[PATCH v6 10/20] appended signatures: Support verifying appended signatures

[Sudhakar Kuppusamy <sudhakar@linux.ibm.com>]

Building on the parsers and the ability to embed X.509 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
secure boot is enabled with enforced mode when the module is loaded,
verification will be enabled and locked automatically. If verification
is enabled, extract trusted keys from the GRUB ELF Note and store them in the db.

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.

Introducing the following GRUB commands.

 1. append_list_db:
      Show the list of trusted certificates from the db list
 2. append_add_db_cert:
      Add the trusted certificate to the db list
 3. append_rm_dbx_cert:
      Remove the distrusted certificate from the db list
 4. append_verify:
      Verify the signed file using db list

Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Sudhakar Kuppusamy <sudhakar@linux.ibm.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Avnish Chouhan <avnish@linux.ibm.com>
---
 grub-core/Makefile.core.def                  |  14 +
 grub-core/commands/appendedsig/appendedsig.c | 691 +++++++++++++++++++
 include/grub/file.h                          |   2 +
 3 files changed, 707 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 b72f322b1..23fddba9a 100644
--- a/grub-core/Makefile.core.def
+++ b/grub-core/Makefile.core.def
@@ -980,6 +980,20 @@ module = {
   cppflags = '$(CPPFLAGS_GCRY)';
 };
 
