schannel: add "best effort" revocation check option

- Implement new option CURLSSLOPT_REVOKE_BEST_EFFORT and
  --ssl-revoke-best-effort to allow a "best effort" revocation check.

A best effort revocation check ignores errors that the revocation check
was unable to take place. The reasoning is described in detail below and
discussed further in the PR.

---

When running e.g. with Fiddler, the schannel backend fails with an
unhelpful error message:

	Unknown error (0x80092012) - The revocation function was unable
	to check revocation for the certificate.

Sadly, many enterprise users who are stuck behind MITM proxies suffer
the very same problem.

This has been discussed in plenty of issues:
https://github.com/curl/curl/issues/3727,
https://github.com/curl/curl/issues/264, for example.

In the latter, a Microsoft Edge developer even made the case that the
common behavior is to ignore issues when a certificate has no recorded
distribution point for revocation lists, or when the server is offline.
This is also known as "best effort" strategy and addresses the Fiddler
issue.

Unfortunately, this strategy was not chosen as the default for schannel
(and is therefore a backend-specific behavior: OpenSSL seems to happily
ignore the offline servers and missing distribution points).

To maintain backward-compatibility, we therefore add a new flag
(`CURLSSLOPT_REVOKE_BEST_EFFORT`) and a new option
(`--ssl-revoke-best-effort`) to select the new behavior.

Due to the many related issues Git for Windows and GitHub Desktop, the
plan is to make this behavior the default in these software packages.

The test 2070 was added to verify this behavior, adapted from 310.

Based-on-work-by: georgeok <giorgos.n.oikonomou@gmail.com>
Co-authored-by: Markus Olsson <j.markus.olsson@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Closes https://github.com/curl/curl/pull/4981

lib/doh.c

@@ -318,6 +318,9 @@ static CURLcode dohprobe(struct Curl_easy *data,
     }
     if(data->set.proxy_ssl.no_revoke)
       ERROR_CHECK_SETOPT(CURLOPT_PROXY_SSL_OPTIONS, CURLSSLOPT_NO_REVOKE);
+    else if(data->set.proxy_ssl.revoke_best_effort)
+      ERROR_CHECK_SETOPT(CURLOPT_PROXY_SSL_OPTIONS,
+                         CURLSSLOPT_REVOKE_BEST_EFFORT);
     if(data->set.str[STRING_SSL_CAPATH_PROXY]) {
       ERROR_CHECK_SETOPT(CURLOPT_PROXY_CAPATH,
         data->set.str[STRING_SSL_CAPATH_PROXY]);
@@ -351,6 +354,8 @@ static CURLcode dohprobe(struct Curl_easy *data,
     }
     if(data->set.ssl.no_revoke)
       ERROR_CHECK_SETOPT(CURLOPT_SSL_OPTIONS, CURLSSLOPT_NO_REVOKE);
+    else if(data->set.ssl.revoke_best_effort)
+      ERROR_CHECK_SETOPT(CURLOPT_SSL_OPTIONS, CURLSSLOPT_REVOKE_BEST_EFFORT);
     if(data->set.ssl.fsslctx)
       ERROR_CHECK_SETOPT(CURLOPT_SSL_CTX_FUNCTION, data->set.ssl.fsslctx);
     if(data->set.ssl.fsslctxp)

lib/setopt.c

@@ -2134,6 +2134,7 @@ CURLcode Curl_vsetopt(struct Curl_easy *data, CURLoption option, va_list param)
       (bool)((arg&CURLSSLOPT_ALLOW_BEAST) ? TRUE : FALSE);
     data->set.ssl.no_revoke = !!(arg & CURLSSLOPT_NO_REVOKE);
     data->set.ssl.no_partialchain = !!(arg & CURLSSLOPT_NO_PARTIALCHAIN);
+    data->set.ssl.revoke_best_effort = !!(arg & CURLSSLOPT_REVOKE_BEST_EFFORT);
     break;
 
 #ifndef CURL_DISABLE_PROXY
@@ -2143,6 +2144,8 @@ CURLcode Curl_vsetopt(struct Curl_easy *data, CURLoption option, va_list param)
       (bool)((arg&CURLSSLOPT_ALLOW_BEAST) ? TRUE : FALSE);
     data->set.proxy_ssl.no_revoke = !!(arg & CURLSSLOPT_NO_REVOKE);
     data->set.proxy_ssl.no_partialchain = !!(arg & CURLSSLOPT_NO_PARTIALCHAIN);
+    data->set.proxy_ssl.revoke_best_effort =
+      !!(arg & CURLSSLOPT_REVOKE_BEST_EFFORT);
     break;
 #endif
 

lib/urldata.h

@@ -258,6 +258,8 @@ struct ssl_config_data {
   BIT(enable_beast); /* allow this flaw for interoperability's sake*/
   BIT(no_revoke);    /* disable SSL certificate revocation checks */
   BIT(no_partialchain); /* don't accept partial certificate chains */
+  BIT(revoke_best_effort); /* ignore SSL revocation offline/missing revocation
+                              list errors */
 };
 
 struct ssl_general_config {

lib/vtls/schannel.c

@@ -520,8 +520,15 @@ schannel_connect_step1(struct connectdata *conn, int sockindex)
         DEBUGF(infof(data, "schannel: disabled server certificate revocation "
                      "checks\n"));
       }
+      else if(data->set.ssl.revoke_best_effort) {
+        schannel_cred.dwFlags |= SCH_CRED_IGNORE_NO_REVOCATION_CHECK |
+          SCH_CRED_IGNORE_REVOCATION_OFFLINE | SCH_CRED_REVOCATION_CHECK_CHAIN;
+
+        DEBUGF(infof(data, "schannel: ignore revocation offline errors"));
+      }
       else {
         schannel_cred.dwFlags |= SCH_CRED_REVOCATION_CHECK_CHAIN;
+
         DEBUGF(infof(data,
                      "schannel: checking server certificate revocation\n"));
       }

lib/vtls/schannel_verify.c

@@ -636,6 +636,15 @@ CURLcode Curl_verify_certificate(struct connectdata *conn, int sockindex)
       CERT_SIMPLE_CHAIN *pSimpleChain = pChainContext->rgpChain[0];
       DWORD dwTrustErrorMask = ~(DWORD)(CERT_TRUST_IS_NOT_TIME_NESTED);
       dwTrustErrorMask &= pSimpleChain->TrustStatus.dwErrorStatus;
+
+      if(data->set.ssl.revoke_best_effort) {
+        /* Ignore errors when root certificates are missing the revocation
+         * list URL, or when the list could not be downloaded because the
+         * server is currently unreachable. */
+        dwTrustErrorMask &= ~(DWORD)(CERT_TRUST_REVOCATION_STATUS_UNKNOWN |
+          CERT_TRUST_IS_OFFLINE_REVOCATION);
+      }
+
       if(dwTrustErrorMask) {
         if(dwTrustErrorMask & CERT_TRUST_IS_REVOKED)
           failf(data, "schannel: CertGetCertificateChain trust error"