linux-kernel - [PATCH v10 1/1] serial: 8250: fix panic due to PSLVERR

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Message-Id: <20250723023322.464-2-cuiyunhui@bytedance.com>
Date: Wed, 23 Jul 2025 10:33:22 +0800
From: Yunhui Cui <cuiyunhui@...edance.com>
To: gregkh@...uxfoundation.org,
	jirislaby@...nel.org,
	ilpo.jarvinen@...ux.intel.com,
	john.ogness@...utronix.de,
	andriy.shevchenko@...ux.intel.com,
	matt.porter@...aro.org,
	tim.kryger@...aro.org,
	markus.mayer@...aro.org,
	heikki.krogerus@...ux.intel.com,
	linux-kernel@...r.kernel.org,
	linux-serial@...r.kernel.org
Cc: Yunhui Cui <cuiyunhui@...edance.com>,
	stable@...r.kernel.org
Subject: [PATCH v10 1/1] serial: 8250: fix panic due to PSLVERR

When the PSLVERR_RESP_EN parameter is set to 1, the device generates
an error response if an attempt is made to read an empty RBR (Receive
Buffer Register) while the FIFO is enabled.

In serial8250_do_startup(), calling serial_port_out(port, UART_LCR,
UART_LCR_WLEN8) triggers dw8250_check_lcr(), which invokes
dw8250_force_idle() and serial8250_clear_and_reinit_fifos(). The latter
function enables the FIFO via serial_out(p, UART_FCR, p->fcr).
Execution proceeds to the serial_port_in(port, UART_RX).
This satisfies the PSLVERR trigger condition.

When another CPU (e.g., using printk()) is accessing the UART (UART
is busy), the current CPU fails the check (value & ~UART_LCR_SPAR) ==
(lcr & ~UART_LCR_SPAR) in dw8250_check_lcr(), causing it to enter
dw8250_force_idle().

Put serial_port_out(port, UART_LCR, UART_LCR_WLEN8) under the port->lock
to fix this issue.

Panic backtrace:
[    0.442336] Oops - unknown exception [#1]
[    0.442343] epc : dw8250_serial_in32+0x1e/0x4a
[    0.442351]  ra : serial8250_do_startup+0x2c8/0x88e
...
[    0.442416] console_on_rootfs+0x26/0x70

Fixes: c49436b657d0 ("serial: 8250_dw: Improve unwritable LCR workaround")
Link: https://lore.kernel.org/all/84cydt5peu.fsf@jogness.linutronix.de/T/
Signed-off-by: Yunhui Cui <cuiyunhui@...edance.com>
Reviewed-by: John Ogness <john.ogness@...utronix.de>
Cc: stable@...r.kernel.org
---
 drivers/tty/serial/8250/8250_port.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 7eddcab318b4b..2da9db960d09f 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -2269,9 +2269,9 @@ static void serial8250_initialize(struct uart_port *port)
 {
 	unsigned long flags;

+	uart_port_lock_irqsave(port, &flags);
 	serial_port_out(port, UART_LCR, UART_LCR_WLEN8);

-	uart_port_lock_irqsave(port, &flags);
 	serial8250_init_mctrl(port);
 	serial8250_iir_txen_test(port);
 	uart_port_unlock_irqrestore(port, flags);
-- 
2.39.5