub-core/commands/appendedsig/appendedsig.c @@ -0,0 +1,645 @@ +/* + * GRUB -- GRand Unified Bootloader + * Copyright (C) 2020-2021 IBM Corporation. + * + * GRUB is free software: you can redistribute it and/or modify + * it under the terms of the GNU General Public License as published by + * the Free Software Foundation, either version 3 of the License, or + * (at your option) any later version. + * + * GRUB is distributed in the hope that it will be useful, + * but WITHOUT ANY WARRANTY; without even the implied warranty of + * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the + * GNU General Public License for more details. + * + * You should have received a copy of the GNU General Public License + * along with GRUB. If not, see . + */ + +#include +#include +#include +#include +#include +#include +#include +#include +#include +#include +#include +#include +#include +#include +#include +#include +#include + +#include "appendedsig.h" + +GRUB_MOD_LICENSE ("GPLv3+"); + +const char magic[] = "~Module signature appended~\n"; + +/* + * This structure is extracted from scripts/sign-file.c in the linux kernel + * source. It was licensed as LGPLv2.1+, which is GPLv3+ compatible. + */ +struct module_signature +{ + grub_uint8_t algo; /* Public-key crypto algorithm [0] */ + grub_uint8_t hash; /* Digest algorithm [0] */ + grub_uint8_t id_type; /* Key identifier type [PKEY_ID_PKCS7] */ + grub_uint8_t signer_len; /* Length of signer's name [0] */ + grub_uint8_t key_id_len; /* Length of key identifier [0] */ + grub_uint8_t __pad[3]; + grub_uint32_t sig_len; /* Length of signature data */ +} GRUB_PACKED; + + +/* This represents an entire, parsed, appended signature */ +struct grub_appended_signature +{ + grub_size_t signature_len; /* Length of PKCS#7 data + + * metadata + magic */ + + struct module_signature sig_metadata; /* Module signature metadata */ + struct pkcs7_signedData pkcs7; /* Parsed PKCS#7 data */ +}; + +/* Trusted certificates for verifying appended signatures */ +struct x509_certificate *grub_trusted_key; + +/* + * Force gcry_rsa to be a module dependency. + * + * If we use grub_crypto_pk_rsa, then then the gcry_rsa module won't be built + * in if you add 'appendedsig' to grub-install --modules. You would need to + * add 'gcry_rsa' too. That's confusing and seems suboptimal, especially when + * we only support RSA. + * + * Dynamic loading also causes some concerns. We can't load gcry_rsa from the + * the filesystem after we install the verifier - we won't be able to verify + * it without having it already present. We also shouldn't load it before we + * install the verifier, because that would mean it wouldn't be verified - an + * attacker could insert any code they wanted into the module. + * + * So instead, reference the internal symbol from gcry_rsa. That creates a + * direct dependency on gcry_rsa, so it will be built in when this module + * is built in. Being built in (assuming the core image is itself signed!) + * also resolves our concerns about loading from the filesystem. + */ +extern gcry_pk_spec_t _gcry_pubkey_spec_rsa; + +static int check_sigs = 0; + +static const char * +grub_env_read_sec (struct grub_env_var *var __attribute__ ((unused)), + const char *val __attribute__ ((unused))) +{ + if (check_sigs == 2) + return "forced"; + else if (check_sigs == 1) + return "enforce"; + else + return "no"; +} + +static char * +grub_env_write_sec (struct grub_env_var *var __attribute__((unused)), + const char *val) +{ + /* Do not allow the value to be changed if set to forced */ + if (check_sigs == 2) + return grub_strdup ("forced"); + + if ((*val == '2') || (*val == 'f')) + check_sigs = 2; + else if ((*val == '1') || (*val == 'e')) + check_sigs = 1; + else if ((*val == '0') || (*val == 'n')) + check_sigs = 0; + + return grub_strdup (grub_env_read_sec (NULL, NULL)); +} + +static grub_err_t +read_cert_from_file (grub_file_t f, struct x509_certificate *certificate) +{ + grub_err_t err; + grub_uint8_t *buf = NULL; + grub_ssize_t read_size; + grub_off_t total_read_size = 0; + grub_off_t file_size = grub_file_size (f); + + + if (file_size == GRUB_FILE_SIZE_UNKNOWN) + return grub_error (GRUB_ERR_BAD_ARGUMENT, + N_("Cannot parse a certificate file of unknown size")); + + buf = grub_zalloc (file_size); + if (!buf) + return grub_error (GRUB_ERR_OUT_OF_MEMORY, + N_("Could not allocate buffer for certificate file contents")); + + while (total_read_size < file_size) + { + read_size = + grub_file_read (f, &buf[total_read_size], + file_size - total_read_size); + if (read_size < 0) + { + err = grub_error (GRUB_ERR_READ_ERROR, + N_("Error reading certificate file")); + goto cleanup_buf; + } + total_read_size += read_size; + } + + err = certificate_import (buf, total_read_size, certificate); + if (err != GRUB_ERR_NONE) + goto cleanup_buf; + + return GRUB_ERR_NONE; + +cleanup_buf: + grub_free (buf); + return err; +} + +static grub_err_t +extract_appended_signature (grub_uint8_t * buf, grub_size_t bufsize, + struct grub_appended_signature *sig) +{ + grub_err_t err; + grub_size_t pkcs7_size; + grub_size_t remaining_len; + grub_uint8_t *appsigdata = buf + bufsize - grub_strlen (magic); + + if (bufsize < grub_strlen (magic)) + return grub_error (GRUB_ERR_BAD_SIGNATURE, + N_("File too short for signature magic")); + + if (grub_memcmp (appsigdata, (grub_uint8_t *) magic, grub_strlen (magic))) + return grub_error (GRUB_ERR_BAD_SIGNATURE, + N_("Missing or invalid signature magic")); + + remaining_len = bufsize - grub_strlen (magic); + + if (remaining_len < sizeof (struct module_signature)) + return grub_error (GRUB_ERR_BAD_SIGNATURE, + N_("File too short for signature metadata")); + + appsigdata -= sizeof (struct module_signature); + + /* extract the metadata */ + grub_memcpy (&(sig->sig_metadata), appsigdata, + sizeof (struct module_signature)); + + remaining_len -= sizeof (struct module_signature); + + if (sig->sig_metadata.id_type != 2) + return grub_error (GRUB_ERR_BAD_SIGNATURE, N_("Wrong signature type")); + +#ifdef GRUB_TARGET_WORDS_BIGENDIAN + pkcs7_size = sig->sig_metadata.sig_len; +#else + pkcs7_size = __builtin_bswap32 (sig->sig_metadata.sig_len); +#endif + + if (pkcs7_size > remaining_len) + return grub_error (GRUB_ERR_BAD_SIGNATURE, + N_("File too short for PKCS#7 message")); + + grub_dprintf ("appendedsig", "sig len %" PRIuGRUB_SIZE "\n", pkcs7_size); + + sig->signature_len = + grub_strlen (magic) + sizeof (struct module_signature) + pkcs7_size; + + /* rewind pointer and parse pkcs7 data */ + appsigdata -= pkcs7_size; + + err = parse_pkcs7_signedData (appsigdata, pkcs7_size, &sig->pkcs7); + if (err != GRUB_ERR_NONE) + return err; + + return GRUB_ERR_NONE; +} + +static grub_err_t +grub_verify_appended_signature (grub_uint8_t * buf, grub_size_t bufsize) +{ + grub_err_t err = GRUB_ERR_NONE; + grub_size_t datasize; + void *context; + unsigned char *hash; + gcry_mpi_t hashmpi; + gcry_err_code_t rc; + struct x509_certificate *pk; + struct grub_appended_signature sig; + + if (!grub_trusted_key) + return grub_error (GRUB_ERR_BAD_SIGNATURE, + N_("No trusted keys to verify against")); + + err = extract_appended_signature (buf, bufsize, &sig); + if (err != GRUB_ERR_NONE) + return err; + + datasize = bufsize - sig.signature_len; + + context = grub_zalloc (sig.pkcs7.hash->contextsize); + if (!context) + return grub_errno; + + sig.pkcs7.hash->init (context); + sig.pkcs7.hash->write (context, buf, datasize); + sig.pkcs7.hash->final (context); + hash = sig.pkcs7.hash->read (context); + grub_dprintf ("appendedsig", + "data size %" PRIxGRUB_SIZE ", hash %02x%02x%02x%02x...