grub-core/loader/efi/chainloader.c: clear free memory before loading image

This is meant to make measurements of DLME more deterministic in the
face of firmware not bothering to zero area containing padding.

Signed-off-by: Sergii Dmytruk <[email protected]>

Author: SergiiDmytruk

grub-core/loader/efi/chainloader.c

@@ -37,6 +37,7 @@
 #include <grub/i18n.h>
 #include <grub/net.h>
 #include <grub/slaunch.h>
+#include <grub/time.h>
 #if defined (__i386__) || defined (__x86_64__)
 #include <grub/macho.h>
 #include <grub/i386/macho.h>
@@ -250,6 +251,57 @@ make_file_path (grub_efi_device_path_t *dp, const char *filename)
   return file_path;
 }
 
+/* Clear unused memory.  This is meant to make measurements of DLME more
+ * deterministic in the face of firmware not bothering to zero area containing
+ * padding. */
+static grub_err_t
+clear_memory (void)
+{
+  grub_efi_memory_descriptor_t *memory_map, *desc;
+  grub_efi_uintn_t memory_map_size, desc_size;
+  grub_efi_boot_services_t *b;
+  grub_efi_uint64_t total_cleared = 0;
+  int ret;
+
+  b = grub_efi_system_table->boot_services;
+
+  memory_map_size = grub_efi_find_mmap_size ();
+
+  memory_map = grub_malloc (memory_map_size);
+  if (!memory_map)
+    return GRUB_ERR_OUT_OF_MEMORY;
+
+  ret = grub_efi_get_memory_map (&memory_map_size, memory_map, NULL,
+                                 &desc_size, NULL);
+  if (ret < 1)
+    return GRUB_ERR_BUG;
+
+  for (desc = memory_map;
+       (grub_addr_t) desc < ((grub_addr_t) memory_map + memory_map_size);
+       desc = (void *)((grub_uint8_t *)desc + desc_size))
+    {
+      grub_efi_uint64_t size = desc->num_pages << 12;
+
+      if (desc->type != GRUB_EFI_CONVENTIONAL_MEMORY ||
+          desc->physical_start >= (1ULL << 32))
+        continue;
+
+      if (desc->physical_start + size > (1ULL << 32))
+        size = (1ULL << 32) - desc->physical_start;
+
+      grub_dprintf ("chain", "Clearing %lu bytes at 0x%lx...\n", size,
+                    desc->physical_start);
+      b->set_mem ((void *)desc->physical_start, size, 0x00);
+      total_cleared += size;
+    }
+
+  grub_free (memory_map);
+
+  grub_dprintf ("chain", "Cleared %llu bytes in total.\n",
+                (unsigned long long)total_cleared);
+  return GRUB_ERR_NONE;
+}
+
 static grub_err_t
 grub_cmd_chainloader (grub_command_t cmd __attribute__ ((unused)),
 		      int argc, char *argv[])
@@ -265,6 +317,8 @@ grub_cmd_chainloader (grub_command_t cmd __attribute__ ((unused)),
   grub_efi_handle_t dev_handle = 0;
   grub_efi_char16_t *cmdline = NULL;
   grub_efi_handle_t image_handle = NULL;
+  grub_uint64_t before_time, after_time;
+  grub_err_t err;
 
   if (argc == 0)
     return grub_error (GRUB_ERR_BAD_ARGUMENT, N_("filename expected"));
@@ -291,7 +345,6 @@ grub_cmd_chainloader (grub_command_t cmd __attribute__ ((unused)),
       grub_net_network_level_address_t addr;
       struct grub_net_network_level_interface *inf;
       grub_net_network_level_address_t gateway;
-      grub_err_t err;
 
       err = grub_net_resolve_address (dev->net->server, &addr);
       if (err)
@@ -344,6 +397,20 @@ grub_cmd_chainloader (grub_command_t cmd __attribute__ ((unused)),
       goto fail;
     }
 
+  if (grub_slaunch_platform_type () != SLP_NONE)
+    {
+      before_time = grub_get_time_ms ();
+      err = clear_memory();
+      after_time = grub_get_time_ms ();
+      grub_dprintf ("chain", "Took %llu ms to clear the memory\n",
+                    (unsigned long long)(after_time - before_time));
+      if (err != GRUB_ERR_NONE)
+        {
+          grub_dprintf ("chain", "Failed to clear unused memory < 4 GiB\n");
+          goto fail;
+        }
+    }
+
 #if defined (__i386__) || defined (__x86_64__)
   if (size >= (grub_ssize_t) sizeof (struct grub_macho_fat_header))
     {