serial: 8250: Fix race condition in RTS-after-send handling

stable inclusion
from stable-4.19.238
commit 157224dbc7270c1f9af791d96b68b8aa19f0168e
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I5A6BA
CVE: NA

--------------------------------

[ Upstream commit dedab69f ]

Set em485->active_timer = NULL isn't always enough to take out the stop
timer. While there is a check that it acts in the right state (i.e.
waiting for RTS-after-send to pass after sending some chars) but the
following might happen:

 - CPU1: some chars send, shifter becomes empty, stop tx timer armed
 - CPU0: more chars send before RTS-after-send expired
 - CPU0: shifter empty irq, port lock taken
 - CPU1: tx timer triggers, waits for port lock
 - CPU0: em485->active_timer = &em485->stop_tx_timer, hrtimer_start(),
   releases lock()
 - CPU1: get lock, see em485->active_timer == &em485->stop_tx_timer,
   tear down RTS too early

This fix bases on research done by Steffen Trumtrar.

Fixes: b86f86e8 ("serial: 8250: fix potential deadlock in rs485-mode")
Signed-off-by: NUwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20220215160236.344236-1-u.kleine-koenig@pengutronix.deSigned-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: NSasha Levin <sashal@kernel.org>
Signed-off-by: NYongqiang Liu <liuyongqiang13@huawei.com>

drivers/tty/serial/8250/8250_port.c

@@ -1593,6 +1593,18 @@ static inline void start_tx_rs485(struct uart_port *port)
 	if (!(up->port.rs485.flags & SER_RS485_RX_DURING_TX))
 		serial8250_stop_rx(&up->port);
 
+	/*
+	 * While serial8250_em485_handle_stop_tx() is a noop if
+	 * em485->active_timer != &em485->stop_tx_timer, it might happen that
+	 * the timer is still armed and triggers only after the current bunch of
+	 * chars is send and em485->active_timer == &em485->stop_tx_timer again.
+	 * So cancel the timer. There is still a theoretical race condition if
+	 * the timer is already running and only comes around to check for
+	 * em485->active_timer when &em485->stop_tx_timer is armed again.
+	 */
+	if (em485->active_timer == &em485->stop_tx_timer)
+		hrtimer_try_to_cancel(&em485->stop_tx_timer);
+
 	em485->active_timer = NULL;
 
 	mcr = serial8250_in_MCR(up);