De-Revert "Add support for Kerberos credential delegation"

This reverts commit 3d03b24c3 (Revert Add support for Kerberos
credential delegation) which was committed on the grounds of concern
about portability, but on further review and discussion, it's clear that
we are better off explicitly requiring MIT Kerberos as that appears to
be the only GSSAPI library currently that's under proper maintenance
and ongoing development.  The API used for storing credentials was added
to MIT Kerberos over a decade ago while for the other libraries which
appear to be mainly based on Heimdal, which exists explicitly to be a
re-implementation of MIT Kerberos, the API never made it to a released
version (even though it was added to the Heimdal git repo over 5 years
ago..).

This post-feature-freeze change was approved by the RMT.

Discussion: https://postgr.es/m/ZDDO6jaESKaBgej0%40tamriel.snowman.net

2 files changed, 84 insertions, 47 deletions

diff --git a/contrib/dblink/dblink.c b/contrib/dblink/dblink.c
index 78a8bcee6e3..55f75eff361 100644
--- a/contrib/dblink/dblink.c
+++ b/contrib/dblink/dblink.c
@@ -48,6 +48,7 @@
 #include "funcapi.h"
 #include "lib/stringinfo.h"
 #include "libpq-fe.h"
+#include "libpq/libpq-be.h"
 #include "libpq/libpq-be-fe-helpers.h"
 #include "mb/pg_wchar.h"
 #include "miscadmin.h"
@@ -111,7 +112,8 @@ static HeapTuple get_tuple_of_interest(Relation rel, int *pkattnums, int pknumat
 static Relation get_rel_from_relname(text *relname_text, LOCKMODE lockmode, AclMode aclmode);
 static char *generate_relation_name(Relation rel);
 static void dblink_connstr_check(const char *connstr);
-static void dblink_security_check(PGconn *conn, remoteConn *rconn);
+static bool dblink_connstr_has_pw(const char *connstr);
+static void dblink_security_check(PGconn *conn, remoteConn *rconn, const char *connstr);
 static void dblink_res_error(PGconn *conn, const char *conname, PGresult *res,
 							 bool fail, const char *fmt,...) pg_attribute_printf(5, 6);
 static char *get_connect_string(const char *servername);
@@ -213,7 +215,7 @@ dblink_get_conn(char *conname_or_str,
 					 errmsg("could not establish connection"),
 					 errdetail_internal("%s", msg)));
 		}
-		dblink_security_check(conn, rconn);
+		dblink_security_check(conn, rconn, connstr);
 		if (PQclientEncoding(conn) != GetDatabaseEncoding())
 			PQsetClientEncoding(conn, GetDatabaseEncodingName());
 		freeconn = true;
@@ -307,7 +309,7 @@ dblink_connect(PG_FUNCTION_ARGS)
 	}
 
 	/* check password actually used if not superuser */
-	dblink_security_check(conn, rconn);
+	dblink_security_check(conn, rconn, connstr);
 
 	/* attempt to set client encoding to match server encoding, if needed */
 	if (PQclientEncoding(conn) != GetDatabaseEncoding())
@@ -2584,64 +2586,99 @@ deleteConnection(const char *name)
 				 errmsg("undefined connection name")));
 }
 
+/*
+ * We need to make sure that the connection made used credentials
+ * which were provided by the user, so check what credentials were
+ * used to connect and then make sure that they came from the user.
+ */
 static void
-dblink_security_check(PGconn *conn, remoteConn *rconn)
+dblink_security_check(PGconn *conn, remoteConn *rconn, const char *connstr)
 {
-	if (!superuser())
-	{
-		if (!PQconnectionUsedPassword(conn))
-		{
-			libpqsrv_disconnect(conn);
-			if (rconn)
-				pfree(rconn);
+	/* Superuser bypasses security check */
+	if (superuser())
+		return;
 
-			ereport(ERROR,
-					(errcode(ERRCODE_S_R_E_PROHIBITED_SQL_STATEMENT_ATTEMPTED),
-					 errmsg("password is required"),
-					 errdetail("Non-superuser cannot connect if the server does not request a password."),
-					 errhint("Target server's authentication method must be changed.")));
-		}
-	}
+	/* If password was used to connect, make sure it was one provided */
+	if (PQconnectionUsedPassword(conn) && dblink_connstr_has_pw(connstr))
+		return;
+
+#ifdef ENABLE_GSS
+	/* If GSSAPI creds used to connect, make sure it was one delegated */
+	if (PQconnectionUsedGSSAPI(conn) && be_gssapi_get_deleg(MyProcPort))
+		return;
+#endif
+
+	/* Otherwise, fail out */
+	libpqsrv_disconnect(conn);
+	if (rconn)
+		pfree(rconn);
+
+	ereport(ERROR,
+			(errcode(ERRCODE_S_R_E_PROHIBITED_SQL_STATEMENT_ATTEMPTED),
+			 errmsg("password or GSSAPI delegated credentials required"),
+			 errdetail("Non-superusers may only connect using credentials they provide, eg: password in connection string or delegated GSSAPI credentials"),
+			 errhint("Ensure provided credentials match target server's authentication method.")));
 }
 