+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;
+  enable = powerpc_ieee1275;
+  cflags = '$(CFLAGS_GCRY) -Wno-redundant-decls';
+  cppflags = '$(CPPFLAGS_GCRY) -I$(srcdir)/lib/libtasn1-grub';
+  depends = crypto, gcry_rsa, gcry_sha256, gcry_sha512, mpi, asn1;
+};
+
 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..48f1ec94e
--- /dev/null
+++ b/grub-core/commands/appendedsig/appendedsig.c
@@ -0,0 +1,691 @@
+/*
+ *  GRUB  --  GRand Unified Bootloader
+ *  Copyright (C) 2020, 2021, 2022 Free Software Foundation, Inc.
+ *  Copyright (C) 2020, 2021, 2022, 2025 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/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/ieee1275/ieee1275.h>
+
+#include "appendedsig.h"
+
+GRUB_MOD_LICENSE ("GPLv3+");
+
+/* Public key type. */
+#define GRUB_PKEY_ID_PKCS7 2
+
+/* Appended signature magic string. */
+static 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 [GRUB_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 *db;
+
+/* Appended signature size. */
+static grub_size_t append_sig_len = 0;
+
+/*
+ * Signature verification flag (check_sigs).
+ * check_sigs: false
+ *  - No signature verification. This is the default.
+ * check_sigs: true
+ *  - Enforce signature verification, and if signature verification fails,
+ *    post the errors and stop the boot.
+ */
+static bool check_sigs = false;
+
+static const char *
+grub_env_read_sec (struct grub_env_var *var __attribute__ ((unused)),
+                   const char *val __attribute__ ((unused)))
+{
+  if (check_sigs == true)
+    return "enforce";
+
+  return "no";
+}
+
+static char *
+grub_env_write_sec (struct grub_env_var *var __attribute__ ((unused)), const char *val)
+{
+  char *ret;
+
+  /*
+   * Do not allow the value to be changed if check_sigs is set to enforce and
+   * secure boot is enabled with enforced mode.
+   */
+  if (check_sigs == true && grub_ieee1275_is_secure_boot () == GRUB_SB_ENFORCED)
+    {
+      ret = grub_strdup ("enforce");
+      if (ret == NULL)
+        grub_error (GRUB_ERR_OUT_OF_MEMORY, "could not duplicate a string enforce");
+
+      return ret;
+    }
+
+  if ((*val == '1') || (*val == 'e'))
+    check_sigs = true;
+  else if ((*val == '0') || (*val == 'n'))
+    check_sigs = false;
+
+  ret = grub_strdup (grub_env_read_sec (NULL, NULL));
+  if (ret == NULL)
+    grub_error (GRUB_ERR_OUT_OF_MEMORY, "could not duplicate a string %s",
+                grub_env_read_sec (NULL, NULL));
+
+  return ret;
+}
+
+static bool
+is_cert_match (const struct x509_certificate *distrusted_cert,
+               const struct x509_certificate *db_cert)
+{
+  if (grub_memcmp (distrusted_cert->subject, db_cert->subject, db_cert->subject_len) == 0
+      && grub_memcmp (distrusted_cert->issuer, db_cert->issuer, db_cert->issuer_len) == 0
+      && grub_memcmp (distrusted_cert->serial, db_cert->serial, db_cert->serial_len) == 0
+      && grub_memcmp (distrusted_cert->mpis[0], db_cert->mpis[0], sizeof (db_cert->mpis[0])) == 0
+      && grub_memcmp (distrusted_cert->mpis[1], db_cert->mpis[1], sizeof (db_cert->mpis[1])) == 0)
+    return true;
+
+  return false;
+}
+
+static grub_err_t
+file_read_whole (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,
+                       "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,
+                       "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 == NULL)
+    return grub_error (GRUB_ERR_OUT_OF_MEMORY,
+                       "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,
+                             "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
+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, "file too short for signature magic");
+
+  if (grub_strncmp ((const char *) appsigdata, magic, sizeof (magic) - 1))
+    return grub_error (GRUB_ERR_BAD_SIGNATURE, "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, "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 != GRUB_PKEY_ID_PKCS7)
+    return grub_error (GRUB_ERR_BAD_SIGNATURE, "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, "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);
+}
+
+/*
+ * Given a hash value 'hval', of hash specification 'hash', prepare
+ * the S-expressions (sexp) and perform the signature verification.
+ */
+static grub_err_t
+verify_signature (const gcry_mpi_t *pkmpi, const gcry_mpi_t hmpi,
+                  const gcry_md_spec_t *hash, const grub_uint8_t *hval)
+{
+  gcry_sexp_t hsexp, pubkey, sig;
+  grub_size_t errof;
+
+  if (_gcry_sexp_build(&hsexp, &errof, "(data (flags %s) (hash %s %b))", "pkcs1",
+                       hash->name, hash->mdlen, hval) != GPG_ERR_NO_ERROR)
+    return GRUB_ERR_BAD_SIGNATURE;
+
+  if (_gcry_sexp_build(&pubkey, &errof, "(public-key (dsa (n %M) (e %M)))",
+                       pkmpi[0], pkmpi[1]) != GPG_ERR_NO_ERROR)
+    return GRUB_ERR_BAD_SIGNATURE;
+
+  if (_gcry_sexp_build(&sig, &errof, "(sig-val (rsa (s %M)))", hmpi) != GPG_ERR_NO_ERROR)
