From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Rashmica Gupta <rashmica.g@gmail.com>
Date: Thu, 11 Jun 2020 11:26:23 +1000
Subject: [PATCH] Add suport for signing grub with an appended signature

Add infrastructure to allow firmware to verify the integrity of grub
by use of a Linux-kernel-module-style appended signature. We initially
target powerpc-ieee1275, but the code should be extensible to other
platforms.

Usually these signatures are appended to a file without modifying the
ELF file itself. (This is what the 'sign-file' tool does, for example.)
The verifier loads the signed file from the file system and looks at the
end of the file for the appended signature. However, on powerpc-ieee1275
platforms, the bootloader is often stored directly in the PReP partition
as raw bytes without a file-system. This makes determining the location
of an appended signature more difficult.

To address this, we add a new ELF note.

The name field of shall be the string "Appended-Signature", zero-padded
to 4 byte alignment. The type field shall be 0x41536967 (the ASCII values
for the string "ASig"). It must be the final section in the ELF binary.

The description shall contain the appended signature structure as defined
by the Linux kernel. The description will also be padded to be a multiple
of 4 bytes. The padding shall be added before the appended signature
structure (not at the end) so that the final bytes of a signed ELF file
are the appended signature magic.

A subsequent patch documents how to create a grub core.img validly signed
under this scheme.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Rashmica Gupta <rashmica.g@gmail.com>

---

You can experiment with this code with a patched version of SLOF
that verifies these signatures. You can find one at:
   https://github.com/daxtens/SLOF

I will be proposing this for inclusion in a future Power Architecture
Platform Reference (PAPR).
---
 util/grub-install-common.c  | 16 +++++++++++++---
 util/grub-mkimage.c         | 11 +++++++++++
 util/grub-mkimagexx.c       | 39 ++++++++++++++++++++++++++++++++++++++-
 util/mkimage.c              | 10 +++++-----
 include/grub/util/install.h |  8 ++++++--
 include/grub/util/mkimage.h |  4 ++--
 6 files changed, 75 insertions(+), 13 deletions(-)

diff --git a/util/grub-install-common.c b/util/grub-install-common.c
index cf993c059ad..561e671ff34 100644
--- a/util/grub-install-common.c
+++ b/util/grub-install-common.c
@@ -303,10 +303,12 @@ handle_install_list (struct install_list *il, const char *val,
 static char **pubkeys;
 static size_t npubkeys;
 static grub_compression_t compression;
+static size_t appsig_size;
 
 int
 grub_install_parse (int key, char *arg)
 {
+  const char *end;
   switch (key)
     {
     case 'C':
@@ -395,6 +397,12 @@ grub_install_parse (int key, char *arg)
       grub_util_error (_("Unrecognized compression `%s'"), arg);
     case GRUB_INSTALL_OPTIONS_GRUB_MKIMAGE:
       return 1;
+    case GRUB_INSTALL_OPTIONS_APPENDED_SIGNATURE_SIZE:
+      grub_errno = 0;
+      appsig_size = grub_strtol(arg, &end, 10);
+      if (grub_errno)
+        return 0;
+      return 1;
     default:
       return 0;
     }
@@ -493,10 +501,12 @@ grub_install_make_image_wrap_file (const char *dir, const char *prefix,
   grub_util_info ("grub-mkimage --directory '%s' --prefix '%s'"
 		  " --output '%s' "
 		  " --dtb '%s' "
-		  "--format '%s' --compression '%s' %s %s\n",
+		  "--format '%s' --compression '%s' "
+		  "--appended-signature-size %zu %s %s\n",
 		  dir, prefix,
 		  outname, dtb ? : "", mkimage_target,
-		  compnames[compression], note ? "--note" : "", s);
+		  compnames[compression], appsig_size,
+		  note ? "--note" : "", s);
   free (s);
 
   tgt = grub_install_get_image_target (mkimage_target);
@@ -506,7 +516,7 @@ grub_install_make_image_wrap_file (const char *dir, const char *prefix,
   grub_install_generate_image (dir, prefix, fp, outname,
 			       modules.entries, memdisk_path,
 			       pubkeys, npubkeys, config_path, tgt,
-			       note, compression, dtb);
+			       note, appsig_size, compression, dtb);
   while (dc--)
     grub_install_pop_module ();
 }
diff --git a/util/grub-mkimage.c b/util/grub-mkimage.c
index 98d24cc06ea..65a015d8a04 100644
--- a/util/grub-mkimage.c
+++ b/util/grub-mkimage.c
@@ -82,6 +82,7 @@ static struct argp_option options[] = {
   {"format",  'O', N_("FORMAT"), 0, 0, 0},
   {"compression",  'C', "(xz|none|auto)", 0, N_("choose the compression to use for core image"), 0},
   {"verbose",     'v', 0,      0, N_("print verbose messages."), 0},
+  {"appended-signature-size", 'S', N_("SIZE"), 0, N_("Add a note segment reserving SIZE bytes for an appended signature"), 0},
   { 0, 0, 0, 0, 0, 0 }
 };
 
