Fix privilege check for SET SESSION AUTHORIZATION.

Presently, the privilege check for SET SESSION AUTHORIZATION checks
whether the original authenticated role was a superuser at
connection start time.  Even if the role loses the superuser
attribute, its existing sessions are permitted to change session
authorization to any role.

This commit modifies this privilege check to verify the original
authenticated role currently has superuser.  In the event that the
authenticated role loses superuser within a session authorization
change, the authorization change will remain in effect, which means
the user can still take advantage of the target role's privileges.
However, [RE]SET SESSION AUTHORIZATION will only permit switching
to the original authenticated role.

Author: Joseph Koshakow
Discussion: https://postgr.es/m/CAAvxfHc-HHzONQ2oXdvhFF9ayRnidPwK%2BfVBhRzaBWYYLVQL-g%40mail.gmail.com

doc/src/sgml/ref/set_session_auth.sgml

@@ -51,7 +51,7 @@ RESET SESSION AUTHORIZATION
 
   <para>
    The session user identifier can be changed only if the initial session
-   user (the <firstterm>authenticated user</firstterm>) had the
+   user (the <firstterm>authenticated user</firstterm>) has the
    superuser privilege.  Otherwise, the command is accepted only if it
    specifies the authenticated user name.
   </para>

src/backend/commands/variable.c

@@ -854,7 +854,7 @@ check_session_authorization(char **newval, void **extra, GucSource source)
 	 * authenticated user's superuserness is what matters.
 	 */
 	if (roleid != GetAuthenticatedUserId() &&
-		!GetAuthenticatedUserIsSuperuser())
+		!superuser_arg(GetAuthenticatedUserId()))
 	{
 		if (source == PGC_S_TEST)
 		{

src/backend/utils/init/miscinit.c

@@ -467,8 +467,8 @@ ChangeToDataDir(void)
  * AuthenticatedUserId is determined at connection start and never changes.
  *
  * SessionUserId is initially the same as AuthenticatedUserId, but can be
- * changed by SET SESSION AUTHORIZATION (if AuthenticatedUserIsSuperuser).
+ * changed by SET SESSION AUTHORIZATION (if AuthenticatedUserId is a
- * This is the ID reported by the SESSION_USER SQL function.
+ * superuser).  This is the ID reported by the SESSION_USER SQL function.
  *
  * OuterUserId is the current user ID in effect at the "outer level" (outside
  * any transaction or function).  This is initially the same as SessionUserId,
@@ -492,8 +492,7 @@ static Oid	OuterUserId = InvalidOid;
 static Oid	CurrentUserId = InvalidOid;
 static const char *SystemUser = NULL;
 
-/* We also have to remember the superuser state of some of these levels */
+/* We also have to remember the superuser state of the session user */
-static bool AuthenticatedUserIsSuperuser = false;
 static bool SessionUserIsSuperuser = false;
 
 static int	SecurityRestrictionContext = 0;
@@ -582,16 +581,6 @@ GetAuthenticatedUserId(void)
 	return AuthenticatedUserId;
 }
 
-/*
- * Return whether the authenticated user was superuser at connection start.
- */
-bool
-GetAuthenticatedUserIsSuperuser(void)
-{
-	Assert(OidIsValid(AuthenticatedUserId));
-	return AuthenticatedUserIsSuperuser;
-}
-
 
 /*
  * GetUserIdAndSecContext/SetUserIdAndSecContext - get/set the current user ID
@@ -741,6 +730,7 @@ InitializeSessionUserId(const char *rolename, Oid roleid)
 	HeapTuple	roleTup;
 	Form_pg_authid rform;
 	char	   *rname;
+	bool		is_superuser;
 
 	/*
 	 * Don't do scans if we're bootstrapping, none of the system catalogs
@@ -780,10 +770,10 @@ InitializeSessionUserId(const char *rolename, Oid roleid)
 	rname = NameStr(rform->rolname);
 
 	AuthenticatedUserId = roleid;
-	AuthenticatedUserIsSuperuser = rform->rolsuper;
+	is_superuser = rform->rolsuper;
 
 	/* This sets OuterUserId/CurrentUserId too */
-	SetSessionUserId(roleid, AuthenticatedUserIsSuperuser);
+	SetSessionUserId(roleid, is_superuser);
 
 	/* Also mark our PGPROC entry with the authenticated user id */
 	/* (We assume this is an atomic store so no lock is needed) */
@@ -816,7 +806,7 @@ InitializeSessionUserId(const char *rolename, Oid roleid)
 		 * just document that the connection limit is approximate.
 		 */
 		if (rform->rolconnlimit >= 0 &&
-			!AuthenticatedUserIsSuperuser &&
+			!is_superuser &&
 			CountUserBackends(roleid) > rform->rolconnlimit)
 			ereport(FATAL,
 					(errcode(ERRCODE_TOO_MANY_CONNECTIONS),
@@ -828,7 +818,7 @@ InitializeSessionUserId(const char *rolename, Oid roleid)
 	SetConfigOption("session_authorization", rname,
 					PGC_BACKEND, PGC_S_OVERRIDE);
 	SetConfigOption("is_superuser",
-					AuthenticatedUserIsSuperuser ? "on" : "off",
+					is_superuser ? "on" : "off",
 					PGC_INTERNAL, PGC_S_DYNAMIC_DEFAULT);
 
 	ReleaseSysCache(roleTup);
@@ -851,8 +841,6 @@ InitializeSessionUserIdStandalone(void)
 	Assert(!OidIsValid(AuthenticatedUserId));
 
 	AuthenticatedUserId = BOOTSTRAP_SUPERUSERID;
-	AuthenticatedUserIsSuperuser = true;
-
 	SetSessionUserId(BOOTSTRAP_SUPERUSERID, true);
 }
 

src/include/miscadmin.h

@@ -357,7 +357,6 @@ extern Oid	GetUserId(void);
 extern Oid	GetOuterUserId(void);
 extern Oid	GetSessionUserId(void);
 extern Oid	GetAuthenticatedUserId(void);
-extern bool GetAuthenticatedUserIsSuperuser(void);
 extern void GetUserIdAndSecContext(Oid *userid, int *sec_context);
 extern void SetUserIdAndSecContext(Oid userid, int sec_context);
 extern bool InLocalUserIdChange(void);