Replace handling of negative verification result with SSL_set_retry_verify()

Provide a different mechanism to indicate that the application wants
to retry the verification. The negative result of the callback function
now indicates an error again.

Instead the SSL_set_retry_verify() can be called from the callback
to indicate that the handshake should be suspended.

Fixes #17568

Reviewed-by: David von Oheimb <david.von.oheimb@siemens.com>
Reviewed-by: Viktor Dukhovni <viktor@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/17825)

(cherry picked from commit dfb39f73132edf56daaad189e6791d1bdb57c4db)

5 files changed, 101 insertions, 15 deletions

diff --git a/doc/man3/SSL_CTX_set_cert_verify_callback.pod b/doc/man3/SSL_CTX_set_cert_verify_callback.pod
index fdeeaee6d7..ecd312c350 100644
--- a/doc/man3/SSL_CTX_set_cert_verify_callback.pod
+++ b/doc/man3/SSL_CTX_set_cert_verify_callback.pod
@@ -36,16 +36,18 @@ In server mode, a return value of 0 leads to handshake failure.
 In client mode, the behaviour is as follows.
 All values, including 0, are ignored
 if the verification mode is B<SSL_VERIFY_NONE>.
-Otherwise, when the return value is 0, the handshake will fail.
-
-In client mode I<callback> may also return -1,
-typically on failure verifying the server certificate.
-This makes the handshake suspend and return control to the calling application
-with B<SSL_ERROR_WANT_RETRY_VERIFY>.
-The app can for instance fetch further certificates or cert status information
-needed for the verification.
-Calling L<SSL_connect(3)> again resumes the connection attempt
-by retrying the server certificate verification step.
+Otherwise, when the return value is less than or equal to 0, the handshake will
+fail.
+
+In client mode I<callback> may also call the L<SSL_set_retry_verify(3)>
+function on the B<SSL> object set in the I<x509_store_ctx> ex data (see
+L<SSL_get_ex_data_X509_STORE_CTX_idx(3)>) and return 1. This would be
+typically done in case the certificate verification was not yet able
+to succeed. This makes the handshake suspend and return control to the
+calling application with B<SSL_ERROR_WANT_RETRY_VERIFY>. The app can for
+instance fetch further certificates or cert status information needed for
+the verification. Calling L<SSL_connect(3)> again resumes the connection
+attempt by retrying the server certificate verification step.
 This process may even be repeated if need be.
 
 In any case a viable verification result value must be reflected
@@ -89,6 +91,7 @@ SSL_CTX_set_cert_verify_callback() does not provide diagnostic information.
 L<ssl(7)>, L<SSL_CTX_set_verify(3)>,
 L<X509_STORE_CTX_set_error(3)>,
 L<SSL_get_verify_result(3)>,
+L<SSL_set_retry_verify(3)>,
 L<SSL_CTX_load_verify_locations(3)>
 
 =head1 COPYRIGHT
diff --git a/doc/man3/SSL_CTX_set_verify.pod b/doc/man3/SSL_CTX_set_verify.pod
index e3271aff01..c085097617 100644
--- a/doc/man3/SSL_CTX_set_verify.pod
+++ b/doc/man3/SSL_CTX_set_verify.pod
@@ -44,6 +44,21 @@ L<SSL_new(3)>. Within the callback function,
 B<SSL_get_ex_data_X509_STORE_CTX_idx> can be called to get the data index
 of the current SSL object that is doing the verification.
 
+In client mode B<verify_callback> may also call the L<SSL_set_retry_verify(3)>
+function on the B<SSL> object set in the I<x509_store_ctx> ex data (see
+L<SSL_get_ex_data_X509_STORE_CTX_idx(3)>) and return 1.
+This would be typically done in case the certificate verification was not yet
+able to succeed.
+This makes the handshake suspend and return control to the calling application
+with B<SSL_ERROR_WANT_RETRY_VERIFY>.
+The application can for instance fetch further certificates or cert status
+information needed for the verification.
+Note that the handshake may still be aborted if a subsequent invocation of the
+callback (e.g. at a lower depth, or for a separate error condition) returns 0.
+Calling L<SSL_connect(3)> again resumes the connection attempt by retrying the
+server certificate verification step.
+This process may even be repeated if need be.
+
 SSL_CTX_set_verify_depth() sets the maximum B<depth> for the certificate chain
 verification that shall be allowed for B<ctx>.
 
