Re: [PATCH v2 19/22] appended signatures: support verifying appended signatures

[Daniel Axtens]

>> +static enum
>> +{ check_sigs_no = 0,
>
>
> nit: newline after '{'
>
fixed

>
>> +  check_sigs_enforce = 1,
>> +  check_sigs_forced = 2
>> +} check_sigs = check_sigs_no;
>
>
> What does 'forced' mean?

It means that it cannot be turned of with `set check_appended_signatures=0`
at the grub prompt. I'm open to better names.

>
>> +
>> +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");
>> +
>> +  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));
>> +}
>> +
>> +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"), 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);
>> +  if (err != GRUB_ERR_NONE)
>> +    {
>> +      grub_free (buf);
>> +      return err;
>> +    }
>
> forgot grub-free(buf) ?

Huh, yeah, seems so. Fixed, thank you.

>
>> +
>> +  return GRUB_ERR_NONE;
>> +}
>> +
>> +static grub_err_t
>> +extract_appended_signature (const 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;
>> +  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;
>> +
>> +  err = parse_pkcs7_signedData (appsigdata, pkcs7_size, &sig->pkcs7);
>> +  if (err != GRUB_ERR_NONE)
>> +    return err;
>> +
>> +  return GRUB_ERR_NONE;
> just 'return parse_pkcs7_signedData (...)' ?

Good call.

>> +}
>> +
>> +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);
>> +          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:
>> +  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:
>> +  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 %lu found - only %lu 
>> 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 == check_sigs_no)
>> +    {
>> +      *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 = check_sigs_forced;
>> +
>> +  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;
>
> Is that comment correct? What does an ELF module have to do with type 
> OBJ_TYPE_X509_PUBKEY?

lolol sorry. I probably copied from pgp.c, which looks like it copied
from kern/main.c. The comment would only have been correct in
kern/main.c. Fixed.

Thanks again!

Kind regards,
Daniel
>> +
>> +    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 31567483ccfc..96827a4f8961 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.  */