Commit 1da082f7 authored Feb 15, 2022 by Uwe Kleine-König Committed by Greg Kroah-Hartman Apr 15, 2022

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



[ Upstream commit dedab69fd650ea74710b2e626e63fd35584ef773 ]

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: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20220215160236.344236-1-u.kleine-koenig@pengutronix.de


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>

parent 617d9c0b

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

+12 −0

Original line number	Diff line number	Diff line
		@@ -1545,6 +1545,18 @@ 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);