Add size limit to X509_NAME structure.

This adds an explicit limit to the size of an X509_NAME structure. Some
part of OpenSSL (e.g. TLS) already effectively limit the size due to
restrictions on certificate size.
Reviewed-by: NMatt Caswell <matt@openssl.org>

crypto/x509/x_name.c

@@ -64,6 +64,13 @@
 #include "internal/asn1_int.h"
 #include "x509_lcl.h"
 
+/*
+ * Maximum length of X509_NAME: much larger than anything we should
+ * ever see in practice.
+ */
+
+#define X509_NAME_MAX (1024 * 1024)
+
 static int x509_name_ex_d2i(ASN1_VALUE **val,
                             const unsigned char **in, long len,
                             const ASN1_ITEM *it,
@@ -187,6 +194,10 @@ static int x509_name_ex_d2i(ASN1_VALUE **val,
     int i, j, ret;
     STACK_OF(X509_NAME_ENTRY) *entries;
     X509_NAME_ENTRY *entry;
+    if (len > X509_NAME_MAX) {
+        ASN1err(ASN1_F_X509_NAME_EX_D2I, ASN1_R_TOO_LONG);
+        return 0;
+    }
     q = p;
 
     /* Get internal representation of Name */