s4-drs: allow bypass of writespn checking for some SPNs

this allows accounts (and in particular RODCs) to make SPN updates on
their own account if they take the form SERVICE/hostname

we may be able to remove this in the future after some changes in our
ACL checking for userPrincipalName

Pair-Programmed-With: Andrew Bartlett <abartlet@samba.org>

Autobuild-User: Andrew Tridgell <tridge@samba.org>
Autobuild-Date: Mon Nov  8 08:45:16 UTC 2010 on sn-devel-104

diff --git a/source4/rpc_server/drsuapi/writespn.c b/source4/rpc_server/drsuapi/writespn.c
index 2e7bd762bfa529a614641459550ce92536602a20..d9338f74137c0d32a3e7d8ab148b3b22e152990a 100644 (file)
--- a/source4/rpc_server/drsuapi/writespn.c
+++ b/source4/rpc_server/drsuapi/writespn.c
@@ -25,6 +25,106 @@
 #include "dsdb/samdb/samdb.h"
 #include "dsdb/common/util.h"
 #include "rpc_server/drsuapi/dcesrv_drsuapi.h"
+#include "system/kerberos.h"
+#include "auth/kerberos/kerberos.h"
+#include "libcli/security/session.h"
+#include "libcli/security/security.h"
+#include "auth/session.h"
+
+/*
+  check that the SPN update should be allowed as an override
+  via sam_ctx_system
+
+  This is only called if the client is not a domain controller or
+  administrator
+ */
+static bool writespn_check_spn(struct drsuapi_bind_state *b_state,
+                              struct dcesrv_call_state *dce_call,
+                              struct ldb_dn *dn,
+                              const char *spn)
+{
+       /*
+         we only allow SPN updates if:
+
+            1) they are on the clients own account object
+           2) they are of the form SERVICE/dnshostname
+        */
+       struct dom_sid *user_sid, *sid;
+       TALLOC_CTX *tmp_ctx = talloc_new(dce_call);
+       struct ldb_result *res;
+       const char *attrs[] = { "objectSID", "dnsHostName", NULL };
+       int ret;
+       krb5_context krb_ctx;
+       krb5_error_code kerr;
+       krb5_principal principal;
+       const char *dns_name, *dnsHostName;
+
+       /*
+         get the objectSid of the DN that is being modified, and
+         check it matches the user_sid in their token
+        */
+
+       ret = dsdb_search_dn(b_state->sam_ctx, tmp_ctx, &res, dn, attrs, DSDB_SEARCH_ONE_ONLY);
+       if (ret != LDB_SUCCESS) {
+               talloc_free(tmp_ctx);
+               return false;
+       }
+
+       user_sid = &dce_call->conn->auth_state.session_info->security_token->sids[PRIMARY_USER_SID_INDEX];
+       sid = samdb_result_dom_sid(tmp_ctx, res->msgs[0], "objectSid");
+       if (sid == NULL) {
+               talloc_free(tmp_ctx);
+               return false;
+       }
+
+       dnsHostName = ldb_msg_find_attr_as_string(res->msgs[0], "dnsHostName", NULL);
+       if (dnsHostName == NULL) {
+               talloc_free(tmp_ctx);
+               return false;
+       }
+
+       if (!dom_sid_equal(sid, user_sid)) {
+               talloc_free(tmp_ctx);
+               return false;
+       }
+
+       kerr = smb_krb5_init_context_basic(tmp_ctx, dce_call->conn->dce_ctx->lp_ctx, &krb_ctx);
+       if (kerr != 0) {
+               talloc_free(tmp_ctx);
+               return false;
+       }
+
+       ret = krb5_parse_name_flags(krb_ctx, spn, KRB5_PRINCIPAL_PARSE_NO_REALM, &principal);
+       if (kerr != 0) {
+               krb5_free_context(krb_ctx);
+               talloc_free(tmp_ctx);
+               return false;
+       }
+
+       if (principal->name.name_string.len != 2) {
+               krb5_free_principal(krb_ctx, principal);
+               krb5_free_context(krb_ctx);
+               talloc_free(tmp_ctx);
+               return false;
+       }
+
+       dns_name = principal->name.name_string.val[1];
+
+       if (strcasecmp(dns_name, dnsHostName) != 0) {
+               krb5_free_principal(krb_ctx, principal);
+               krb5_free_context(krb_ctx);
+               talloc_free(tmp_ctx);
+               return false;
+       }
+
+       /* its a simple update on their own account - allow it with
+        * permissions override */
+       krb5_free_principal(krb_ctx, principal);
+       krb5_free_context(krb_ctx);
+       talloc_free(tmp_ctx);
+
+       return true;
+}
 
 /*
   drsuapi_DsWriteAccountSpn
@@ -34,6 +134,7 @@ WERROR dcesrv_drsuapi_DsWriteAccountSpn(struct dcesrv_call_state *dce_call, TALL
 {
        struct drsuapi_bind_state *b_state;
        struct dcesrv_handle *h;
+       enum security_user_level level;
 
        *r->out.level_out = r->in.level;
 
@@ -43,6 +144,8 @@ WERROR dcesrv_drsuapi_DsWriteAccountSpn(struct dcesrv_call_state *dce_call, TALL
        r->out.res = talloc(mem_ctx, union drsuapi_DsWriteAccountSpnResult);
        W_ERROR_HAVE_NO_MEMORY(r->out.res);
 
+       level = security_session_user_level(dce_call->conn->auth_state.session_info, NULL);
+
        switch (r->in.level) {
                case 1: {
                        struct drsuapi_DsWriteAccountSpnRequest1 *req;
@@ -51,6 +154,7 @@ WERROR dcesrv_drsuapi_DsWriteAccountSpn(struct dcesrv_call_state *dce_call, TALL
                        unsigned int i;
                        int ret;
                        unsigned spn_count=0;
+                       bool passed_checks = true;
 
                        req = &r->in.req->req1;
                        count = req->count;
@@ -68,6 +172,12 @@ WERROR dcesrv_drsuapi_DsWriteAccountSpn(struct dcesrv_call_state *dce_call, TALL
 
                        /* construct mods */
                        for (i = 0; i < count; i++) {
+                               if (!writespn_check_spn(b_state,
+                                                      dce_call,
+                                                      msg->dn,
+                                                      req->spn_names[i].str)) {
+                                       passed_checks = false;
+                               }
                                ret = samdb_msg_add_string(b_state->sam_ctx,
                                                           msg, msg, "servicePrincipalName",
                                                           req->spn_names[i].str);
@@ -98,7 +208,7 @@ WERROR dcesrv_drsuapi_DsWriteAccountSpn(struct dcesrv_call_state *dce_call, TALL
                        }
 
                        /* Apply to database */
-                       ret = dsdb_modify(b_state->sam_ctx, msg,
+                       ret = dsdb_modify(passed_checks?b_state->sam_ctx_system:b_state->sam_ctx, msg,
                                          DSDB_MODIFY_PERMISSIVE);
                        if (ret != LDB_SUCCESS) {
                                DEBUG(0,("Failed to modify SPNs on %s: %s\n",