Ban role pg_signal_backend from more superuser backend types.

Documentation says it cannot signal "a backend owned by a superuser".
On the contrary, it could signal background workers, including the
logical replication launcher.  It could signal autovacuum workers and
the autovacuum launcher.  Block all that.  Signaling autovacuum workers
and those two launchers doesn't stall progress beyond what one could
achieve other ways.  If a cluster uses a non-core extension with a
background worker that does not auto-restart, this could create a denial
of service with respect to that background worker.  A background worker
with bugs in its code for responding to terminations or cancellations
could experience those bugs at a time the pg_signal_backend member
chooses.  Back-patch to v11 (all supported versions).

Reviewed by Jelte Fennema-Nio.  Reported by Hemanth Sandrana and
Mahendrakar Srinivasarao.

Security: CVE-2023-5870

diff --git a/src/backend/utils/adt/misc.c b/src/backend/utils/adt/misc.c
index 8915e23089e7d37675d65f766e0040508d90ffda..f0a39b0c9dfa58febc287512396e9eeb1e3d9a3d 100644 (file)
--- a/src/backend/utils/adt/misc.c
+++ b/src/backend/utils/adt/misc.c
@@ -242,8 +242,13 @@ pg_signal_backend(int pid, int sig)
        return SIGNAL_BACKEND_ERROR;
    }
 
-   /* Only allow superusers to signal superuser-owned backends. */
-   if (superuser_arg(proc->roleId) && !superuser())
+   /*
+    * Only allow superusers to signal superuser-owned backends.  Any process
+    * not advertising a role might have the importance of a superuser-owned
+    * backend, so treat it that way.
+    */
+   if ((!OidIsValid(proc->roleId) || superuser_arg(proc->roleId)) &&
+       !superuser())
        return SIGNAL_BACKEND_NOSUPERUSER;
 
    /* Users can signal backends they have role membership in. */

diff --git a/src/test/regress/expected/privileges.out b/src/test/regress/expected/privileges.out
index ecd84d5914167f5d47df4110c85282b4c401a301..94beb42d0a384294d54994f016841a33cbe845ba 100644 (file)
--- a/src/test/regress/expected/privileges.out
+++ b/src/test/regress/expected/privileges.out
@@ -1718,6 +1718,24 @@ SELECT * FROM pg_largeobject LIMIT 0;
 SET SESSION AUTHORIZATION regress_priv_user1;
 SELECT * FROM pg_largeobject LIMIT 0;          -- to be denied
 ERROR:  permission denied for table pg_largeobject
+-- pg_signal_backend can't signal superusers
+RESET SESSION AUTHORIZATION;
+BEGIN;
+CREATE OR REPLACE FUNCTION terminate_nothrow(pid int) RETURNS bool
+   LANGUAGE plpgsql SECURITY DEFINER SET client_min_messages = error AS $$
+BEGIN
+   RETURN pg_terminate_backend($1);
+EXCEPTION WHEN OTHERS THEN
+   RETURN false;
+END$$;
+ALTER FUNCTION terminate_nothrow OWNER TO pg_signal_backend;
+SELECT backend_type FROM pg_stat_activity
+WHERE CASE WHEN COALESCE(usesysid, 10) = 10 THEN terminate_nothrow(pid) END;
+ backend_type 
+--------------
+(0 rows)
+
+ROLLBACK;
 -- test default ACLs
 \c -
 CREATE SCHEMA testns;

diff --git a/src/test/regress/sql/privileges.sql b/src/test/regress/sql/privileges.sql
index 34242e36addf8245ddaeecdf74170e5aea71868f..5881dfd90f1a9da9feb4bc772723abdd5c48bf95 100644 (file)
--- a/src/test/regress/sql/privileges.sql
+++ b/src/test/regress/sql/privileges.sql
@@ -1053,6 +1053,21 @@ SELECT * FROM pg_largeobject LIMIT 0;
 SET SESSION AUTHORIZATION regress_priv_user1;
 SELECT * FROM pg_largeobject LIMIT 0;          -- to be denied
 
+-- pg_signal_backend can't signal superusers
+RESET SESSION AUTHORIZATION;
+BEGIN;
+CREATE OR REPLACE FUNCTION terminate_nothrow(pid int) RETURNS bool
+   LANGUAGE plpgsql SECURITY DEFINER SET client_min_messages = error AS $$
+BEGIN
+   RETURN pg_terminate_backend($1);
+EXCEPTION WHEN OTHERS THEN
+   RETURN false;
+END$$;
+ALTER FUNCTION terminate_nothrow OWNER TO pg_signal_backend;
+SELECT backend_type FROM pg_stat_activity
+WHERE CASE WHEN COALESCE(usesysid, 10) = 10 THEN terminate_nothrow(pid) END;
+ROLLBACK;
+
 -- test default ACLs
 \c -