 /*
- * For non-superusers, insist that the connstr specify a password.  This
- * prevents a password from being picked up from .pgpass, a service file,
- * the environment, etc.  We don't want the postgres user's passwords
- * to be accessible to non-superusers.
+ * Function to check if the connection string includes an explicit
+ * password, needed to ensure that non-superuser password-based auth
+ * is using a provided password and not one picked up from the
+ * environment.
  */
-static void
-dblink_connstr_check(const char *connstr)
+static bool
+dblink_connstr_has_pw(const char *connstr)
 {
-	if (!superuser())
-	{
-		PQconninfoOption *options;
-		PQconninfoOption *option;
-		bool		connstr_gives_password = false;
+	PQconninfoOption *options;
+	PQconninfoOption *option;
+	bool		connstr_gives_password = false;
 
-		options = PQconninfoParse(connstr, NULL);
-		if (options)
+	options = PQconninfoParse(connstr, NULL);
+	if (options)
+	{
+		for (option = options; option->keyword != NULL; option++)
 		{
-			for (option = options; option->keyword != NULL; option++)
+			if (strcmp(option->keyword, "password") == 0)
 			{
-				if (strcmp(option->keyword, "password") == 0)
+				if (option->val != NULL && option->val[0] != '\0')
 				{
-					if (option->val != NULL && option->val[0] != '\0')
-					{
-						connstr_gives_password = true;
-						break;
-					}
+					connstr_gives_password = true;
+					break;
 				}
 			}
-			PQconninfoFree(options);
 		}
-
-		if (!connstr_gives_password)
-			ereport(ERROR,
-					(errcode(ERRCODE_S_R_E_PROHIBITED_SQL_STATEMENT_ATTEMPTED),
-					 errmsg("password is required"),
-					 errdetail("Non-superusers must provide a password in the connection string.")));
+		PQconninfoFree(options);
 	}
+
+	return connstr_gives_password;
+}
+
+/*
+ * For non-superusers, insist that the connstr specify a password, except
+ * if GSSAPI credentials have been delegated (and we check that they are used
+ * for the connection in dblink_security_check later).  This prevents a
+ * password or GSSAPI credentials from being picked up from .pgpass, a
+ * service file, the environment, etc.  We don't want the postgres user's
+ * passwords or Kerberos credentials to be accessible to non-superusers.
+ */
+static void
+dblink_connstr_check(const char *connstr)
+{
+	if (superuser())
+		return;
+
+	if (dblink_connstr_has_pw(connstr))
+		return;
+
+#ifdef ENABLE_GSS
+	if (be_gssapi_get_deleg(MyProcPort))
+		return;
+#endif
+
+	ereport(ERROR,
+			(errcode(ERRCODE_S_R_E_PROHIBITED_SQL_STATEMENT_ATTEMPTED),
+			 errmsg("password or GSSAPI delegated credentials required"),
+			 errdetail("Non-superusers must provide a password in the connection string or send delegated GSSAPI credentials.")));
 }
 
 /*
diff --git a/contrib/dblink/expected/dblink.out b/contrib/dblink/expected/dblink.out
index 0f5050b4093..7809f58d96b 100644
--- a/contrib/dblink/expected/dblink.out
+++ b/contrib/dblink/expected/dblink.out
@@ -903,8 +903,8 @@ GRANT EXECUTE ON FUNCTION dblink_connect_u(text, text) TO regress_dblink_user;
 SET SESSION AUTHORIZATION regress_dblink_user;
 -- should fail
 SELECT dblink_connect('myconn', 'fdtest');
-ERROR:  password is required
-DETAIL:  Non-superusers must provide a password in the connection string.
+ERROR:  password or GSSAPI delegated credentials required
+DETAIL:  Non-superusers must provide a password in the connection string or send delegated GSSAPI credentials.
 -- should succeed
 SELECT dblink_connect_u('myconn', 'fdtest');
  dblink_connect_u