From 508acb901e31cf10fcac0ff9304d3d2d33bb2b2f Mon Sep 17 00:00:00 2001
From: Noah Misch <noah@leadboat.com>
Date: Mon, 6 Nov 2023 06:14:13 -0800
Subject: [PATCH] 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
---
 src/backend/storage/ipc/signalfuncs.c    |  9 +++++++--
 src/test/regress/expected/privileges.out | 18 ++++++++++++++++++
 src/test/regress/sql/privileges.sql      | 15 +++++++++++++++
 3 files changed, 40 insertions(+), 2 deletions(-)

diff --git a/src/backend/storage/ipc/signalfuncs.c b/src/backend/storage/ipc/signalfuncs.c
index de69d60e79..b6ff412de4 100644
--- a/src/backend/storage/ipc/signalfuncs.c
+++ b/src/backend/storage/ipc/signalfuncs.c
@@ -69,8 +69,13 @@ 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 b3c3b257d1..edf5f3a6b1 100644
--- a/src/test/regress/expected/privileges.out
+++ b/src/test/regress/expected/privileges.out
@@ -1846,6 +1846,24 @@ 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 pg_database_owner
 RESET SESSION AUTHORIZATION;
 GRANT pg_database_owner TO regress_priv_user1;
diff --git a/src/test/regress/sql/privileges.sql b/src/test/regress/sql/privileges.sql
index af05f95113..f96143e071 100644
--- a/src/test/regress/sql/privileges.sql
+++ b/src/test/regress/sql/privileges.sql
@@ -1133,6 +1133,21 @@ 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 pg_database_owner
 RESET SESSION AUTHORIZATION;
 GRANT pg_database_owner TO regress_priv_user1;