disk/lvm: Sanitize rlocn->offset to prevent wild read

rlocn->offset is read directly from disk and added to the metadatabuf
pointer to create a pointer to a block of metadata. It's a 64-bit
quantity so as long as you don't overflow you can set subsequent
pointers to point anywhere in memory.

Require that rlocn->offset fits within the metadata buffer size.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

grub-core/disk/lvm.c

@@ -248,6 +248,14 @@ grub_lvm_detect (grub_disk_t disk,
     }
 
   rlocn = mdah->raw_locns;
+  if (grub_le_to_cpu64 (rlocn->offset) >= grub_le_to_cpu64 (mda_size))
+    {
+#ifdef GRUB_UTIL
+      grub_util_info ("metadata offset is beyond end of metadata area");
+#endif
+      goto fail2;
+    }
+
   if (grub_le_to_cpu64 (rlocn->offset) + grub_le_to_cpu64 (rlocn->size) >
       grub_le_to_cpu64 (mdah->size))
     {