serial: protect uart_port_dtr_rts() in uart_shutdown() too

JIRA: https://issues.redhat.com/browse/RHEL-63838
CVE: CVE-2024-50058

commit 602baba
Author: Jiri Slaby (SUSE) <jirislaby@kernel.org>
Date:   Mon Aug 5 12:20:35 2024 +0200

    serial: protect uart_port_dtr_rts() in uart_shutdown() too

    Commit af224ca (serial: core: Prevent unsafe uart port access, part
    3) added few uport == NULL checks. It added one to uart_shutdown(), so
    the commit assumes, uport can be NULL in there. But right after that
    protection, there is an unprotected "uart_port_dtr_rts(uport, false);"
    call. That is invoked only if HUPCL is set, so I assume that is the
    reason why we do not see lots of these reports.

    Or it cannot be NULL at this point at all for some reason :P.

    Until the above is investigated, stay on the safe side and move this
    dereference to the if too.

    I got this inconsistency from Coverity under CID 1585130. Thanks.

    Signed-off-by: Jiri Slaby (SUSE) <jirislaby@kernel.org>
    Cc: Peter Hurley <peter@hurleysoftware.com>
    Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    Link: https://lore.kernel.org/r/20240805102046.307511-3-jirislaby@kernel.org
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Signed-off-by: CKI Backport Bot <cki-ci-bot+cki-gitlab-backport-bot@redhat.com>

Author: CKI Backport Bot

drivers/tty/serial/serial_core.c

@@ -374,14 +374,16 @@ static void uart_shutdown(struct tty_struct *tty, struct uart_state *state)
 		/*
 		 * Turn off DTR and RTS early.
 		 */
-		if (uport && uart_console(uport) && tty) {
-			uport->cons->cflag = tty->termios.c_cflag;
-			uport->cons->ispeed = tty->termios.c_ispeed;
-			uport->cons->ospeed = tty->termios.c_ospeed;
-		}
+		if (uport) {
+			if (uart_console(uport) && tty) {
+				uport->cons->cflag = tty->termios.c_cflag;
+				uport->cons->ispeed = tty->termios.c_ispeed;
+				uport->cons->ospeed = tty->termios.c_ospeed;
+			}
 
-		if (!tty || C_HUPCL(tty))
-			uart_port_dtr_rts(uport, false);
+			if (!tty || C_HUPCL(tty))
+				uart_port_dtr_rts(uport, false);
+		}
 
 		uart_port_shutdown(port);
 	}