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

3 files changed, 40 insertions, 2 deletions

diff --git a/src/backend/storage/ipc/signalfuncs.c b/src/backend/storage/ipc/signalfuncs.c
index d822e82cb98..126ceac6ed6 100644
--- a/src/backend/storage/ipc/signalfuncs.c
+++ b/src/backend/storage/ipc/signalfuncs.c
@@ -68,8 +68,13 @@ 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 1a62dde37b6..5cff2e4695c 100644
--- a/src/test/regress/expected/privileges.out
+++ b/src/test/regress/expected/privileges.out
@@ -1793,6 +1793,24 @@ 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 ebfa9d7d554..c5c950f493f 100644
--- a/src/test/regress/sql/privileges.sql
+++ b/src/test/regress/sql/privileges.sql
@@ -1099,6 +1099,21 @@ 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 -