libfdt: Check that the root-node name is empty

The root node is supposed to have an empty name, but at present this is
not checked. The behaviour of such a tree is not well defined. Most
software rightly assumes that the root node is at offset 0 and does not
check the name. This oddity was discovered as part of a security
investigation into U-Boot verified boot.

Add a check for this to fdt_check_full().

Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Arie Haenel <arie.haenel@intel.com>
Reported-by: Julien Lenoir <julien.lenoir@intel.com>
Message-Id: <20210323010410.3222701-2-sjg@chromium.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

libfdt/fdt_check.c

@@ -59,6 +59,16 @@ int fdt_check_full(const void *fdt, size_t bufsize)
 			depth++;
 			if (depth > INT_MAX)
 				return -FDT_ERR_BADSTRUCTURE;
+
+			/* The root node must have an empty name */
+			if (depth == 1) {
+				const char *name;
+				int len;
+
+				name = fdt_get_name(fdt, offset, &len);
+				if (*name || len)
+					return -FDT_ERR_BADSTRUCTURE;
+			}
 			break;
 
 		case FDT_END_NODE:

tests/Makefile.tests

@@ -33,7 +33,7 @@ LIB_TESTS_L = get_mem_rsv \
 LIB_TESTS = $(LIB_TESTS_L:%=$(TESTS_PREFIX)%)
 
 LIBTREE_TESTS_L = truncated_property truncated_string truncated_memrsv \
-	two_roots
+	two_roots named_root
 
 LIBTREE_TESTS = $(LIBTREE_TESTS_L:%=$(TESTS_PREFIX)%)
 

tests/dumptrees.c

@@ -24,7 +24,8 @@ static struct {
 	TREE(ovf_size_strings),
 	TREE(truncated_property), TREE(truncated_string),
 	TREE(truncated_memrsv),
-	TREE(two_roots)
+	TREE(two_roots),
+	TREE(named_root)
 };
 
 #define NUM_TREES	(sizeof(trees) / sizeof(trees[0]))

tests/run_tests.sh

@@ -518,7 +518,7 @@ libfdt_tests () {
 	run_test check_full $good
     done
     for bad in truncated_property.dtb truncated_string.dtb \
-		truncated_memrsv.dtb two_roots.dtb; do
+		truncated_memrsv.dtb two_roots.dtb named_root.dtb; do
 	run_test check_full -n $bad
     done
 }

tests/testdata.h

@@ -56,4 +56,5 @@ extern struct fdt_header ovf_size_strings;
 extern struct fdt_header truncated_string;
 extern struct fdt_header truncated_memrsv;
 extern struct fdt_header two_roots;
+extern struct fdt_header named_root;
 #endif /* ! __ASSEMBLY */

tests/trees.S

@@ -298,3 +298,18 @@ two_roots_strings_end:
 
 two_roots_end:
 
+
+        /* root node with a non-empty name */
+        TREE_HDR(named_root)
+	EMPTY_RSVMAP(named_root)
+
+named_root_struct:
+	BEGIN_NODE("fake")
+	END_NODE
+	FDTLONG(FDT_END)
+named_root_struct_end:
+
+named_root_strings:
+named_root_strings_end:
+
+named_root_end: