From 7d93bda31ce0b4e0e22c6e464c9138800dcf8b1c Mon Sep 17 00:00:00 2001
From: Alexander Bokovoy <abokovoy@redhat.com>
Date: Fri, 26 Nov 2021 11:13:51 +0200
Subject: [PATCH] ipa-kdb: fix requester SID check according to MS-KILE and
 MS-SFU updates

New versions of MS-KILE and MS-SFU after Windows Server November 2021
security updates add PAC_REQUESTER_SID buffer check behavior:

 - PAC_REQUESTER_SID should only be added for TGT requests

 - if PAC_REQUESTER_SID is present, KDC must verify that the cname on
   the ticket resolves to the account with the same SID as the
   PAC_REQUESTER_SID. If it doesn't KDC must respond with
   KDC_ERR_TKT_REVOKED

Change requester SID check to skip exact check for non-local
PAC_REQUESTER_SID but harden to ensure it comes from the trusted domains
we know about.

If requester SID is the same as in PAC, we already do cname vs PAC SID
verification.

With these changes FreeIPA works against Windows Server 2019 with
November 2021 security fixes in cross-realm S4U2Self operations.

Fixes: https://pagure.io/freeipa/issue/9031

Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>
Reviewed-By: Rob Crittenden <rcritten@redhat.com>
---
 daemons/ipa-kdb/ipa_kdb_mspac.c | 47 ++++++++++++++++++++++++---------
 1 file changed, 34 insertions(+), 13 deletions(-)

diff --git a/daemons/ipa-kdb/ipa_kdb_mspac.c b/daemons/ipa-kdb/ipa_kdb_mspac.c
index 538cfbba9..1b972c167 100644
--- a/daemons/ipa-kdb/ipa_kdb_mspac.c
+++ b/daemons/ipa-kdb/ipa_kdb_mspac.c
@@ -1697,7 +1697,7 @@ static krb5_error_code check_logon_info_consistent(krb5_context context,
                                       "local [%s], PAC [%s]",
                                       dom ? dom : "<failed to display>",
                                       sid ? sid : "<failed to display>");
-            return KRB5KDC_ERR_POLICY;
+            return KRB5KDC_ERR_TGT_REVOKED;
         }
     }
 
@@ -1709,7 +1709,7 @@ static krb5_error_code check_logon_info_consistent(krb5_context context,
     kerr = ipadb_get_principal(context, client_princ, flags, &client_actual);
     if (kerr != 0) {
         krb5_klog_syslog(LOG_ERR, "PAC issue: ipadb_get_principal failed.");
-        return KRB5KDC_ERR_POLICY;
+        return KRB5KDC_ERR_TGT_REVOKED;
     }
 
     ied = (struct ipadb_e_data *)client_actual->e_data;
@@ -1743,7 +1743,7 @@ static krb5_error_code check_logon_info_consistent(krb5_context context,
                                   "local [%s] vs PAC [%s]",
                                   local_sid ? local_sid : "<failed to display>",
                                   pac_sid ? pac_sid : "<failed to display>");
-        kerr = KRB5KDC_ERR_POLICY;
+        kerr = KRB5KDC_ERR_TGT_REVOKED;
         goto done;
     }
 
@@ -2005,22 +2005,43 @@ static krb5_error_code ipadb_check_logon_info(krb5_context context,
     /* Check that requester SID is the same as in the PAC entry */
     if (requester_sid != NULL) {
         struct dom_sid client_sid;
+        bool is_from_trusted_domain = false;
         kerr = ipadb_get_sid_from_pac(tmpctx, info.info, &client_sid);
         if (kerr) {
             goto done;
         }
         result = dom_sid_check(&client_sid, requester_sid, true);
         if (!result) {
-            /* memctx is freed by the caller */
-            char *pac_sid = dom_sid_string(tmpctx, &client_sid);
-            char *req_sid = dom_sid_string(tmpctx, requester_sid);
-            krb5_klog_syslog(LOG_ERR, "PAC issue: PAC has a SID "
-                                      "different from what PAC requester claims. "
-                                      "PAC [%s] vs PAC requester [%s]",
-                                      pac_sid ? pac_sid : "<failed to display>",
-                                      req_sid ? req_sid : "<failed to display>");
-            kerr = KRB5KDC_ERR_POLICY;
-            goto done;
+            struct ipadb_context *ipactx = ipadb_get_context(context);
+            if (!ipactx || !ipactx->mspac) {
+                return KRB5_KDB_DBNOTINITED;
+            }
+            /* In S4U case we might be dealing with the PAC issued by the trusted domain */
+            if (is_s4u && (ipactx->mspac->trusts != NULL)) {
+                /* Iterate through list of trusts and check if this SID belongs to
+                * one of the domains we trust */
+                for(int i = 0 ; i < ipactx->mspac->num_trusts ; i++) {
+                    result = dom_sid_check(&ipactx->mspac->trusts[i].domsid,
+                                           requester_sid, false);
+                    if (result) {
+                        is_from_trusted_domain = true;
+                        break;
+                    }
+                }
+            }
+
+            if (!is_from_trusted_domain) {
+                /* memctx is freed by the caller */
+                char *pac_sid = dom_sid_string(tmpctx, &client_sid);
+                char *req_sid = dom_sid_string(tmpctx, requester_sid);
+                krb5_klog_syslog(LOG_ERR, "PAC issue: PAC has a SID "
+                                        "different from what PAC requester claims. "
+                                        "PAC [%s] vs PAC requester [%s]",
+                                        pac_sid ? pac_sid : "<failed to display>",
+                                        req_sid ? req_sid : "<failed to display>");
+                kerr = KRB5KDC_ERR_TGT_REVOKED;
+                goto done;
+            }
         }
     }
 
-- 
2.31.1