libpq: Be strict about cancel key lengths

The protocol documentation states that the maximum length of a cancel
key is 256 bytes. This starts checking for that limit in
libpq. Otherwise third party backend implementations will probably
start using more bytes anyway. We also start requiring that a protocol
3.0 connection does not send a longer cancel key, to make sure that
servers don't start breaking old 3.0-only clients by accident. Finally
this also restricts the minimum key length to 4 bytes (both in the
protocol spec and in the libpq implementation).

Author: Jelte Fennema-Nio <[email protected]>
Discussion: https://www.postgresql.org/message-id/[email protected]

Author: hlinnaka

doc/src/sgml/protocol.sgml

@@ -4136,7 +4136,7 @@ psql "dbname=postgres replication=database" -c "IDENTIFY_SYSTEM;"
          message, indicated by the length field.
         </para>
         <para>
-          The maximum key length is 256 bytes. The
+          The minimum and maximum key length are 4 and 256 bytes, respectively. The
           <productname>PostgreSQL</productname> server only sends keys up to
           32 bytes, but the larger maximum size allows for future server
           versions, as well as connection poolers and other middleware, to use

src/interfaces/libpq/fe-protocol3.c

@@ -1569,6 +1569,27 @@ getBackendKeyData(PGconn *conn, int msgLength)
 
 	cancel_key_len = 5 + msgLength - (conn->inCursor - conn->inStart);
 
+	if (cancel_key_len != 4 && conn->pversion == PG_PROTOCOL(3, 0))
+	{
+		libpq_append_conn_error(conn, "received invalid BackendKeyData message: cancel key length %d is different from 4, which is not supported in version 3.0 of the protocol", cancel_key_len);
+		handleFatalError(conn);
+		return 0;
+	}
+
+	if (cancel_key_len < 4)
+	{
+		libpq_append_conn_error(conn, "received invalid BackendKeyData message: cancel key length %d is below minimum of 4 bytes", cancel_key_len);
+		handleFatalError(conn);
+		return 0;
+	}
+
+	if (cancel_key_len > 256)
+	{
+		libpq_append_conn_error(conn, "received invalid BackendKeyData message: cancel key length %d exceeds maximum of 256 bytes", cancel_key_len);
+		handleFatalError(conn);
+		return 0;
+	}
+
 	conn->be_cancel_key = malloc(cancel_key_len);
 	if (conn->be_cancel_key == NULL)
 	{