On Thu, Mar 27, 2025 at 01:02:31AM +0530, Sudhakar Kuppusamy wrote:
> From: Daniel Axtens <[email protected]>
>
> 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 <[email protected]>
> Signed-off-by: Sudhakar Kuppusamy <[email protected]>
> Reviewed-by: Stefan Berger <[email protected]>
> Reviewed-by: Avnish Chouhan <[email protected]>
> ---
> grub-core/Makefile.core.def | 14 +
> grub-core/commands/appendedsig/appendedsig.c | 618 ++++++++++++++++++
> grub-core/commands/appendedsig/appendedsig.h | 2 +-
> grub-core/commands/appendedsig/asn1util.c | 2 +-
> .../commands/appendedsig/gnutls_asn1_tab.c | 2 +-
> .../commands/appendedsig/pkix_asn1_tab.c | 2 +-
> grub-core/commands/appendedsig/x509.c | 2 +-
Please take into account Gary's comment...
> include/grub/file.h | 2 +
> 8 files changed, 639 insertions(+), 5 deletions(-)
> create mode 100644 grub-core/commands/appendedsig/appendedsig.c
>
> diff --git a/grub-core/Makefile.core.def b/grub-core/Makefile.core.def
> index 60db2adc5..d693986c6 100644
> --- a/grub-core/Makefile.core.def
> +++ b/grub-core/Makefile.core.def
> @@ -979,6 +979,20 @@ module = {
> cppflags = '-I$(srcdir)/lib/posix_wrap';
> };
>
> +module = {
> + name = appendedsig;
> + common = commands/appendedsig/appendedsig.c;
> + common = commands/appendedsig/x509.c;
> + common = commands/appendedsig/pkcs7.c;
> + common = commands/appendedsig/asn1util.c;
> + common = commands/appendedsig/gnutls_asn1_tab.c;
> + common = commands/appendedsig/pkix_asn1_tab.c;
> +
> + // posix wrapper required for gcry to get sys/types.h
> + cflags = '$(CFLAGS_POSIX)';
> + cppflags = '-I$(srcdir)/lib/posix_wrap -I$(srcdir)/lib/libtasn1-grub';
> +};
> +
> 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..cbd227a3c
> --- /dev/null
> +++ b/grub-core/commands/appendedsig/appendedsig.c
> @@ -0,0 +1,618 @@
> +/*
> + * GRUB -- GRand Unified Bootloader
> + * Copyright (C) 2020, 2021, 2022 Free Software Foundation, Inc.
> + * Copyright (C) 2020, 2021, 2022 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 <http://www.gnu.org/licenses/>.
> + */
> +
> +#include <grub/types.h>
> +#include <grub/misc.h>
> +#include <grub/mm.h>
> +#include <grub/err.h>
> +#include <grub/dl.h>
> +#include <grub/file.h>
> +#include <grub/command.h>
> +#include <grub/crypto.h>
> +#include <grub/pkcs1_v15.h>
> +#include <grub/i18n.h>
> +#include <grub/gcrypt/gcrypt.h>
> +#include <grub/kernel.h>
> +#include <grub/extcmd.h>
> +#include <grub/verify.h>
> +#include <libtasn1.h>
> +#include <grub/env.h>
> +#include <grub/lockdown.h>
> +
> +#include "appendedsig.h"
> +
> +GRUB_MOD_LICENSE ("GPLv3+");
> +
> +const char magic[] = "~Module signature appended~\n";
static const char magic[]...?
> +/*
> + * 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;
Please take a look at the commit 6744840b1 (build: Track explicit module
dependencies in Makefile.core.def) and use "depends" instead.
I think I saw similar things in earlier patches. If it is the same case
please fix the other issues as above...
> +static enum
> +{
> + check_sigs_no = 0,
> + check_sigs_enforce = 1,
> + check_sigs_forced = 2
Please use uppercase for all these constants.
> +} check_sigs = check_sigs_no;
> +
> +static const char *
> +grub_env_read_sec (struct grub_env_var *var __attribute__ ((unused)),
> + const char *val __attribute__ ((unused)))
> +{
> + if (check_sigs == check_sigs_forced)
> + return "forced";
> + else if (check_sigs == check_sigs_enforce)
> + 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 == check_sigs_forced)
> + return grub_strdup ("forced");
grub_strdup() may fail...
> +
> + if ((*val == '2') || (*val == 'f'))
> + check_sigs = check_sigs_forced;
> + else if ((*val == '1') || (*val == 'e'))
> + check_sigs = check_sigs_enforce;
> + else if ((*val == '0') || (*val == 'n'))
> + check_sigs = check_sigs_no;
> +
> + return grub_strdup (grub_env_read_sec (NULL, NULL));
Ditto...
