grub2/grub-core/kern/efi/sb.c

/*
 *  GRUB  --  GRand Unified Bootloader
 *  Copyright (C) 2020  Free Software Foundation, Inc.
 *
 *  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/>.
 *
 *  UEFI Secure Boot related checkings.
 */

#include <grub/efi/efi.h>
#include <grub/efi/pe32.h>
#include <grub/efi/sb.h>
#include <grub/err.h>
#include <grub/file.h>
#include <grub/i386/linux.h>
#include <grub/mm.h>
#include <grub/types.h>
#include <grub/verify.h>

static grub_efi_guid_t shim_lock_guid = GRUB_EFI_SHIM_LOCK_GUID;

/* List of modules which cannot be loaded if UEFI secure boot mode is enabled. */
static const char * const disabled_mods[] = {"iorw", "memrw", "wrmsr", NULL};

/*
 * Determine whether we're in secure boot mode.
 *
 * Please keep the logic in sync with the Linux kernel,
 * drivers/firmware/efi/libstub/secureboot.c:efi_get_secureboot().
 */
grub_uint8_t
grub_efi_get_secureboot (void)
{
  static grub_efi_guid_t efi_variable_guid = GRUB_EFI_GLOBAL_VARIABLE_GUID;
  static grub_efi_guid_t efi_shim_lock_guid = GRUB_EFI_SHIM_LOCK_GUID;
  grub_efi_status_t status;
  grub_efi_uint32_t attr = 0;
  grub_size_t size = 0;
  grub_uint8_t *secboot = NULL;
  grub_uint8_t *setupmode = NULL;
  grub_uint8_t *moksbstate = NULL;
  grub_uint8_t secureboot = GRUB_EFI_SECUREBOOT_MODE_UNKNOWN;
  const char *secureboot_str = "UNKNOWN";

  status = grub_efi_get_variable ("SecureBoot", &efi_variable_guid,
				  &size, (void **) &secboot);

  if (status == GRUB_EFI_NOT_FOUND)
    {
      secureboot = GRUB_EFI_SECUREBOOT_MODE_DISABLED;
      goto out;
    }

  if (status != GRUB_EFI_SUCCESS)
    goto out;

  status = grub_efi_get_variable ("SetupMode", &efi_variable_guid,
				  &size, (void **) &setupmode);

  if (status != GRUB_EFI_SUCCESS)
    goto out;

  if ((*secboot == 0) || (*setupmode == 1))
    {
      secureboot = GRUB_EFI_SECUREBOOT_MODE_DISABLED;
      goto out;
    }

  /*
   * See if a user has put the shim into insecure mode. If so, and if the
   * variable doesn't have the runtime attribute set, we might as well
   * honor that.
   */
  status = grub_efi_get_variable_with_attributes ("MokSBState", &efi_shim_lock_guid,
						  &size, (void **) &moksbstate, &attr);

  /* If it fails, we don't care why. Default to secure. */
  if (status != GRUB_EFI_SUCCESS)
    {
      secureboot = GRUB_EFI_SECUREBOOT_MODE_ENABLED;
      goto out;
    }

  if (!(attr & GRUB_EFI_VARIABLE_RUNTIME_ACCESS) && *moksbstate == 1)
    {
      secureboot = GRUB_EFI_SECUREBOOT_MODE_DISABLED;
      goto out;
    }

  secureboot = GRUB_EFI_SECUREBOOT_MODE_ENABLED;

 out:
  grub_free (moksbstate);
  grub_free (setupmode);
  grub_free (secboot);

  if (secureboot == GRUB_EFI_SECUREBOOT_MODE_DISABLED)
    secureboot_str = "Disabled";
  else if (secureboot == GRUB_EFI_SECUREBOOT_MODE_ENABLED)
    secureboot_str = "Enabled";

  grub_dprintf ("efi", "UEFI Secure Boot state: %s\n", secureboot_str);

  return secureboot;
}

static grub_err_t
shim_lock_verifier_init (grub_file_t io __attribute__ ((unused)),
			 enum grub_file_type type,
			 void **context __attribute__ ((unused)),
			 enum grub_verify_flags *flags)
{
  const char *b, *e;
  int i;

  *flags = GRUB_VERIFY_FLAGS_SKIP_VERIFICATION;

  switch (type & GRUB_FILE_TYPE_MASK)
    {
    case GRUB_FILE_TYPE_GRUB_MODULE:
      /* Establish GRUB module name. */
      b = grub_strrchr (io->name, '/');
      e = grub_strrchr (io->name, '.');

      b = b ? (b + 1) : io->name;
      e = e ? e : io->name + grub_strlen (io->name);
      e = (e > b) ? e : io->name + grub_strlen (io->name);

      for (i = 0; disabled_mods[i]; i++)
	if (!grub_strncmp (b, disabled_mods[i], grub_strlen (b) - grub_strlen (e)))
	  {
	    grub_error (GRUB_ERR_ACCESS_DENIED,
			N_("module cannot be loaded in UEFI secure boot mode: %s"),
			io->name);
	    return GRUB_ERR_ACCESS_DENIED;
	  }

      /* Fall through. */

    case GRUB_FILE_TYPE_ACPI_TABLE:
    case GRUB_FILE_TYPE_DEVICE_TREE_IMAGE:
      *flags = GRUB_VERIFY_FLAGS_DEFER_AUTH;

      return GRUB_ERR_NONE;

    case GRUB_FILE_TYPE_LINUX_KERNEL:
    case GRUB_FILE_TYPE_MULTIBOOT_KERNEL:
    case GRUB_FILE_TYPE_BSD_KERNEL:
    case GRUB_FILE_TYPE_XNU_KERNEL:
    case GRUB_FILE_TYPE_PLAN9_KERNEL:
      for (i = 0; disabled_mods[i]; i++)
	if (grub_dl_get (disabled_mods[i]))
	  {
	    grub_error (GRUB_ERR_ACCESS_DENIED,
			N_("cannot boot due to dangerous module in memory: %s"),
			disabled_mods[i]);
	    return GRUB_ERR_ACCESS_DENIED;
	  }

      *flags = GRUB_VERIFY_FLAGS_SINGLE_CHUNK;

      /* Fall through. */

    default:
      return GRUB_ERR_NONE;
    }
}

static grub_err_t
shim_lock_verifier_write (void *context __attribute__ ((unused)), void *buf, grub_size_t size)
{
  grub_efi_shim_lock_protocol_t *sl = grub_efi_locate_protocol (&shim_lock_guid, 0);

  if (!sl)
    return grub_error (GRUB_ERR_ACCESS_DENIED, N_("shim_lock protocol not found"));

  if (sl->verify (buf, size) != GRUB_EFI_SUCCESS)
    return grub_error (GRUB_ERR_BAD_SIGNATURE, N_("bad shim signature"));

  return GRUB_ERR_NONE;
}

struct grub_file_verifier shim_lock_verifier =
  {
    .name = "shim_lock_verifier",
    .init = shim_lock_verifier_init,
    .write = shim_lock_verifier_write
  };

void
grub_shim_lock_verifier_setup (void)
{
  grub_efi_shim_lock_protocol_t *sl =
    grub_efi_locate_protocol (&shim_lock_guid, 0);

  if (!sl)
    return;

  if (grub_efi_get_secureboot () != GRUB_EFI_SECUREBOOT_MODE_ENABLED)
    return;

  grub_verifier_register (&shim_lock_verifier);
}