Fix verify algorithm.

Disable loop checking when we retry verification with an alternative path.
This fixes the case where an intermediate CA is explicitly trusted and part
of the untrusted certificate list. By disabling loop checking for this case
the untrusted CA can be replaced by the explicitly trusted case and
verification will succeed.
Reviewed-by: NMatt Caswell <matt@openssl.org>

crypto/x509/x509_vfy.c

@@ -365,8 +365,16 @@ int X509_verify_cert(X509_STORE_CTX *ctx)
             && !(ctx->param->flags & X509_V_FLAG_TRUSTED_FIRST)
             && !(ctx->param->flags & X509_V_FLAG_NO_ALT_CHAINS)) {
             while (j-- > 1) {
+                STACK_OF(X509) *chtmp = ctx->chain;
                 xtmp2 = sk_X509_value(ctx->chain, j - 1);
+                /*
+                 * Temporarily set chain to NULL so we don't discount
+                 * duplicates: the same certificate could be an untrusted
+                 * CA found in the trusted store.
+                 */
+                ctx->chain = NULL;
                 ok = ctx->get_issuer(&xtmp, ctx, xtmp2);
+                ctx->chain = chtmp;
                 if (ok < 0)
                     goto end;
                 /* Check if we found an alternate chain */