Fix invalid handling of verify errors in libssl

In the event that X509_verify() returned an internal error result then
libssl would mishandle this and set rwstate to SSL_RETRY_VERIFY. This
subsequently causes SSL_get_error() to return SSL_ERROR_WANT_RETRY_VERIFY.
That return code is supposed to only ever be returned if an application
is using an app verify callback to complete replace the use of
X509_verify(). Applications may not be written to expect that return code
and could therefore crash (or misbehave in some other way) as a result.

CVE-2021-4044

Reviewed-by: Tomas Mraz <tomas@openssl.org>

Author: mattcaswell

ssl/ssl_cert.c

@@ -362,6 +362,13 @@ void ssl_cert_set_cert_cb(CERT *c, int (*cb) (SSL *ssl, void *arg), void *arg)
     c->cert_cb_arg = arg;
 }
 
+/*
+ * Verify a certificate chain
+ * Return codes:
+ *  1: Verify success
+ *  0: Verify failure or error
+ * -1: Retry required
+ */
 int ssl_verify_cert_chain(SSL *s, STACK_OF(X509) *sk)
 {
     X509 *x;
@@ -423,10 +430,14 @@ int ssl_verify_cert_chain(SSL *s, STACK_OF(X509) *sk)
     if (s->verify_callback)
         X509_STORE_CTX_set_verify_cb(ctx, s->verify_callback);
 
-    if (s->ctx->app_verify_callback != NULL)
+    if (s->ctx->app_verify_callback != NULL) {
         i = s->ctx->app_verify_callback(ctx, s->ctx->app_verify_arg);
-    else
+    } else {
         i = X509_verify_cert(ctx);
+        /* We treat an error in the same way as a failure to verify */
+        if (i < 0)
+            i = 0;
+    }
 
     s->verify_result = X509_STORE_CTX_get_error(ctx);
     sk_X509_pop_free(s->verified_chain, X509_free);

ssl/statem/statem_clnt.c

@@ -1878,7 +1878,7 @@ WORK_STATE tls_post_process_server_certificate(SSL *s, WORK_STATE wst)
      * (less clean) historic behaviour of performing validation if any flag is
      * set. The *documented* interface remains the same.
      */
-    if (s->verify_mode != SSL_VERIFY_NONE && i <= 0) {
+    if (s->verify_mode != SSL_VERIFY_NONE && i == 0) {
         SSLfatal(s, ssl_x509err2alert(s->verify_result),
                  SSL_R_CERTIFICATE_VERIFY_FAILED);
         return WORK_ERROR;