@@ -124,6 +125,7 @@ struct arguments
   char *font;
   char *config;
   int note;
+  size_t appsig_size;
   const struct grub_install_image_target_desc *image_target;
   grub_compression_t comp;
 };
@@ -134,6 +136,7 @@ argp_parser (int key, char *arg, struct argp_state *state)
   /* Get the input argument from argp_parse, which we
      know is a pointer to our arguments structure. */
   struct arguments *arguments = state->input;
+  const char* end;
 
   switch (key)
     {
@@ -166,6 +169,13 @@ argp_parser (int key, char *arg, struct argp_state *state)
       arguments->note = 1;
       break;
 
+    case 'S':
+      grub_errno = 0;
+      arguments->appsig_size = grub_strtol(arg, &end, 10);
+      if (grub_errno)
+        return 0;
+      break;
+
     case 'm':
       if (arguments->memdisk)
 	free (arguments->memdisk);
@@ -309,6 +319,7 @@ main (int argc, char *argv[])
 			       arguments.memdisk, arguments.pubkeys,
 			       arguments.npubkeys, arguments.config,
 			       arguments.image_target, arguments.note,
+			       arguments.appsig_size,
 			       arguments.comp, arguments.dtb);
 
   grub_util_file_sync  (fp);
diff --git a/util/grub-mkimagexx.c b/util/grub-mkimagexx.c
index f9aa1a033b5..1bb5eb84c14 100644
--- a/util/grub-mkimagexx.c
+++ b/util/grub-mkimagexx.c
@@ -82,6 +82,15 @@ struct grub_ieee1275_note
   struct grub_ieee1275_note_desc descriptor;
 };
 
+#define GRUB_APPENDED_SIGNATURE_NOTE_NAME "Appended-Signature"
+#define GRUB_APPENDED_SIGNATURE_NOTE_TYPE 0x41536967 /* "ASig" */
+
+struct grub_appended_signature_note
+{
+  Elf32_Nhdr header;
+  char name[ALIGN_UP(sizeof (GRUB_APPENDED_SIGNATURE_NOTE_NAME), 4)];
+};
+
 #define GRUB_XEN_NOTE_NAME "Xen"
 
 struct fixup_block_list
@@ -205,7 +214,7 @@ grub_arm_reloc_jump24 (grub_uint32_t *target, Elf32_Addr sym_addr)
 
 void
 SUFFIX (grub_mkimage_generate_elf) (const struct grub_install_image_target_desc *image_target,
-				    int note, char **core_img, size_t *core_size,
+				    int note, size_t appsig_size, char **core_img, size_t *core_size,
 				    Elf_Addr target_addr,
 				    struct grub_mkimage_layout *layout)
 {
@@ -219,6 +228,12 @@ SUFFIX (grub_mkimage_generate_elf) (const struct grub_install_image_target_desc
   int shnum = 4;
   int string_size = sizeof (".text") + sizeof ("mods") + 1;
 
+  if (appsig_size)
+    {
+      phnum++;
+      footer_size += ALIGN_UP(sizeof (struct grub_appended_signature_note) + appsig_size, 4);
+    }
+
   if (image_target->id != IMAGE_LOONGSON_ELF)
     phnum += 2;
 
@@ -449,6 +464,28 @@ SUFFIX (grub_mkimage_generate_elf) (const struct grub_install_image_target_desc
       phdr->p_offset = grub_host_to_target32 (header_size + program_size);
     }
 
