0033-gpt-be-more-careful-about-relocating-backup-header.patch

From 53e4e887d6ca11fc302bae2c417ecab94ced1252 Mon Sep 17 00:00:00 2001
From: Michael Marineau <michael.marineau@coreos.com>
Date: Wed, 21 Sep 2016 13:52:52 -0700
Subject: [PATCH] gpt: be more careful about relocating backup header

The header was being relocated without checking the new location is
actually safe. If the BIOS thinks the disk is smaller than the OS then
repair may relocate the header into allocated space, failing the final
validation check. So only move it if the disk has grown.

Additionally, if the backup is valid then we can assume its current
location is good enough and leave it as-is.
---
 grub-core/lib/gpt.c | 16 ++++++++++------
 1 file changed, 10 insertions(+), 6 deletions(-)

diff --git a/grub-core/lib/gpt.c b/grub-core/lib/gpt.c
index 3c6ff35..35e65d8 100644
--- a/grub-core/lib/gpt.c
+++ b/grub-core/lib/gpt.c
@@ -599,7 +599,17 @@ grub_gpt_repair (grub_disk_t disk, grub_gpt_t gpt)
   if (grub_gpt_primary_valid (gpt))
     {
       grub_dprintf ("gpt", "primary GPT is valid\n");
+
+      /* Relocate backup to end if disk if the disk has grown.  */
       backup_header = grub_le_to_cpu64 (gpt->primary.alternate_lba);
+      if (grub_gpt_disk_size_valid (disk) &&
+	  disk->total_sectors - 1 > backup_header)
+	{
+	  backup_header = disk->total_sectors - 1;
+	  grub_dprintf ("gpt", "backup GPT header relocated to 0x%llx\n",
+			(unsigned long long) backup_header);
+	}
+
       grub_memcpy (&gpt->backup, &gpt->primary, sizeof (gpt->backup));
     }
   else if (grub_gpt_backup_valid (gpt))
@@ -611,12 +621,6 @@ grub_gpt_repair (grub_disk_t disk, grub_gpt_t gpt)
   else
     return grub_error (GRUB_ERR_BUG, "No valid GPT");
 
-  /* Relocate backup to end if disk whenever possible.  */
-  if (grub_gpt_disk_size_valid(disk))
-    backup_header = disk->total_sectors - 1;
-  grub_dprintf ("gpt", "backup GPT header will be located at 0x%llx\n",
-		(unsigned long long) backup_header);
-
   backup_entries = backup_header -
     grub_gpt_size_to_sectors (gpt, gpt->entries_size);
   grub_dprintf ("gpt", "backup GPT entries will be located at 0x%llx\n",
-- 
2.21.3