+    return GRUB_ERR_BAD_SIGNATURE;
+
+  _gcry_sexp_dump(sig);
+  _gcry_sexp_dump(hsexp);
+  _gcry_sexp_dump(pubkey);
+
+  if (grub_crypto_pk_rsa->verify (sig, hsexp, pubkey) != GPG_ERR_NO_ERROR)
+    return GRUB_ERR_BAD_SIGNATURE;
+
+  return GRUB_ERR_NONE;
+}
+
+static grub_err_t
+grub_verify_appended_signature (const grub_uint8_t *buf, grub_size_t bufsize)
+{
+  grub_err_t err;
+  grub_size_t datasize;
+  void *context;
+  unsigned char *hash;
+  struct x509_certificate *pk;
+  struct grub_appended_signature sig;
+  struct pkcs7_signerInfo *si;
+  int i;
+
+  if (db == NULL)
+    return grub_error (GRUB_ERR_BAD_SIGNATURE, "no trusted keys to verify against");
+
+  err = extract_appended_signature (buf, bufsize, &sig);
+  if (err != GRUB_ERR_NONE)
+    return err;
+
+  append_sig_len = sig.signature_len;
+  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 == NULL)
+        return grub_errno;
+
+      si->hash->init (context, 0);
+      si->hash->write (context, buf, datasize);
+      si->hash->final (context);
+      hash = si->hash->read (context);
+
+      grub_dprintf ("appendedsig", "data size %" PRIuGRUB_SIZE ", signer %d hash %02x%02x%02x%02x...\n",
+                    datasize, i, hash[0], hash[1], hash[2], hash[3]);
+
+      for (pk = db; pk != NULL; pk = pk->next)
+        {
+          err = verify_signature (pk->mpis, si->sig_mpi, si->hash, hash);
+          if (err == GRUB_ERR_NONE)
+            {
+              grub_dprintf ("appendedsig", "verify signer %d with key '%s' succeeded\n",
+                            i, pk->subject);
+              break;
+            }
+
+          grub_dprintf ("appendedsig", "verify signer %d with key '%s' failed\n",
+                        i, pk->subject);
+        }
+
+      grub_free (context);
+      if (err == GRUB_ERR_NONE)
+        break;
+    }
+
+  pkcs7_signedData_release (&sig.pkcs7);
+
+  if (err != GRUB_ERR_NONE)
+    return grub_error (err, "failed to verify signature against a trusted key");
+
+  return err;
+}
+
+static grub_err_t
+read_cert_from_file (grub_file_t cert_file, struct x509_certificate **cert)
+{
+  grub_err_t err;
+  grub_uint8_t *buf = NULL;
+  grub_size_t buf_size = 0;
+
+  *cert = grub_zalloc (sizeof (struct x509_certificate));
+  if (*cert == NULL)
+      return grub_error (GRUB_ERR_OUT_OF_MEMORY, "could not allocate memory for certificate");
+
+  err = file_read_whole (cert_file, &buf, &buf_size);
+  if (err != GRUB_ERR_NONE)
+    return err;
+
+  if (check_sigs == true)
+    buf_size -= append_sig_len;
+
+  err = parse_x509_certificate (buf, buf_size, *cert);
+  grub_free (buf);
+
+  return err;
+}
+
+static bool
+is_cert_present_in_db (const struct x509_certificate *cert_in)
+{
+  struct x509_certificate *cert;
+
+  for (cert = db; cert; cert = cert->next)
+    {
+      if (is_cert_match (cert, cert_in) == true)
+        return true;
+    }
+
+  return false;
+}
+
+static bool
+is_cert_removed_from_db (const struct x509_certificate *cert)
+{
+  int i = 1;
+  struct x509_certificate *curr_cert, *prev_cert;
+
+  for (curr_cert = prev_cert = db; curr_cert != NULL; curr_cert = curr_cert->next)
+    {
+      if (is_cert_match (curr_cert, cert) == true)
+        {
+          if (i == 1) /* Match with first certificate in the db list. */
+            db = curr_cert->next;
+          else
+            prev_cert->next = curr_cert->next;
+
+          grub_dprintf ("appendedsig",
+                        "removed certificate with CN: %s from the db list\n", curr_cert->subject);
+          curr_cert->next = NULL;
+          certificate_release (curr_cert);
+          grub_free (curr_cert);
+          return true;
+	}
+      else
+        prev_cert = curr_cert;
+
+      i++;
+    }
+
+  return false;
+}
+
+static grub_err_t
+grub_cmd_verify_signature (grub_command_t cmd __attribute__ ((unused)), int argc, char **args)
+{
+  grub_file_t signed_file;
+  grub_err_t err;
+  grub_uint8_t *signed_data = NULL;
+  grub_size_t signed_data_size = 0;
+
+  if (argc != 1)
+    return grub_error (GRUB_ERR_BAD_ARGUMENT,
+                       "a signed file is expected\nExample:\n\tappend_verify <SIGNED FILE>\n");
+
+  if (*args == NULL)
+    return grub_error (GRUB_ERR_BAD_FILENAME, "missing signed file");
+
+  grub_dprintf ("appendedsig", "verifying %s\n", args[0]);
+
+  signed_file = grub_file_open (args[0], GRUB_FILE_TYPE_VERIFY_SIGNATURE);
+  if (signed_file == NULL)
+    return grub_error (GRUB_ERR_FILE_NOT_FOUND, "could not open %s file", args[0]);
+
+  err = file_read_whole (signed_file, &signed_data, &signed_data_size);
+  if (err == GRUB_ERR_NONE)
+    err = grub_verify_appended_signature (signed_data, signed_data_size);
+
+  grub_file_close (signed_file);
+  grub_free (signed_data);
+
+  return err;
+}
+
+static grub_err_t
+grub_cmd_db_cert (grub_command_t cmd __attribute__ ((unused)), int argc, char **args)
+{
+  grub_err_t err;
+  grub_file_t cert_file;
+  struct x509_certificate *cert = NULL;
