libfdt: fdt_get_alias_namelen: Validate aliases

Ensure that the alias found matches the device tree specification v0.4:

    Each property of the /aliases node defines an alias. The property
    name specifies the alias name. The property value specifies the full
    path to a node in the devicetree.

This protects against a stack overflow caused by

    fdt_path_offset_namelen(fdt, path, namelen)

calling

    fdt_path_offset(fdt, fdt_get_alias_namelen(fdt, path, namelen))

leading to infinite recursion on DTs with "circular" aliases.

This fix was originally written by Mike McTernan for Android in [1].

[1]: 9308e7f977

Signed-off-by: Pierre-Clément Tosi <ptosi@google.com>
Acked-by: Mike McTernan <mikemcternan@google.com>
Message-ID: <20231010092725.63h7c45p2fnmj577@google.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

libfdt/fdt_ro.c

@@ -537,7 +537,16 @@ static const void *fdt_path_getprop_namelen(const void *fdt, const char *path,
 const char *fdt_get_alias_namelen(const void *fdt,
 				  const char *name, int namelen)
 {
-	return fdt_path_getprop_namelen(fdt, "/aliases", name, namelen, NULL);
+	int len;
+	const char *alias;
+
+	alias = fdt_path_getprop_namelen(fdt, "/aliases", name, namelen, &len);
+
+	if (!can_assume(VALID_DTB) &&
+	    !(alias && len > 0 && alias[len - 1] == '\0' && *alias == '/'))
+		return NULL;
+
+	return alias;
 }
 
 const char *fdt_get_alias(const void *fdt, const char *name)

tests/aliases.dts

@@ -5,6 +5,10 @@
 	#size-cells = <0>;
 
 	aliases {
+		empty = "";
+		loop = "loop";
+		nonull = [626164];
+		relative = "s1/subsubnode";
 		s1 = &sub1;
 		ss1 = &subsub1;
 		sss1 = &subsubsub1;

tests/get_alias.c

@@ -21,9 +21,16 @@ static void check_alias(void *fdt, const char *path, const char *alias)
 
 	aliaspath = fdt_get_alias(fdt, alias);
 
-	if (path && !aliaspath)
+	if (!path && !aliaspath)
+		return;
+
+	if (!aliaspath)
 		FAIL("fdt_get_alias(%s) failed\n", alias);
 
+	if (!path)
+		FAIL("fdt_get_alias(%s) returned %s instead of NULL",
+		     alias, aliaspath);
+
 	if (strcmp(aliaspath, path) != 0)
 		FAIL("fdt_get_alias(%s) returned %s instead of %s\n",
 		     alias, aliaspath, path);
@@ -36,9 +43,14 @@ int main(int argc, char *argv[])
 	test_init(argc, argv);
 	fdt = load_blob_arg(argc, argv);
 
+	check_alias(fdt, NULL, "empty");
+	check_alias(fdt, NULL, "nonull");
+	check_alias(fdt, NULL, "relative");
 	check_alias(fdt, "/subnode@1", "s1");
 	check_alias(fdt, "/subnode@1/subsubnode", "ss1");
 	check_alias(fdt, "/subnode@1/subsubnode/subsubsubnode", "sss1");
 
+	check_alias(fdt, NULL, "loop"); // Might trigger a stack overflow
+
 	PASS();
 }