serial: 8250: lock port in startup() callbacks

uart_ops startup() callback is called without interrupts
disabled and without port->lock locked, relatively late during the
boot process (from the call path of console_on_rootfs()). If the
device is a console, it was already previously registered and could
be actively printing messages.

The console printing function serial8250_console_write() modifies
the interrupt register (UART_IER) under the port->lock with the
pattern: read, clear, restore.

Since some startup() callbacks are modifying UART_IER without the
port->lock locked, it is possible that the value intended to be
written by the startup() callback will get overwritten and be
lost.

CPU0                           CPU1
serial8250_console_write       omap_8250_startup
--------------------------     -----------------
spin_lock(port->lock)
oldval = read(UART_IER)
uart_console_write()
                               write(newval, UART_IER)
write(oldval, UART_IER)
spin_unlock(port->lock)

Add port->lock synchronization to the 8250 startup() callbacks
where they need to access UART_IER. This avoids racing with
serial8250_console_write().

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Link: https://lore.kernel.org/r/20230525093159.223817-2-john.ogness@linutronix.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

1 files changed, 16 insertions, 2 deletions

diff --git a/drivers/tty/serial/8250/8250_port.c b/drivers/tty/serial/8250/8250_port.c
index 2d8f87f4e40f..ecfcdc02eeaf 100644
--- a/drivers/tty/serial/8250/8250_port.c
+++ b/drivers/tty/serial/8250/8250_port.c
@@ -2150,7 +2150,12 @@ int serial8250_do_startup(struct uart_port *port)
 
 	serial8250_rpm_get(up);
 	if (port->type == PORT_16C950) {
-		/* Wake up and initialize UART */
+		/*
+		 * Wake up and initialize UART
+		 *
+		 * Synchronize UART_IER access against the console.
+		 */
+		spin_lock_irqsave(&port->lock, flags);
 		up->acr = 0;
 		serial_port_out(port, UART_LCR, UART_LCR_CONF_MODE_B);
 		serial_port_out(port, UART_EFR, UART_EFR_ECB);
@@ -2160,12 +2165,19 @@ int serial8250_do_startup(struct uart_port *port)
 		serial_port_out(port, UART_LCR, UART_LCR_CONF_MODE_B);
 		serial_port_out(port, UART_EFR, UART_EFR_ECB);
 		serial_port_out(port, UART_LCR, 0);
+		spin_unlock_irqrestore(&port->lock, flags);
 	}
 
 	if (port->type == PORT_DA830) {
-		/* Reset the port */
+		/*
+		 * Reset the port
+		 *
+		 * Synchronize UART_IER access against the console.
+		 */
+		spin_lock_irqsave(&port->lock, flags);
 		serial_port_out(port, UART_IER, 0);
 		serial_port_out(port, UART_DA830_PWREMU_MGMT, 0);
+		spin_unlock_irqrestore(&port->lock, flags);
 		mdelay(10);
 
 		/* Enable Tx, Rx and free run mode */
@@ -2276,6 +2288,8 @@ int serial8250_do_startup(struct uart_port *port)
 		 * this interrupt whenever the transmitter is idle and
 		 * the interrupt is enabled.  Delays are necessary to
 		 * allow register changes to become visible.
+		 *
+		 * Synchronize UART_IER access against the console.
 		 */
 		spin_lock_irqsave(&port->lock, flags);