Add flag to inhibit checking for alternate certificate chains. Setting this

behaviour will force behaviour as per previous versions of OpenSSL
Reviewed-by: NDr. Stephen Henson <steve@openssl.org>

crypto/x509/x509_vfy.c

@@ -357,11 +357,13 @@ int X509_verify_cert(X509_STORE_CTX *ctx)
         /*
          * If it's not explicitly trusted then check if there is an alternative
          * chain that could be used. We only do this if we haven't already
-         * checked via TRUSTED_FIRST
+         * checked via TRUSTED_FIRST and the user hasn't switched off alternate
+         * chain checking
          */
         retry = 0;
         if (i != X509_TRUST_TRUSTED
-            && !(ctx->param->flags & X509_V_FLAG_TRUSTED_FIRST)) {
+            && !(ctx->param->flags & X509_V_FLAG_TRUSTED_FIRST)
+            && !(ctx->param->flags & X509_V_FLAG_NO_ALT_CHAINS)) {
             while (j-- > 1) {
                 xtmp2 = sk_X509_value(ctx->chain, j - 1);
                 ok = ctx->get_issuer(&xtmp, ctx, xtmp2);

crypto/x509/x509_vfy.h

@@ -419,6 +419,12 @@ void X509_STORE_CTX_set_depth(X509_STORE_CTX *ctx, int depth);
 # define X509_V_FLAG_SUITEB_128_LOS              0x30000
 /* Allow partial chains if at least one certificate is in trusted store */
 # define X509_V_FLAG_PARTIAL_CHAIN               0x80000
+/*
+ * If the initial chain is not trusted, do not attempt to build an alternative
+ * chain. Alternate chain checking was introduced in 1.1.0. Setting this flag
+ * will force the behaviour to match that of previous versions.
+ */
+# define X509_V_FLAG_NO_ALT_CHAINS               0x100000
 
 # define X509_VP_FLAG_DEFAULT                    0x1
 # define X509_VP_FLAG_OVERWRITE                  0x2