powerpc/ieee1275: Add support 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: Rashmica Gupta <rashmica.g@gmail.com>
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>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

util/grub-mkimagexx.c

@@ -115,6 +115,14 @@ struct grub_sbat_note {
   char name[ALIGN_UP(sizeof(GRUB_SBAT_NOTE_NAME), 4)];
 };
 
+#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)];
+};
+
 static int
 is_relocatable (const struct grub_install_image_target_desc *image_target)
 {
@@ -216,7 +224,7 @@ 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 *sbat, char **core_img, size_t *core_size,
+				    int note, char *sbat, size_t appsig_size, char **core_img, size_t *core_size,
 				    Elf_Addr target_addr,
 				    struct grub_mkimage_layout *layout)
 {
@@ -237,6 +245,12 @@ SUFFIX (grub_mkimage_generate_elf) (const struct grub_install_image_target_desc
       footer_size += ALIGN_UP (sizeof (struct grub_sbat_note) + layout->sbat_size, 4);
     }
 
+  if (appsig_size)
+    {
+      phnum++;
+      footer_size += ALIGN_UP (sizeof (struct grub_appended_signature_note), 4);
+    }
+
   if (image_target->id != IMAGE_LOONGSON_ELF)
     phnum += 2;
 
@@ -518,6 +532,30 @@ SUFFIX (grub_mkimage_generate_elf) (const struct grub_install_image_target_desc
       memcpy (note_ptr->name, GRUB_SBAT_NOTE_NAME, sizeof (GRUB_SBAT_NOTE_NAME));
       memcpy ((char *)(note_ptr + 1), sbat, layout->sbat_size);
 
+      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 + footer_offset);
+      footer += note_size;
+      footer_offset += note_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 *) footer;
+
+      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);