Message-Id: <20250403090336.16643-1-cuiyunhui@bytedance.com>
Date: Thu,  3 Apr 2025 17:03:36 +0800
From: Yunhui Cui <cuiyunhui@...edance.com>
To: gregkh@...uxfoundation.org,
	jirislaby@...nel.org,
	john.ogness@...utronix.de,
	pmladek@...e.com,
	arnd@...db.de,
	andriy.shevchenko@...ux.intel.com,
	namcao@...utronix.de,
	benjamin.larsson@...exis.eu,
	schnelle@...ux.ibm.com,
	linux-kernel@...r.kernel.org,
	linux-serial@...r.kernel.org
Cc: Yunhui Cui <cuiyunhui@...edance.com>
Subject: [PATCH] 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 dont_test_tx_en label:
...
serial_port_in(port, UART_RX);
This satisfies the PSLVERR trigger condition.

Because 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), causing it to enter dw8250_force_idle().

To resolve this issue, relevant serial_port_out operations should be
placed in a critical section, and UART_RX data should only be read
when the UART_LSR DR bit is set.

Panic message:
[    0.442336] Oops - unknown exception [#1]
[    0.442337] Modules linked in:
[    0.442339] CPU: 3 UID: 0 PID: 1 Comm: swapper/0 Tainted: G        W          6.12.13-00102-gf1f43e345877 #1
[    0.442342] Tainted: [W]=WARN
[    0.442343] epc : dw8250_serial_in32+0x1e/0x4a
[    0.442351]  ra : serial8250_do_startup+0x2c8/0x88e
[    0.442354] epc : ffffffff8064efca ra : ffffffff8064af28 sp : ffff8f8000103990
[    0.442355]  gp : ffffffff815bad28 tp : ffffaf807e36d400 t0 : ffffaf80804cf080
[    0.442356]  t1 : 0000000000000001 t2 : 0000000000000000 s0 : ffff8f80001039a0
[    0.442358]  s1 : ffffffff81626fc0 a0 : ffffffff81626fc0 a1 : 0000000000000000
[    0.442359]  a2 : 0000000000000000 a3 : 0000000000000000 a4 : ffffffff81626fc0
[    0.442360]  a5 : ffff8f800012d900 a6 : 000000000000000f a7 : 000000000fc648c1
[    0.442361]  s2 : 0000000000000000 s3 : 0000000200000022 s4 : 0000000000000000
[    0.442362]  s5 : ffffffff81626fc0 s6 : ffffaf8085227000 s7 : ffffffff81073c58
[    0.442363]  s8 : 0000000000500000 s9 : ffffaf80851a5a60 s10: ffffaf80851a5a60
[    0.442365]  s11: ffffffff80e85980 t3 : ffffaf807e324600 t4 : 0000000000000002
[    0.442365]  t5 : 0000000000000003 t6 : ffffaf80804cf072
[    0.442366] status: 0000000200000120 badaddr: 0000000000000000 cause: 0000000000000013
[    0.442368] [<ffffffff8064efca>] dw8250_serial_in32+0x1e/0x4a
[    0.442371] [<ffffffff8064af28>] serial8250_do_startup+0x2c8/0x88e
[    0.442373] [<ffffffff8064b514>] serial8250_startup+0x26/0x2e
[    0.442375] [<ffffffff806428a2>] uart_startup+0x13a/0x308
[    0.442377] [<ffffffff80642aa4>] uart_port_activate+0x34/0x50
[    0.442378] [<ffffffff8062ab6a>] tty_port_open+0xb4/0x110
[    0.442383] [<ffffffff8063f548>] uart_open+0x22/0x36
[    0.442389] [<ffffffff806234b4>] tty_open+0x1be/0x5e6
[    0.442396] [<ffffffff802f2d52>] chrdev_open+0x10a/0x2a8
[    0.442400] [<ffffffff802e7ab6>] do_dentry_open+0xf6/0x34e
[    0.442405] [<ffffffff802e9456>] vfs_open+0x2a/0xb4
[    0.442408] [<ffffffff80300124>] path_openat+0x676/0xf36
[    0.442410] [<ffffffff80300a58>] do_filp_open+0x74/0xfa
[    0.442412] [<ffffffff802e9900>] file_open_name+0x84/0x144
[    0.442414] [<ffffffff802e99f6>] filp_open+0x36/0x54
[    0.442416] [<ffffffff80a01232>] console_on_rootfs+0x26/0x70
[    0.442420] [<ffffffff80a0154e>] kernel_init_freeable+0x2d2/0x30e
[    0.442422] [<ffffffff8099c730>] kernel_init+0x2a/0x15e
[    0.442427] [<ffffffff809a7666>] ret_from_fork+0xe/0x1c
[    0.442430] Code: e022 e406 0800 4683 0c15 691c 872a 96bb 00d5 97b6 (439c) 851b
[    0.442432] ---[ end trace 0000000000000000 ]---
[    0.442434] Kernel panic - not syncing: Fatal exception in interrupt
[    0.442435] SMP: stopping secondary CPUs
[    0.451111] ---[ end Kernel panic - not syncing: Fatal exception in interrupt ]---

Signed-off-by: Yunhui Cui <cuiyunhui@...edance.com>
---
 drivers/tty/serial/8250/8250_port.c | 14 +++++++++-----
 1 file changed, 9 insertions(+), 5 deletions(-)

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 3f256e96c722..6909c81109db 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -2264,13 +2264,16 @@ int serial8250_do_startup(struct uart_port *port)
 	 * Clear the FIFO buffers and disable them.
 	 * (they will be reenabled in set_termios())
 	 */
+	uart_port_lock_irqsave(port, &flags);
 	serial8250_clear_fifos(up);
+	uart_port_unlock_irqrestore(port, flags);
 
 	/*
 	 * Clear the interrupt registers.
 	 */
-	serial_port_in(port, UART_LSR);
-	serial_port_in(port, UART_RX);
+	lsr = serial_port_in(port, UART_LSR);
+	if (lsr & UART_LSR_DR)
+		serial_port_in(port, UART_RX);
 	serial_port_in(port, UART_IIR);
 	serial_port_in(port, UART_MSR);
 
@@ -2380,9 +2383,9 @@ int serial8250_do_startup(struct uart_port *port)
 	/*
 	 * Now, initialize the UART
 	 */
+	uart_port_lock_irqsave(port, &flags);
 	serial_port_out(port, UART_LCR, UART_LCR_WLEN8);
 
-	uart_port_lock_irqsave(port, &flags);
 	if (up->port.flags & UPF_FOURPORT) {
 		if (!up->port.irq)
 			up->port.mctrl |= TIOCM_OUT1;
@@ -2435,8 +2438,9 @@ int serial8250_do_startup(struct uart_port *port)
 	 * saved flags to avoid getting false values from polling
 	 * routines or the previous session.
 	 */
-	serial_port_in(port, UART_LSR);
-	serial_port_in(port, UART_RX);
+	lsr = serial_port_in(port, UART_LSR);
+	if (lsr & UART_LSR_DR)
+		serial_port_in(port, UART_RX);
 	serial_port_in(port, UART_IIR);
 	serial_port_in(port, UART_MSR);
 	up->lsr_saved_flags = 0;
-- 
2.39.2

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