diff --git a/doc/man3/SSL_set_retry_verify.pod b/doc/man3/SSL_set_retry_verify.pod
new file mode 100644
index 0000000000..4eb7f449ed
--- /dev/null
+++ b/doc/man3/SSL_set_retry_verify.pod
@@ -0,0 +1,70 @@
+=pod
+
+=head1 NAME
+
+SSL_set_retry_verify - indicate that certificate verification should be retried
+
+=head1 SYNOPSIS
+
+ #include <openssl/ssl.h>
+
+ int SSL_set_retry_verify(SSL *ssl);
+
+=head1 DESCRIPTION
+
+SSL_set_retry_verify() should be called from the certificate verification
+callback on a client when the application wants to indicate that the handshake
+should be suspended and the control should be returned to the application.
+L<SSL_want_retry_verify(3)> will return 1 as a consequence until the handshake
+is resumed again by the application, retrying the verification step.
+
+Please refer to L<SSL_CTX_set_cert_verify_callback(3)> for further details.
+
+=head1 NOTES
+
+The effect of calling SSL_set_retry_verify() outside of the certificate
+verification callback on the client side is undefined.
+
+=head1 RETURN VALUES
+
+SSL_set_retry verify() returns 1 on success, 0 otherwise.
+
+=head1 EXAMPLES
+
+The following code snippet shows how to obtain the B<SSL> object associated
+with the B<X509_STORE_CTX> to call the SSL_set_retry_verify() function:
+
+    int idx = SSL_get_ex_data_X509_STORE_CTX_idx();
+    SSL *ssl;
+
+    /* this should not happen but check anyway */
+    if (idx < 0
+        || (ssl = X509_STORE_CTX_get_ex_data(ctx, idx)) == NULL) 
+        return 0;
+
+    if (/* we need to retry verification callback */)
+        return SSL_set_retry_verify(ssl);
+
+    /* do normal processing of the verification callback */
+
+=head1 SEE ALSO
+
+L<ssl(7)>, L<SSL_connect(3)>, L<SSL_CTX_set_cert_verify_callback(3)>,
+L<SSL_want_retry_verify(3)>
+
+=head1 HISTORY
+
+SSL_set_retry_verify() was added in OpenSSL 3.0.2 to replace backwards
+incompatible handling of a negative return value from the verification
+callback.
+
+=head1 COPYRIGHT
+
+Copyright 2022 The OpenSSL Project Authors. All Rights Reserved.
+
+Licensed under the Apache License 2.0 (the "License").  You may not use
+this file except in compliance with the License.  You can obtain a copy
+in the file LICENSE in the source distribution or at
+L<https://www.openssl.org/source/license.html>.
+
+=cut
diff --git a/doc/man3/SSL_want.pod b/doc/man3/SSL_want.pod
index 831094ae0a..22920135b9 100644
--- a/doc/man3/SSL_want.pod
+++ b/doc/man3/SSL_want.pod
@@ -70,8 +70,8 @@ A call to L<SSL_get_error(3)> should return B<SSL_ERROR_WANT_X509_LOOKUP>.
 
 =item SSL_RETRY_VERIFY
 
-The operation did not complete because an application callback set by
-SSL_CTX_set_cert_verify_callback() has asked to be called again.
+The operation did not complete because a certificate verification callback
+has asked to be called again via L<SSL_set_retry_verify(3)>.
 A call to L<SSL_get_error(3)> should return B<SSL_ERROR_WANT_RETRY_VERIFY>.
 
 =item SSL_ASYNC_PAUSED
diff --git a/doc/man3/X509_verify_cert.pod b/doc/man3/X509_verify_cert.pod
index a14a0b25c4..6172ea5b1f 100644
--- a/doc/man3/X509_verify_cert.pod
+++ b/doc/man3/X509_verify_cert.pod
@@ -52,9 +52,7 @@ A negative return value from X509_verify_cert() can occur if it is invoked
 incorrectly, such as with no certificate set in I<ctx>, or when it is called
 twice in succession without reinitialising I<ctx> for the second call.
 A negative return value can also happen due to internal resource problems
-or because an internal inconsistency has been detected
-or if a retry operation is requested during internal lookups
-(which never happens with standard lookup methods).
+or because an internal inconsistency has been detected.
 Applications must interpret any return value <= 0 as an error.
 
 The X509_STORE_CTX_verify() behaves like X509_verify_cert() except that its