Re: [PATCH v2] serial: 8250: fix panic due to PSLVERR

Greg KH
2025-04-08T06:25:01Z

On Tue, Apr 08, 2025 at 02:12:00PM +0800, Yunhui Cui wrote:
> 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 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@bytedance.com>
> ---
> drivers/tty/serial/8250/8250_port.c | 20 ++++++++++++++------
> 1 file changed, 14 insertions(+), 6 deletions(-)
>
> diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
> index 3f256e96c722..3df358008489 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 +2383,10 @@ int serial8250_do_startup(struct uart_port *port)
> /*
> * Now, initialize the UART
> */
> - serial_port_out(port, UART_LCR, UART_LCR_WLEN8);
>
> uart_port_lock_irqsave(port, &flags);
> + serial_port_out(port, UART_LCR, UART_LCR_WLEN8);
> +
> if (up->port.flags & UPF_FOURPORT) {
> if (!up->port.irq)
> up->port.mctrl |= TIOCM_OUT1;
> @@ -2435,8 +2439,9 @@ 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;
> @@ -2492,6 +2497,7 @@ void serial8250_do_shutdown(struct uart_port *port)
> {
> struct uart_8250_port *up = up_to_u8250p(port);
> unsigned long flags;
> + u16 lsr;
>
> serial8250_rpm_get(up);
> /*
> @@ -2538,7 +2544,9 @@ void serial8250_do_shutdown(struct uart_port *port)
> * Read data port to reset things, and then unlink from
> * the IRQ chain.
> */
> - serial_port_in(port, UART_RX);
> + lsr = serial_port_in(port, UART_LSR);
> + if (lsr & UART_LSR_DR)
> + serial_port_in(port, UART_RX);
> serial8250_rpm_put(up);
>
> up->ops->release_irq(up);
> --
> 2.39.2
>
>

Hi,

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