+  if (appsig_size) {
+    int note_size = ALIGN_UP(sizeof (struct grub_appended_signature_note) + appsig_size, 4);
+    struct grub_appended_signature_note *note_ptr = (struct grub_appended_signature_note *)
+      (elf_img + program_size + header_size + (note ? sizeof (struct grub_ieee1275_note) : 0));
+
+    note_ptr->header.n_namesz = grub_host_to_target32 (sizeof (GRUB_APPENDED_SIGNATURE_NOTE_NAME));
+    /* needs to sit at the end, so we round this up and sign some zero padding */
+    note_ptr->header.n_descsz = grub_host_to_target32 (ALIGN_UP(appsig_size, 4));
+    note_ptr->header.n_type = grub_host_to_target32 (GRUB_APPENDED_SIGNATURE_NOTE_TYPE);
+    strcpy (note_ptr->name, GRUB_APPENDED_SIGNATURE_NOTE_NAME);
+
+    phdr++;
+    phdr->p_type = grub_host_to_target32 (PT_NOTE);
+    phdr->p_flags = grub_host_to_target32 (PF_R);
+    phdr->p_align = grub_host_to_target32 (image_target->voidp_sizeof);
+    phdr->p_vaddr = 0;
+    phdr->p_paddr = 0;
+    phdr->p_filesz = grub_host_to_target32 (note_size);
+    phdr->p_memsz = 0;
+    phdr->p_offset = grub_host_to_target32 (header_size + program_size + (note ? sizeof (struct grub_ieee1275_note) : 0));
+  }
+
   {
     char *str_start = (elf_img + sizeof (*ehdr) + phnum * sizeof (*phdr)
 		       + shnum * sizeof (*shdr));
diff --git a/util/mkimage.c b/util/mkimage.c
index e22d82afa61..a81120f26be 100644
--- a/util/mkimage.c
+++ b/util/mkimage.c
@@ -777,7 +777,7 @@ grub_install_generate_image (const char *dir, const char *prefix,
 			     char *memdisk_path, char **pubkey_paths,
 			     size_t npubkeys, char *config_path,
 			     const struct grub_install_image_target_desc *image_target,
-			     int note, grub_compression_t comp, const char *dtb_path)
+			     int note, size_t appsig_size, grub_compression_t comp, const char *dtb_path)
 {
   char *kernel_img, *core_img;
   size_t total_module_size, core_size;
@@ -1694,11 +1694,11 @@ grub_install_generate_image (const char *dir, const char *prefix,
 	else
 	  target_addr = image_target->link_addr;
 	if (image_target->voidp_sizeof == 4)
-	  grub_mkimage_generate_elf32 (image_target, note, &core_img, &core_size,
-				       target_addr, &layout);
+	  grub_mkimage_generate_elf32 (image_target, note, appsig_size, &core_img,
+				       &core_size, target_addr, &layout);
 	else
-	  grub_mkimage_generate_elf64 (image_target, note, &core_img, &core_size,
-				       target_addr, &layout);
+	  grub_mkimage_generate_elf64 (image_target, note, appsig_size, &core_img,
+				       &core_size, target_addr, &layout);
       }
       break;
     }
diff --git a/include/grub/util/install.h b/include/grub/util/install.h
index 0dba8b67f93..ba5e6a2ea8f 100644
--- a/include/grub/util/install.h
+++ b/include/grub/util/install.h
@@ -63,6 +63,9 @@
     /* TRANSLATORS: "embed" is a verb (command description).  "*/	\
   { "pubkey",   'k', N_("FILE"), 0,					\
       N_("embed FILE as public key for signature checking"), 0},	\
+  { "appended-signature-size", GRUB_INSTALL_OPTIONS_APPENDED_SIGNATURE_SIZE,\
+    "SIZE", 0, N_("Add a note segment reserving SIZE bytes for an appended signature"), \
+    1},                                                                 \
   { "verbose", 'v', 0, 0,						\
     N_("print verbose messages."), 1 }
 
@@ -119,7 +122,8 @@ enum grub_install_options {
   GRUB_INSTALL_OPTIONS_THEMES_DIRECTORY,
   GRUB_INSTALL_OPTIONS_GRUB_MKIMAGE,
   GRUB_INSTALL_OPTIONS_INSTALL_CORE_COMPRESS,
-  GRUB_INSTALL_OPTIONS_DTB
+  GRUB_INSTALL_OPTIONS_DTB,
+  GRUB_INSTALL_OPTIONS_APPENDED_SIGNATURE_SIZE
 };
 
 extern char *grub_install_source_directory;
@@ -179,7 +183,7 @@ grub_install_generate_image (const char *dir, const char *prefix,
 			     size_t npubkeys,
 			     char *config_path,
 			     const struct grub_install_image_target_desc *image_target,
-			     int note,
+			     int note, size_t appsig_size,
 			     grub_compression_t comp, const char *dtb_file);
 
 const struct grub_install_image_target_desc *
diff --git a/include/grub/util/mkimage.h b/include/grub/util/mkimage.h
index b3a5ca132bc..cef7fffa7ae 100644
--- a/include/grub/util/mkimage.h
+++ b/include/grub/util/mkimage.h
@@ -50,12 +50,12 @@ grub_mkimage_load_image64 (const char *kernel_path,
 			   const struct grub_install_image_target_desc *image_target);
 void
 grub_mkimage_generate_elf32 (const struct grub_install_image_target_desc *image_target,
-			     int note, char **core_img, size_t *core_size,
+			     int note, size_t appsig_size, char **core_img, size_t *core_size,
 			     Elf32_Addr target_addr,
 			     struct grub_mkimage_layout *layout);
 void
 grub_mkimage_generate_elf64 (const struct grub_install_image_target_desc *image_target,
-			     int note, char **core_img, size_t *core_size,
+			     int note, size_t appsig_size, char **core_img, size_t *core_size,
 			     Elf64_Addr target_addr,
 			     struct grub_mkimage_layout *layout);
 