+
+  if (argc != 1)
+    return grub_error (GRUB_ERR_BAD_ARGUMENT,
+                       "a trusted X.509 certificate file is expected in DER format\n"
+                       "Example:\n\tappend_add_db_cert <X509_CERTIFICATE>\n");
+
+  if (*args == NULL)
+    return grub_error (GRUB_ERR_BAD_FILENAME, "missing trusted X.509 certificate file");
+
+  cert_file = grub_file_open (args[0],
+                              GRUB_FILE_TYPE_CERTIFICATE_TRUST | GRUB_FILE_TYPE_NO_DECOMPRESS);
+  if (cert_file == NULL)
+    return grub_error (GRUB_ERR_BAD_FILE_TYPE, "could not open %s file", args[0]);
+
+  err = read_cert_from_file (cert_file, &cert);
+  grub_file_close (cert_file);
+  if (err != GRUB_ERR_NONE)
+    {
+      grub_free (cert);
+      return err;
+    }
+
+  if (is_cert_present_in_db (cert) == true)
+    {
+      certificate_release (cert);
+      grub_free (cert);
+      return grub_error (GRUB_ERR_STILL_REFERENCED,
+                         "could not add the certificate, as it is present in the db list");
+    }
+
+  grub_dprintf ("appendedsig", "added certificate with CN: %s\n", cert->subject);
+
+  cert->next = db;
+  db = cert;
+
+  return GRUB_ERR_NONE;
+}
+
+static grub_err_t
+grub_cmd_dbx_cert (grub_command_t cmd __attribute__ ((unused)), int argc, char **args)
+{
+  grub_err_t err;
+  grub_file_t cert_file;
+  struct x509_certificate *cert = NULL;
+
+  if (argc != 1)
+    return grub_error (GRUB_ERR_BAD_ARGUMENT,
+                       "a distrusted X.509 certificate file is expected in DER format\n"
+                       "Example:\n\tappend_rm_dbx_cert <X509_CERTIFICATE>\n");
+
+  if (*args == NULL)
+    return grub_error (GRUB_ERR_BAD_FILENAME, "missing distrusted X.509 certificate file");
+
+  cert_file = grub_file_open (args[0],
+                              GRUB_FILE_TYPE_CERTIFICATE_TRUST | GRUB_FILE_TYPE_NO_DECOMPRESS);
+  if (cert_file == NULL)
+    return grub_error (GRUB_ERR_BAD_FILE_TYPE, "could not open %s file", args[0]);
+
+  err = read_cert_from_file (cert_file, &cert);
+  grub_file_close (cert_file);
+  if (err != GRUB_ERR_NONE)
+    grub_free (cert);
+
+  if (is_cert_removed_from_db (cert) == false)
+    err = grub_error (GRUB_ERR_EOF,
+                      "not found certificate with CN:%s in the db list", cert->subject);
+
+  certificate_release (cert);
+  grub_free (cert);
+
+  return err;
+}
+
+static grub_err_t
+grub_cmd_list_db (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 = db; cert != NULL; cert = cert->next)
+    {
+      grub_printf ("Certificate %d:\n", cert_num);
+      grub_printf ("\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 ("\tissuer: %s\n", cert->issuer);
+      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 == false)
+    {
+      *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.
+         */
+      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_db, cmd_dbx_cert, cmd_db_cert;
+
+GRUB_MOD_INIT (appendedsig)
+{
+  int rc;
+  struct grub_module_header *header;
+
+  /*
+   * If secure boot is enabled with enforced mode,
+   * enable signature verification.
+   */
+  if (grub_ieee1275_is_secure_boot () == GRUB_SB_ENFORCED)
+    check_sigs = true;
+
+  db = 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));
+
+  /*
+   * If signature verification is enabled,
+   * extract trusted keys from ELF Note and store them in the db.
+   */
+  if (check_sigs == true)
+    {
+      FOR_MODULES (header)
+      {
+        struct grub_file pseudo_file;
+        struct x509_certificate *pk = NULL;
+        grub_err_t err;
+
+        /* Not an X.509 certificate, 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);
+
+        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 = db;
+        db = pk;
+      }
+    }
+
+  cmd_db_cert = grub_register_command ("append_add_db_cert", grub_cmd_db_cert, N_("X509_CERTIFICATE"),
+                                       N_("Add trusted X509_CERTIFICATE to the db list"));
+  cmd_dbx_cert = grub_register_command ("append_rm_dbx_cert", grub_cmd_dbx_cert, N_("X509_CERTIFICATE"),
+                                        N_("Remove distrusted X509_CERTIFICATE from the db list"));
+  cmd_list_db = grub_register_command ("append_list_db", grub_cmd_list_db, 0,
+                                       N_("Show the list of trusted X.509 certificates from the db list"));
+  cmd_verify = grub_register_command ("append_verify", grub_cmd_verify_signature, N_("SIGNED_FILE"),
+                                      N_("Verify SIGNED_FILE against the trusted X.509 certificates in the db list"));
+
+  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_db);
+  grub_unregister_command (cmd_db_cert);
+  grub_unregister_command (cmd_dbx_cert);
+}
diff --git a/include/grub/file.h b/include/grub/file.h
index a5bf3a792..d678de063 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.  */
-- 
2.39.5 (Apple Git-154)


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