> +}
> +
> +static grub_err_t
> +file_read_all (grub_file_t file, grub_uint8_t **buf, grub_size_t *len)
> +{
> + grub_off_t full_file_size;
> + grub_size_t file_size, total_read_size = 0;
> + grub_ssize_t read_size;
> +
> + full_file_size = grub_file_size (file);
> + if (full_file_size == GRUB_FILE_SIZE_UNKNOWN)
> + return grub_error (GRUB_ERR_BAD_ARGUMENT,
> + N_("Cannot read a file of unknown size into a
> buffer"));
> +
> + if (full_file_size > GRUB_SIZE_MAX)
> + return grub_error (GRUB_ERR_OUT_OF_RANGE,
> + N_("File is too large to read: %" PRIuGRUB_UINT64_T "
> bytes"),
> + full_file_size);
> +
> + file_size = (grub_size_t) full_file_size;
> +
> + *buf = grub_malloc (file_size);
> + if (!*buf)
> + return grub_error (GRUB_ERR_OUT_OF_MEMORY,
> + N_("Could not allocate file data buffer size %"
> PRIuGRUB_SIZE),
> + file_size);
> +
> + while (total_read_size < file_size)
> + {
> + read_size = grub_file_read (file, *buf + total_read_size, file_size -
> total_read_size);
> +
> + if (read_size < 0)
> + {
> + grub_free (*buf);
> + return grub_errno;
> + }
> + else if (read_size == 0)
> + {
> + grub_free (*buf);
> + return grub_error (GRUB_ERR_IO,
> + N_("Could not read full file size "
> + "(%" PRIuGRUB_SIZE "), only %" PRIuGRUB_SIZE
> " bytes read"),
Be careful, you cannot use PRIuGRUB_SIZE conversions in N_(). Though
I would suggest to drop N_() here and from cryptic error messages which
should be targeted only for experts. And probably it would be better to
add file path to this error message.
> + file_size, total_read_size);
> + }
> +
> + total_read_size += read_size;
> + }
> + *len = file_size;
> + return GRUB_ERR_NONE;
> +}
> +
> +static grub_err_t
> +read_cert_from_file (grub_file_t f, struct x509_certificate *certificate)
> +{
> + grub_err_t err;
> + grub_uint8_t *buf;
> + grub_size_t file_size;
> +
> + err = file_read_all (f, &buf, &file_size);
> + if (err != GRUB_ERR_NONE)
> + return err;
> +
> + err = parse_x509_certificate (buf, file_size, certificate);
> + grub_free (buf);
> +
> + return err;
> +}
> +
> +static grub_err_t
> +extract_appended_signature (const grub_uint8_t *buf, grub_size_t bufsize,
> + struct grub_appended_signature *sig)
> +{
> + grub_size_t pkcs7_size;
> + grub_size_t remaining_len;
> + const 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"));
> +
> + pkcs7_size = grub_be_to_cpu32 (sig->sig_metadata.sig_len);
> +
> + 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;
> +
> + return parse_pkcs7_signedData (appsigdata, pkcs7_size, &sig->pkcs7);
> +}
> +
> +static grub_err_t
> +grub_verify_appended_signature (const 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;
> + struct pkcs7_signerInfo *si;
> + int i;
> +
> + 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;
> +
> + for (i = 0; i < sig.pkcs7.signerInfo_count; i++)
> + {
> + /*
> + * This could be optimised in a couple of ways:
> + * - we could only compute hashes once per hash type
> + * - we could track signer information and only verify where IDs match
> + * For now we do the naive O(trusted keys * pkcs7 signers) approach.
> + */
> + si = &sig.pkcs7.signerInfos[i];
> + context = grub_zalloc (si->hash->contextsize);
> + if (!context)
> + return grub_errno;
> +
> + si->hash->init (context);
> + si->hash->write (context, buf, datasize);
> + si->hash->final (context);
> + hash = si->hash->read (context);
> +
> + grub_dprintf ("appendedsig", "data size %" PRIxGRUB_SIZE ", signer %d
> hash %02x%02x%02x%02x...\n",
> + datasize, i, 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, si->hash, pk->mpis[0]);
> + if (rc)
> + {
> + err = grub_error (GRUB_ERR_BAD_SIGNATURE,
> + N_("Error padding hash for RSA verification:
> %d"), rc);
Is gcry_err_code_t always equal int type? Even if yes it would be nice
to cast rc to int. And I would drop N_() here.
> + grub_free (context);
> + goto cleanup;
> + }
> +
> + rc = _gcry_pubkey_spec_rsa.verify (0, hashmpi, &si->sig_mpi,
> pk->mpis, NULL, NULL);
> + gcry_mpi_release (hashmpi);
> +
> + if (rc == 0)
> + {
> + grub_dprintf ("appendedsig", "verify signer %d with key '%s'
> succeeded\n",
> + i, pk->subject);
> + err = GRUB_ERR_NONE;
> + break;
> + }
> +
> + grub_dprintf ("appendedsig", "verify signer %d with key '%s'
> failed with %d\n",
> + i, pk->subject, rc);
> + }
> +
> + grub_free (context);
> +
> + if (err == GRUB_ERR_NONE)
> + break;
> + }
> +
> + /* 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:
Missing space before label.
> + 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_size_t file_size;
> +
> + 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;
> + }
> +
> + err = file_read_all (f, &data, &file_size);
> + if (err != GRUB_ERR_NONE)
> + goto cleanup;
> +
> + err = grub_verify_appended_signature (data, file_size);
> +
> + grub_free (data);
> +
> +cleanup:
Ditto...
> + 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;
This check is unreliable. Please take a look at commit ac8a37dda
(net/http: Allow use of non-standard TCP/IP ports) and fix checks
for all grub_strtoul() et consortes calls.
Daniel
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