Re: [PATCH v2 2/2] tty: use tty_init_dev_retry() to workaround a race condition

[Sudip Mukherjee]

Hi Jiri,

On Fri, Nov 22, 2019 at 10:05:09AM +0100, Jiri Slaby wrote:
> On 21. 11. 19, 22:01, Sudip Mukherjee wrote:
> > Hi Greg,
> > 
> > On Thu, Nov 21, 2019 at 05:41:38PM +0100, Greg Kroah-Hartman wrote:
> >> On Thu, Nov 21, 2019 at 03:22:39PM +0000, Sudip Mukherjee wrote:
> >>> There seems to be a race condition in tty drivers and I could see on
> >>> many boot cycles a NULL pointer dereference as tty_init_dev() tries to
> >>> do 'tty->port->itty = tty' even though tty->port is NULL.
<snip>
> >>>
> >>> uart_add_one_port() registers the console, as soon as it registers, the
> >>> userspace tries to use it and that leads to tty_open() but
> >>> uart_add_one_port() has not yet done tty_port_link_device() and so
> >>> tty->port is not yet configured when control reaches tty_init_dev().
> >>
> >> Shouldn't we do tty_port_link_device() before uart_add_one_port() to
> >> remove that race?  Once you register the console, yes, tty_open() can
> >> happen, so the driver had better be ready to go at that point in time.
> >>
> > 
> > But tty_port_link_device() is done by uart_add_one_port() itself.
> > After registering the console uart_add_one_port() will call
> > tty_port_register_device_attr_serdev() and tty_port_link_device() is
> > called from this. Thats still tty core.
> 
> Interferences of console vs tty code are ugly. Does it help to simply
> put tty_port_link_device to uart_add_one_port before uart_configure_port?

sorry for the late response, got busy with an out-of-tree driver.

It fixes the problem if I put tty_port_link_device() before
uart_configure_port(). Please check the attached patch and that
completely fixes the problem. Do you want me to send a proper patch for
it or do you want me to check more into it?

--
Regards
Sudip
diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 351843f847c0..006d478a63be 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -2820,6 +2820,7 @@ int uart_add_one_port(struct uart_driver *drv, struct uart_port *uport)
 	if (uport->cons && uport->dev)
 		of_console_check(uport->dev->of_node, uport->cons->name, uport->line);
 
+	tty_port_link_device(port, drv->tty_driver, uport->line);
 	uart_configure_port(drv, state, uport);
 
 	port->console = uart_console(uport);
diff --git a/drivers/tty/tty_port.c b/drivers/tty/tty_port.c
index a9e12b3bc31d..a4f85fc75539 100644
--- a/drivers/tty/tty_port.c
+++ b/drivers/tty/tty_port.c
@@ -89,7 +89,8 @@ void tty_port_link_device(struct tty_port *port,
 {
 	if (WARN_ON(index >= driver->num))
 		return;
-	driver->ports[index] = port;
+	if (!driver->ports[index])
+		driver->ports[index] = port;
 }
 EXPORT_SYMBOL_GPL(tty_port_link_device);