kern: Add lockdown support

When the GRUB starts on a secure boot platform, some commands can be
used to subvert the protections provided by the verification mechanism and
could lead to booting untrusted system.

To prevent that situation, allow GRUB to be locked down. That way the code
may check if GRUB has been locked down and further restrict the commands
that are registered or what subset of their functionality could be used.

The lockdown support adds the following components:

* The grub_lockdown() function which can be used to lockdown GRUB if,
  e.g., UEFI Secure Boot is enabled.

* The grub_is_lockdown() function which can be used to check if the GRUB
  was locked down.

* A verifier that flags OS kernels, the GRUB modules, Device Trees and ACPI
  tables as GRUB_VERIFY_FLAGS_DEFER_AUTH to defer verification to other
  verifiers. These files are only successfully verified if another registered
  verifier returns success. Otherwise, the whole verification process fails.

  For example, PE/COFF binaries verification can be done by the shim_lock
  verifier which validates the signatures using the shim_lock protocol.
  However, the verification is not deferred directly to the shim_lock verifier.
  The shim_lock verifier is hooked into the verification process instead.

* A set of grub_{command,extcmd}_lockdown functions that can be used by
  code registering command handlers, to only register unsafe commands if
  the GRUB has not been locked down.

Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

conf/Makefile.common

@@ -84,7 +84,9 @@ CPPFLAGS_PARTTOOL_LIST = -Dgrub_parttool_register=PARTTOOL_LIST_MARKER
 CPPFLAGS_TERMINAL_LIST = '-Dgrub_term_register_input(...)=INPUT_TERMINAL_LIST_MARKER(__VA_ARGS__)'
 CPPFLAGS_TERMINAL_LIST += '-Dgrub_term_register_output(...)=OUTPUT_TERMINAL_LIST_MARKER(__VA_ARGS__)'
 CPPFLAGS_COMMAND_LIST = '-Dgrub_register_command(...)=COMMAND_LIST_MARKER(__VA_ARGS__)'
+CPPFLAGS_COMMAND_LIST += '-Dgrub_register_command_lockdown(...)=COMMAND_LOCKDOWN_LIST_MARKER(__VA_ARGS__)'
 CPPFLAGS_COMMAND_LIST += '-Dgrub_register_extcmd(...)=EXTCOMMAND_LIST_MARKER(__VA_ARGS__)'
+CPPFLAGS_COMMAND_LIST += '-Dgrub_register_extcmd_lockdown(...)=EXTCOMMAND_LOCKDOWN_LIST_MARKER(__VA_ARGS__)'
 CPPFLAGS_COMMAND_LIST += '-Dgrub_register_command_p1(...)=P1COMMAND_LIST_MARKER(__VA_ARGS__)'
 CPPFLAGS_FDT_LIST := '-Dgrub_fdtbus_register(...)=FDT_DRIVER_LIST_MARKER(__VA_ARGS__)'
 CPPFLAGS_MARKER = $(CPPFLAGS_FS_LIST) $(CPPFLAGS_VIDEO_LIST) \

docs/grub-dev.texi

@@ -86,6 +86,7 @@ This edition documents version @value{VERSION}.
 * PFF2 Font File Format::
 * Graphical Menu Software Design::
 * Verifiers framework::
+* Lockdown framework::
 * Copying This Manual::         Copying This Manual
 * Index::
 @end menu
@@ -2123,6 +2124,32 @@ Optionally at the end of the file @samp{fini}, if it exists, is called with just
 the context. If you return no error during any of @samp{init}, @samp{write} and
 @samp{fini} then the file is considered as having succeded verification.
 
+@node Lockdown framework
+@chapter Lockdown framework
+
+The GRUB can be locked down, which is a restricted mode where some operations
+are not allowed. For instance, some commands cannot be used when the GRUB is
+locked down.
+
+The function
+@code{grub_lockdown()} is used to lockdown GRUB and the function
+@code{grub_is_lockdown()} function can be used to check whether lockdown is
+enabled or not. When enabled, the function returns @samp{GRUB_LOCKDOWN_ENABLED}
+and @samp{GRUB_LOCKDOWN_DISABLED} when is not enabled.
+
+The following functions can be used to register the commands that can only be
+used when lockdown is disabled:
+
+@itemize
+
+@item @code{grub_cmd_lockdown()} registers command which should not run when the
+GRUB is in lockdown mode.
+
+@item @code{grub_cmd_lockdown()} registers extended command which should not run
+when the GRUB is in lockdown mode.
+
+@end itemize
+
 @node Copying This Manual
 @appendix Copying This Manual
 

docs/grub.texi

@@ -5749,6 +5749,7 @@ environment variables and commands are listed in the same order.
 * Using digital signatures::         Booting digitally signed code
 * UEFI secure boot and shim::        Booting digitally signed PE files
 * Measured Boot::                    Measuring boot components
+* Lockdown::                         Lockdown when booting on a secure setup
 @end menu
 
 @node Authentication and authorisation
@@ -5958,6 +5959,13 @@ into @file{core.img} in order to avoid a potential gap in measurement between
 
 Measured boot is currently only supported on EFI platforms.
 
