linux: Fix integer overflows in initrd size handling

These could be triggered by a crafted filesystem with very large files.

Fixes: CVE-2020-15707

Signed-off-by: Colin Watson <cjwatson@debian.org>
Reviewed-by: Jan Setje-Eilers <jan.setjeeilers@oracle.com>

Patch-Name: linux-initrd-overflow.patch

grub-core/loader/linux.c

@@ -4,6 +4,7 @@
 #include <grub/misc.h>
 #include <grub/file.h>
 #include <grub/mm.h>
+#include <grub/safemath.h>
 
 struct newc_head
 {
@@ -98,13 +99,13 @@ free_dir (struct dir *root)
   grub_free (root);
 }
 
-static grub_size_t
+static grub_err_t
 insert_dir (const char *name, struct dir **root,
-	    grub_uint8_t *ptr)
+	    grub_uint8_t *ptr, grub_size_t *size)
 {
   struct dir *cur, **head = root;
   const char *cb, *ce = name;
-  grub_size_t size = 0;
+  *size = 0;
   while (1)
     {
       for (cb = ce; *cb == '/'; cb++);
@@ -130,14 +131,22 @@ insert_dir (const char *name, struct dir **root,
 	      ptr = make_header (ptr, name, ce - name,
 				 040777, 0);
 	    }
-	  size += ALIGN_UP ((ce - (char *) name)
-			    + sizeof (struct newc_head), 4);
+	  if (grub_add (*size,
+		        ALIGN_UP ((ce - (char *) name)
+				  + sizeof (struct newc_head), 4),
+			size))
+	    {
+	      grub_error (GRUB_ERR_OUT_OF_RANGE, N_("overflow is detected"));
+	      grub_free (n->name);
+	      grub_free (n);
+	      return grub_errno;
+	    }
 	  *head = n;
 	  cur = n;
 	}
       root = &cur->next;
     }
-  return size;
+  return GRUB_ERR_NONE;
 }
 
 grub_err_t
@@ -173,26 +182,33 @@ grub_initrd_init (int argc, char *argv[],
 	  eptr = grub_strchr (ptr, ':');
 	  if (eptr)
 	    {
+	      grub_size_t dir_size, name_len;
+
 	      initrd_ctx->components[i].newc_name = grub_strndup (ptr, eptr - ptr);
-	      if (!initrd_ctx->components[i].newc_name)
+	      if (!initrd_ctx->components[i].newc_name ||
+		  insert_dir (initrd_ctx->components[i].newc_name, &root, 0,
+			      &dir_size))
 		{
 		  grub_initrd_close (initrd_ctx);
 		  return grub_errno;
 		}
-	      initrd_ctx->size
-		+= ALIGN_UP (sizeof (struct newc_head)
-			    + grub_strlen (initrd_ctx->components[i].newc_name),
-			     4);
-	      initrd_ctx->size += insert_dir (initrd_ctx->components[i].newc_name,
-					      &root, 0);
+	      name_len = grub_strlen (initrd_ctx->components[i].newc_name);
+	      if (grub_add (initrd_ctx->size,
+			    ALIGN_UP (sizeof (struct newc_head) + name_len, 4),
+			    &initrd_ctx->size) ||
+		  grub_add (initrd_ctx->size, dir_size, &initrd_ctx->size))
+		goto overflow;
 	      newc = 1;
 	      fname = eptr + 1;
 	    }
 	}
       else if (newc)
 	{
-	  initrd_ctx->size += ALIGN_UP (sizeof (struct newc_head)
-					+ sizeof ("TRAILER!!!") - 1, 4);
+	  if (grub_add (initrd_ctx->size,
+			ALIGN_UP (sizeof (struct newc_head)
+				  + sizeof ("TRAILER!!!") - 1, 4),
+			&initrd_ctx->size))
+	    goto overflow;
 	  free_dir (root);
 	  root = 0;
 	  newc = 0;
@@ -208,19 +224,29 @@ grub_initrd_init (int argc, char *argv[],
       initrd_ctx->nfiles++;
       initrd_ctx->components[i].size
 	= grub_file_size (initrd_ctx->components[i].file);
-      initrd_ctx->size += initrd_ctx->components[i].size;
+      if (grub_add (initrd_ctx->size, initrd_ctx->components[i].size,
+		    &initrd_ctx->size))
+	goto overflow;
     }
 
   if (newc)
     {
       initrd_ctx->size = ALIGN_UP (initrd_ctx->size, 4);
-      initrd_ctx->size += ALIGN_UP (sizeof (struct newc_head)
-				    + sizeof ("TRAILER!!!") - 1, 4);
+      if (grub_add (initrd_ctx->size,
+		    ALIGN_UP (sizeof (struct newc_head)
+			      + sizeof ("TRAILER!!!") - 1, 4),
+		    &initrd_ctx->size))
+	goto overflow;
       free_dir (root);
       root = 0;
     }
   
   return GRUB_ERR_NONE;
+
+overflow:
+  free_dir (root);
+  grub_initrd_close (initrd_ctx);
+  return grub_error (GRUB_ERR_OUT_OF_RANGE, N_("overflow is detected"));
 }
 
 grub_size_t
@@ -261,8 +287,16 @@ grub_initrd_load (struct grub_linux_initrd_context *initrd_ctx,
 
       if (initrd_ctx->components[i].newc_name)
 	{
-	  ptr += insert_dir (initrd_ctx->components[i].newc_name,
-			     &root, ptr);
+	  grub_size_t dir_size;
+
+	  if (insert_dir (initrd_ctx->components[i].newc_name, &root, ptr,
+			  &dir_size))
+	    {
+	      free_dir (root);
+	      grub_initrd_close (initrd_ctx);
+	      return grub_errno;
+	    }
+	  ptr += dir_size;
 	  ptr = make_header (ptr, initrd_ctx->components[i].newc_name,
 			     grub_strlen (initrd_ctx->components[i].newc_name),
 			     0100777,