[PATCH 4/4] serial: sc16is7xx: fix invalid FIFO access with special register set

[Hugo Villeneuve <hugo@xxxxxxxxxxx>]

From: Hugo Villeneuve <hvilleneuve@xxxxxxxxxxxx>

When enabling access to the special register set, Receiver time-out and
RHR interrupts can happen. In this case, the IRQ handler will try to read
from the FIFO thru the RHR register at address 0x00, but address 0x00 is
mapped to DLL register, resulting in erroneous FIFO reading.

Call graph example:
    sc16is7xx_startup(): entry
    sc16is7xx_ms_proc(): entry
    sc16is7xx_set_termios(): entry
    sc16is7xx_set_baud(): DLH/DLL = $009C --> access special register set
    sc16is7xx_port_irq() entry            --> IIR is 0x0C
    sc16is7xx_handle_rx() entry
    sc16is7xx_fifo_read(): --> unable to access FIFO (RHR) because it is
                               mapped to DLL (LCR=LCR_CONF_MODE_A)
    sc16is7xx_set_baud(): exit --> Restore access to general register set

Fix the problem by claiming the efr_lock mutex when accessing the Special
register set.

Reworked to fix conflicts when backporting to linux-5.15.y.

Fixes: dfeae619d781 ("serial: sc16is7xx")
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Hugo Villeneuve <hvilleneuve@xxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20240723125302.1305372-3-hugo@xxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 drivers/tty/serial/sc16is7xx.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/tty/serial/sc16is7xx.c b/drivers/tty/serial/sc16is7xx.c
index 928c701f0c35a..3b7c238793ef4 100644
--- a/drivers/tty/serial/sc16is7xx.c
+++ b/drivers/tty/serial/sc16is7xx.c
@@ -537,6 +537,8 @@ static int sc16is7xx_set_baud(struct uart_port *port, int baud)
 			      prescaler == 1 ? 0 : SC16IS7XX_MCR_CLKSEL_BIT);
 
 	/* Open the LCR divisors for configuration */
+	mutex_lock(&one->efr_lock);
+
 	sc16is7xx_port_write(port, SC16IS7XX_LCR_REG,
 			     SC16IS7XX_LCR_CONF_MODE_A);
 
@@ -549,6 +551,8 @@ static int sc16is7xx_set_baud(struct uart_port *port, int baud)
 	/* Put LCR back to the normal mode */
 	sc16is7xx_port_write(port, SC16IS7XX_LCR_REG, lcr);
 
+	mutex_unlock(&one->efr_lock);
+
 	return DIV_ROUND_CLOSEST((clk / prescaler) / 16, div);
 }
 
-- 
2.39.5