+@node Lockdown
+@section Lockdown when booting on a secure setup
+
+The GRUB can be locked down when booted on a secure boot environment, for example
+if the UEFI secure boot is enabled. On a locked down configuration, the GRUB will
+be restricted and some operations/commands cannot be executed.
+
 @node Platform limitations
 @chapter Platform limitations
 

grub-core/Makefile.am

@@ -80,6 +80,7 @@ KERNEL_HEADER_FILES += $(top_srcdir)/include/grub/fs.h
 KERNEL_HEADER_FILES += $(top_srcdir)/include/grub/i18n.h
 KERNEL_HEADER_FILES += $(top_srcdir)/include/grub/kernel.h
 KERNEL_HEADER_FILES += $(top_srcdir)/include/grub/list.h
+KERNEL_HEADER_FILES += $(top_srcdir)/include/grub/lockdown.h
 KERNEL_HEADER_FILES += $(top_srcdir)/include/grub/misc.h
 if COND_emu
 KERNEL_HEADER_FILES += $(top_srcdir)/include/grub/compiler-rt-emu.h
@@ -377,8 +378,10 @@ command.lst: $(MARKER_FILES)
 	  b=`basename $$pp .marker`; \
 	  sed -n \
 	    -e "/EXTCOMMAND_LIST_MARKER *( *\"/{s/.*( *\"\([^\"]*\)\".*/*\1: $$b/;p;}" \
+	    -e "/EXTCOMMAND_LOCKDOWN_LIST_MARKER *( *\"/{s/.*( *\"\([^\"]*\)\".*/*\1: $$b/;p;}" \
 	    -e "/P1COMMAND_LIST_MARKER *( *\"/{s/.*( *\"\([^\"]*\)\".*/*\1: $$b/;p;}" \
-	    -e "/COMMAND_LIST_MARKER *( *\"/{s/.*( *\"\([^\"]*\)\".*/\1: $$b/;p;}" $$pp; \
+	    -e "/COMMAND_LIST_MARKER *( *\"/{s/.*( *\"\([^\"]*\)\".*/\1: $$b/;p;}" \
+	    -e "/COMMAND_LOCKDOWN_LIST_MARKER *( *\"/{s/.*( *\"\([^\"]*\)\".*/\1: $$b/;p;}" $$pp; \
 	done) | sort -u > $@
 platform_DATA += command.lst
 CLEANFILES += command.lst

grub-core/Makefile.core.def

@@ -205,6 +205,7 @@ kernel = {
   efi = kern/acpi.c;
   efi = kern/efi/acpi.c;
   efi = kern/efi/sb.c;
+  efi = kern/lockdown.c;
   i386_coreboot = kern/i386/pc/acpi.c;
   i386_multiboot = kern/i386/pc/acpi.c;
   i386_coreboot = kern/acpi.c;

grub-core/commands/extcmd.c

@@ -19,6 +19,7 @@
 
 #include <grub/mm.h>
 #include <grub/list.h>
+#include <grub/lockdown.h>
 #include <grub/misc.h>
 #include <grub/extcmd.h>
 #include <grub/script_sh.h>
@@ -110,6 +111,28 @@ grub_register_extcmd (const char *name, grub_extcmd_func_t func,
 				    summary, description, parser, 1);
 }
 
+static grub_err_t
+grub_extcmd_lockdown (grub_extcmd_context_t ctxt __attribute__ ((unused)),
+                      int argc __attribute__ ((unused)),
+                      char **argv __attribute__ ((unused)))
+{
+  return grub_error (GRUB_ERR_ACCESS_DENIED,
+                     N_("%s: the command is not allowed when lockdown is enforced"),
+                     ctxt->extcmd->cmd->name);
+}
+
+grub_extcmd_t
+grub_register_extcmd_lockdown (const char *name, grub_extcmd_func_t func,
+                               grub_command_flags_t flags, const char *summary,
+                               const char *description,
+                               const struct grub_arg_option *parser)
+{
+  if (grub_is_lockdown () == GRUB_LOCKDOWN_ENABLED)
+    func = grub_extcmd_lockdown;
+
+  return grub_register_extcmd (name, func, flags, summary, description, parser);
+}
+
 void
 grub_unregister_extcmd (grub_extcmd_t ext)
 {

grub-core/kern/command.c

@@ -17,6 +17,7 @@
  *  along with GRUB.  If not, see <http://www.gnu.org/licenses/>.
  */
 
+#include <grub/lockdown.h>
 #include <grub/mm.h>
 #include <grub/command.h>
 
@@ -77,6 +78,29 @@ grub_register_command_prio (const char *name,
   return cmd;
 }
 