\n", + datasize, hash[0], hash[1], hash[2], hash[3]); + + err = GRUB_ERR_BAD_SIGNATURE; + for (pk = grub_trusted_key; pk; pk = pk->next) + { + rc = grub_crypto_rsa_pad (&hashmpi, hash, sig.pkcs7.hash, pk->mpis[0]); + if (rc) + { + err = grub_error (GRUB_ERR_BAD_SIGNATURE, + N_("Error padding hash for RSA verification: %d"), + rc); + goto cleanup; + } + + rc = _gcry_pubkey_spec_rsa.verify (0, hashmpi, &sig.pkcs7.sig_mpi, + pk->mpis, NULL, NULL); + gcry_mpi_release (hashmpi); + + if (rc == 0) + { + grub_dprintf ("appendedsig", "verify with key '%s' succeeded\n", + pk->subject); + err = GRUB_ERR_NONE; + break; + } + + grub_dprintf ("appendedsig", "verify with key '%s' failed with %d\n", + pk->subject, rc); + } + + /* If we didn't verify, provide a neat message */ + if (err != GRUB_ERR_NONE) + err = grub_error (GRUB_ERR_BAD_SIGNATURE, + N_("Failed to verify signature against a trusted key")); + +cleanup: + grub_free (context); + pkcs7_signedData_release (&sig.pkcs7); + + return err; +} + +static grub_err_t +grub_cmd_verify_signature (grub_command_t cmd __attribute__((unused)), + int argc, char **args) +{ + grub_file_t f; + grub_err_t err = GRUB_ERR_NONE; + grub_uint8_t *data; + grub_ssize_t read_size; + grub_off_t file_size, total_read_size = 0; + + if (argc < 1) + return grub_error (GRUB_ERR_BAD_ARGUMENT, N_("one argument expected")); + + grub_dprintf ("appendedsig", "verifying %s\n", args[0]); + + f = grub_file_open (args[0], GRUB_FILE_TYPE_VERIFY_SIGNATURE); + if (!f) + { + err = grub_errno; + goto cleanup; + } + + file_size = grub_file_size (f); + if (file_size == GRUB_FILE_SIZE_UNKNOWN) + return grub_error (GRUB_ERR_BAD_ARGUMENT, + N_("Cannot verify the signature of a file of unknown size")); + + data = grub_malloc (file_size); + if (!data) + return grub_error (GRUB_ERR_OUT_OF_MEMORY, + N_("Could not allocate data buffer size %" + PRIuGRUB_UINT64_T " for verification"), file_size); + + while (total_read_size < file_size) + { + read_size = + grub_file_read (f, &data[total_read_size], + file_size - total_read_size); + if (read_size < 0) + { + err = grub_error (GRUB_ERR_READ_ERROR, + N_("Error reading file to verify")); + goto cleanup_data; + } + total_read_size += read_size; + } + + err = grub_verify_appended_signature (data, file_size); + +cleanup_data: + grub_free (data); +cleanup: + if (f) + grub_file_close (f); + return err; +} + +static grub_err_t +grub_cmd_distrust (grub_command_t cmd __attribute__((unused)), + int argc, char **args) +{ + unsigned long cert_num, i; + struct x509_certificate *cert, *prev; + + if (argc != 1) + return grub_error (GRUB_ERR_BAD_ARGUMENT, N_("One argument expected")); + + grub_errno = GRUB_ERR_NONE; + cert_num = grub_strtoul (args[0], NULL, 10); + if (grub_errno != GRUB_ERR_NONE) + return grub_errno; + + if (cert_num < 1) + return grub_error (GRUB_ERR_BAD_ARGUMENT, + N_("Certificate number too small - numbers start at 1")); + + if (cert_num == 1) + { + cert = grub_trusted_key; + grub_trusted_key = cert->next; + + certificate_release (cert); + grub_free (cert); + return GRUB_ERR_NONE; + } + i = 2; + prev = grub_trusted_key; + cert = grub_trusted_key->next; + while (cert) + { + if (i == cert_num) + { + prev->next = cert->next; + certificate_release (cert); + grub_free (cert); + return GRUB_ERR_NONE; + } + i++; + prev = cert; + cert = cert->next; + } + + return grub_error (GRUB_ERR_BAD_ARGUMENT, + N_("No certificate number %d found - only %d certificates in the store"), + cert_num, i - 1); +} + +static grub_err_t +grub_cmd_trust (grub_command_t cmd __attribute__((unused)), + int argc, char **args) +{ + grub_file_t certf; + struct x509_certificate *cert = NULL; + grub_err_t err; + + if (argc != 1) + return grub_error (GRUB_ERR_BAD_ARGUMENT, N_("one argument expected")); + + certf = grub_file_open (args[0], + GRUB_FILE_TYPE_CERTIFICATE_TRUST + | GRUB_FILE_TYPE_NO_DECOMPRESS); + if (!certf) + return grub_errno; + + + cert = grub_zalloc (sizeof (struct x509_certificate)); + if (!cert) + return grub_error (GRUB_ERR_OUT_OF_MEMORY, + N_("Could not allocate memory for certificate")); + + err = read_cert_from_file (certf, cert); + grub_file_close (certf); + if (err != GRUB_ERR_NONE) + { + grub_free (cert); + return err; + } + grub_dprintf ("appendedsig", "Loaded certificate with CN: %s\n", + cert->subject); + + cert->next = grub_trusted_key; + grub_trusted_key = cert; + + return GRUB_ERR_NONE; +} + +static grub_err_t +grub_cmd_list (grub_command_t cmd __attribute__((unused)), + int argc __attribute__((unused)), + char **args __attribute__((unused))) +{ + struct x509_certificate *cert; + int cert_num = 1; + grub_size_t i; + + for (cert = grub_trusted_key; cert; cert = cert->next) + { + grub_printf (N_("Certificate %d:\n"), cert_num); + + grub_printf (N_("\tSerial: ")); + for (i = 0; i < cert->serial_len - 1; i++) + { + grub_printf ("%02x:", cert->serial[i]); + } + grub_printf ("%02x\n", cert->serial[cert->serial_len - 1]); + + grub_printf ("\tCN: %s\n\n", cert->subject); + cert_num++; + + } + + return GRUB_ERR_NONE; +} + +static grub_err_t +appendedsig_init (grub_file_t io __attribute__((unused)), + enum grub_file_type type, + void **context __attribute__((unused)), + enum grub_verify_flags *flags) +{ + if (!check_sigs) + { + *flags = GRUB_VERIFY_FLAGS_SKIP_VERIFICATION; + return GRUB_ERR_NONE; + } + + switch (type & GRUB_FILE_TYPE_MASK) + { + case GRUB_FILE_TYPE_CERTIFICATE_TRUST: + /* + * This is a certificate to add to trusted keychain. + * + * This needs to be verified or blocked. Ideally we'd write an x509 + * verifier, but we lack the hubris required to take this on. Instead, + * require that it have an appended signature. + */ + + /* Fall through */ + + case GRUB_FILE_TYPE_LINUX_KERNEL: + case GRUB_FILE_TYPE_GRUB_MODULE: + /* + * Appended signatures are only defined for ELF binaries. + * Out of an abundance of caution, we only verify Linux kernels and + * GRUB modules at this point. + */ + *flags = GRUB_VERIFY_FLAGS_SINGLE_CHUNK; + return GRUB_ERR_NONE; + + case GRUB_FILE_TYPE_ACPI_TABLE: + case GRUB_FILE_TYPE_DEVICE_TREE_IMAGE: + /* + * It is possible to use appended signature verification without + * lockdown - like the PGP verifier. When combined with an embedded + * config file in a signed grub binary, this could still be a meaningful + * secure-boot chain - so long as it isn't subverted by something like a + * rouge ACPI table or DT image. Defer them explicitly. + */ + *flags = GRUB_VERIFY_FLAGS_DEFER_AUTH; + return GRUB_ERR_NONE; + + default: + *flags = GRUB_VERIFY_FLAGS_SKIP_VERIFICATION; + return GRUB_ERR_NONE; + } +} + +static grub_err_t +appendedsig_write (void *ctxt __attribute__((unused)), + void *buf, grub_size_t size) +{ + return grub_verify_appended_signature (buf, size); +} + +struct grub_file_verifier grub_appendedsig_verifier = { + .name = "appendedsig", + .init = appendedsig_init, + .write = appendedsig_write, +}; + +static grub_ssize_t +pseudo_read (struct grub_file *file, char *buf, grub_size_t len) +{ + grub_memcpy (buf, (grub_uint8_t *) file->data + file->offset, len); + return len; +} + +/* Filesystem descriptor. */ +static struct grub_fs pseudo_fs = { + .name = "pseudo", + .fs_read = pseudo_read +}; + +static grub_command_t cmd_verify, cmd_list, cmd_distrust, cmd_trust; + +GRUB_MOD_INIT (appendedsig) +{ + int rc; + struct grub_module_header *header; + + /* If in lockdown, immediately enter forced mode */ + if (grub_is_lockdown () == GRUB_LOCKDOWN_ENABLED) + check_sigs = 2; + + grub_trusted_key = NULL; + + grub_register_variable_hook ("check_appended_signatures", + grub_env_read_sec, + grub_env_write_sec); + grub_env_export ("check_appended_signatures"); + + rc = asn1_init (); + if (rc) + grub_fatal ("Error initing ASN.1 data structures: %d: %s\n", rc, + asn1_strerror (rc)); + + FOR_MODULES (header) + { + struct grub_file pseudo_file; + struct x509_certificate *pk = NULL; + grub_err_t err; + + /* Not an ELF module, skip. */ + if (header->type != OBJ_TYPE_X509_PUBKEY) + continue; + + grub_memset (&pseudo_file, 0, sizeof (pseudo_file)); + pseudo_file.fs = &pseudo_fs; + pseudo_file.size = header->size - sizeof (struct grub_module_header); + pseudo_file.data = (char *) header + sizeof (struct grub_module_header); + + grub_dprintf ("appendedsig", + "Found an x509 key, size=%" PRIuGRUB_UINT64_T "\n", + pseudo_file.size); + + pk = grub_zalloc (sizeof (struct x509_certificate)); + if (!pk) + { + grub_fatal ("Out of memory loading initial certificates"); + } + + err = read_cert_from_file (&pseudo_file, pk); + if (err != GRUB_ERR_NONE) + grub_fatal ("Error loading initial key: %s", grub_errmsg); + + grub_dprintf ("appendedsig", "loaded certificate CN='%s'\n", pk->subject); + + pk->next = grub_trusted_key; + grub_trusted_key = pk; + } + + cmd_trust = + grub_register_command ("trust_certificate", grub_cmd_trust, + N_("X509_CERTIFICATE"), + N_("Add X509_CERTIFICATE to trusted certificates.")); + cmd_list = + grub_register_command ("list_certificates", grub_cmd_list, 0, + N_("Show the list of trusted x509 certificates.")); + cmd_verify = + grub_register_command ("verify_appended", grub_cmd_verify_signature, + N_("FILE"), + N_("Verify FILE against the trusted x509 certificates.")); + cmd_distrust = + grub_register_command ("distrust_certificate", grub_cmd_distrust, + N_("CERT_NUMBER"), + N_("Remove CERT_NUMBER (as listed by list_certificates) from trusted certificates.")); + + grub_verifier_register (&grub_appendedsig_verifier); + grub_dl_set_persistent (mod); +} + +GRUB_MOD_FINI (appendedsig) +{ + /* + * grub_dl_set_persistent should prevent this from actually running, but + * it does still run under emu. + */ + + grub_verifier_unregister (&grub_appendedsig_verifier); + grub_unregister_command (cmd_verify); + grub_unregister_command (cmd_list); + grub_unregister_command (cmd_trust); + grub_unregister_command (cmd_distrust); +} diff --git a/include/grub/file.h b/include/grub/file.h index 31567483c..96827a4f8 100644 --- a/include/grub/file.h +++ b/include/grub/file.h @@ -80,6 +80,8 @@ enum grub_file_type GRUB_FILE_TYPE_PUBLIC_KEY, /* File holding public key to add to trused keys. */ GRUB_FILE_TYPE_PUBLIC_KEY_TRUST, + /* File holding x509 certificiate to add to trusted keys. */ + GRUB_FILE_TYPE_CERTIFICATE_TRUST, /* File of which we intend to print a blocklist to the user. */ GRUB_FILE_TYPE_PRINT_BLOCKLIST, /* File we intend to use for test loading or testing speed. */