image-sig: Ensure that hashed-nodes is null-terminated

A specially crafted FIT image leads to memory corruption in the stack
when using the verified boot feature. The function fit_config_check_sig
has a logic error that makes it possible to write past the end of the
stack allocated array node_inc. This could potentially be used to bypass
the signature check when using verified boot.

This change ensures that the number of strings is correct when counted.

Signed-off-by: Konrad Beckmann <konrad.beckmann@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

common/image-sig.c

@@ -334,6 +334,11 @@ int fit_config_check_sig(const void *fit, int noffset, int required_keynode,
 		return -1;
 	}
 
+	if (prop && prop_len > 0 && prop[prop_len - 1] != '\0') {
+		*err_msgp = "hashed-nodes property must be null-terminated";
+		return -1;
+	}
+
 	/* Add a sanity check here since we are using the stack */
 	if (count > IMAGE_MAX_HASHED_NODES) {
 		*err_msgp = "Number of hashed nodes exceeds maximum";