From cf6b16f113b1b5e6efce79b569be1de3e504de8f 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 04/23] 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).
---
 include/grub/util/install.h |  8 ++++++--
 include/grub/util/mkimage.h |  4 ++--
 util/grub-install-common.c  | 15 +++++++++++---
 util/grub-mkimage.c         | 11 +++++++++++
 util/grub-mkimagexx.c       | 39 ++++++++++++++++++++++++++++++++++++-
 util/mkimage.c              | 13 +++++++------
 6 files changed, 76 insertions(+), 14 deletions(-)

--- a/include/grub/util/install.h
+++ b/include/grub/util/install.h
@@ -67,6 +67,9 @@
       N_("SBAT metadata"), 0 },						\
   { "disable-shim-lock", GRUB_INSTALL_OPTIONS_DISABLE_SHIM_LOCK, 0, 0,	\
       N_("disable shim_lock verifier"), 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 }
 
@@ -130,7 +133,8 @@
   GRUB_INSTALL_OPTIONS_INSTALL_CORE_COMPRESS,
   GRUB_INSTALL_OPTIONS_DTB,
   GRUB_INSTALL_OPTIONS_SBAT,
-  GRUB_INSTALL_OPTIONS_DISABLE_SHIM_LOCK
+  GRUB_INSTALL_OPTIONS_DISABLE_SHIM_LOCK,
+  GRUB_INSTALL_OPTIONS_APPENDED_SIGNATURE_SIZE
 };
 
 extern char *grub_install_source_directory;
@@ -190,7 +194,7 @@
 			     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 char *sbat_path, const int disable_shim_lock);
 
--- a/include/grub/util/mkimage.h
+++ b/include/grub/util/mkimage.h
@@ -51,12 +51,12 @@
 			   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);
 
--- a/util/grub-install-common.c
+++ b/util/grub-install-common.c
@@ -466,10 +466,12 @@
 static char *sbat;
 static int disable_shim_lock;
 static grub_compression_t compression;
+static size_t appsig_size;
 
 int
 grub_install_parse (int key, char *arg)
 {
+  const char *end;
   switch (key)
     {
     case GRUB_INSTALL_OPTIONS_INSTALL_CORE_COMPRESS:
@@ -567,6 +569,12 @@
       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;
     }
@@ -679,9 +687,11 @@
   *p = '\0';
 
   grub_util_info ("grub-mkimage --directory '%s' --prefix '%s' --output '%s'"
-		  " --format '%s' --compression '%s'%s%s%s\n",
+		  " --format '%s' --compression '%s'"
+		  " --appended-signature-size %zu%s%s%s\n",
 		  dir, prefix, outname,
 		  mkimage_target, compnames[compression],
+		  appsig_size,
 		  note ? " --note" : "",
 		  disable_shim_lock ? " --disable-shim-lock" : "", s);
   free (s);
@@ -693,7 +703,7 @@
   grub_install_generate_image (dir, prefix, fp, outname,
 			       modules.entries, memdisk_path,
 			       pubkeys, npubkeys, config_path, tgt,
-			       note, compression, dtb, sbat,
+			       note, appsig_size, compression, dtb, sbat,
 			       disable_shim_lock);
   while (dc--)
     grub_install_pop_module ();
--- a/util/grub-mkimage.c
+++ b/util/grub-mkimage.c
@@ -84,6 +84,7 @@
   {"sbat", 's', N_("FILE"), 0, N_("SBAT metadata"), 0},
   {"disable-shim-lock", GRUB_INSTALL_OPTIONS_DISABLE_SHIM_LOCK, 0, 0, N_("disable shim_lock verifier"), 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 }
 };
 
@@ -128,6 +129,7 @@
   char *sbat;
   int note;
   int disable_shim_lock;
+  size_t appsig_size;
   const struct grub_install_image_target_desc *image_target;
   grub_compression_t comp;
 };
@@ -138,6 +140,7 @@
   /* 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)
     {
@@ -170,6 +173,13 @@
       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);
@@ -324,6 +334,7 @@
 			       arguments.memdisk, arguments.pubkeys,
 			       arguments.npubkeys, arguments.config,
 			       arguments.image_target, arguments.note,
+			       arguments.appsig_size,
 			       arguments.comp, arguments.dtb,
 			       arguments.sbat, arguments.disable_shim_lock);
 
--- a/util/grub-mkimagexx.c
+++ b/util/grub-mkimagexx.c
@@ -85,6 +85,15 @@
   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
@@ -208,7 +217,7 @@
 
 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)
 {
@@ -222,6 +231,12 @@
   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;
 
@@ -485,6 +500,28 @@
       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));
--- a/util/mkimage.c
+++ b/util/mkimage.c
@@ -885,8 +885,9 @@
 			     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,
-			     const char *sbat_path, int disable_shim_lock)
+			     int note, size_t appsig_size, grub_compression_t comp,
+			     const char *dtb_path, const char *sbat_path, 
+			     int disable_shim_lock)
 {
   char *kernel_img, *core_img;
   size_t total_module_size, core_size;
@@ -1810,11 +1811,11 @@
 	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;
     }