+static grub_err_t
+grub_cmd_lockdown (grub_command_t cmd __attribute__ ((unused)),
+                   int argc __attribute__ ((unused)),
+                   char **argv __attribute__ ((unused)))
+
+{
+  return grub_error (GRUB_ERR_ACCESS_DENIED,
+                     N_("%s: the command is not allowed when lockdown is enforced"),
+                     cmd->name);
+}
+
+grub_command_t
+grub_register_command_lockdown (const char *name,
+                                grub_command_func_t func,
+                                const char *summary,
+                                const char *description)
+{
+  if (grub_is_lockdown () == GRUB_LOCKDOWN_ENABLED)
+    func = grub_cmd_lockdown;
+
+  return grub_register_command_prio (name, func, summary, description, 0);
+}
+
 void
 grub_unregister_command (grub_command_t cmd)
 {

grub-core/kern/lockdown.c

@@ -0,0 +1,80 @@
+/*
+ *  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/>.
+ *
+ */
+
+#include <grub/dl.h>
+#include <grub/file.h>
+#include <grub/lockdown.h>
+#include <grub/verify.h>
+
+static int lockdown = GRUB_LOCKDOWN_DISABLED;
+
+static grub_err_t
+lockdown_verifier_init (grub_file_t io __attribute__ ((unused)),
+                    enum grub_file_type type,
+                    void **context __attribute__ ((unused)),
+                    enum grub_verify_flags *flags)
+{
+  *flags = GRUB_VERIFY_FLAGS_SKIP_VERIFICATION;
+
+  switch (type & GRUB_FILE_TYPE_MASK)
+    {
+    case GRUB_FILE_TYPE_GRUB_MODULE:
+    case GRUB_FILE_TYPE_LINUX_KERNEL:
+    case GRUB_FILE_TYPE_MULTIBOOT_KERNEL:
+    case GRUB_FILE_TYPE_XEN_HYPERVISOR:
+    case GRUB_FILE_TYPE_BSD_KERNEL:
+    case GRUB_FILE_TYPE_XNU_KERNEL:
+    case GRUB_FILE_TYPE_PLAN9_KERNEL:
+    case GRUB_FILE_TYPE_NTLDR:
+    case GRUB_FILE_TYPE_TRUECRYPT:
+    case GRUB_FILE_TYPE_FREEDOS:
+    case GRUB_FILE_TYPE_PXECHAINLOADER:
+    case GRUB_FILE_TYPE_PCCHAINLOADER:
+    case GRUB_FILE_TYPE_COREBOOT_CHAINLOADER:
+    case GRUB_FILE_TYPE_EFI_CHAINLOADED_IMAGE:
+    case GRUB_FILE_TYPE_ACPI_TABLE:
+    case GRUB_FILE_TYPE_DEVICE_TREE_IMAGE:
+      *flags = GRUB_VERIFY_FLAGS_DEFER_AUTH;
+
+      /* Fall through. */
+
+    default:
+      return GRUB_ERR_NONE;
+    }
+}
+
+struct grub_file_verifier lockdown_verifier =
+  {
+    .name = "lockdown_verifier",
+    .init = lockdown_verifier_init,
+  };
+
+void
+grub_lockdown (void)
+{
+  lockdown = GRUB_LOCKDOWN_ENABLED;
+
+  grub_verifier_register (&lockdown_verifier);
+}
+
+int
+grub_is_lockdown (void)
+{
+  return lockdown;
+}

include/grub/command.h

@@ -86,6 +86,11 @@ EXPORT_FUNC(grub_register_command_prio) (const char *name,
 					 const char *summary,
 					 const char *description,
 					 int prio);
+grub_command_t
+EXPORT_FUNC(grub_register_command_lockdown) (const char *name,
+                                             grub_command_func_t func,
+                                             const char *summary,
+                                             const char *description);
 void EXPORT_FUNC(grub_unregister_command) (grub_command_t cmd);
 
 static inline grub_command_t

include/grub/extcmd.h

@@ -62,6 +62,13 @@ grub_extcmd_t EXPORT_FUNC(grub_register_extcmd) (const char *name,
 						 const char *description,
 						 const struct grub_arg_option *parser);
 
+grub_extcmd_t EXPORT_FUNC(grub_register_extcmd_lockdown) (const char *name,
+                                                          grub_extcmd_func_t func,
+                                                          grub_command_flags_t flags,
+                                                          const char *summary,
+                                                          const char *description,
+                                                          const struct grub_arg_option *parser);
+
 grub_extcmd_t EXPORT_FUNC(grub_register_extcmd_prio) (const char *name,
 						      grub_extcmd_func_t func,
 						      grub_command_flags_t flags,

include/grub/lockdown.h

@@ -0,0 +1,44 @@
+/*
+ *  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/>.
+ */
+
+#ifndef GRUB_LOCKDOWN_H
+#define GRUB_LOCKDOWN_H 1
+
+#include <grub/symbol.h>
+
+#define GRUB_LOCKDOWN_DISABLED       0
+#define GRUB_LOCKDOWN_ENABLED        1
+
+#ifdef GRUB_MACHINE_EFI
+extern void
+EXPORT_FUNC (grub_lockdown) (void);
+extern int
+EXPORT_FUNC (grub_is_lockdown) (void);
+#else
+static inline void
+grub_lockdown (void)
+{
+}
+
+static inline int
+grub_is_lockdown (void)
+{
+  return GRUB_LOCKDOWN_DISABLED;
+}
+#endif
+#endif /* ! GRUB_LOCKDOWN_H */