[PATCH] tty: link tty and port before configuring it as console

[Sudip Mukherjee]

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.
'tty->port' will be set by the driver and if the driver has not yet done
it before we open the tty device we can get to this situation. By adding
some extra debug prints, I noticed that:

6.650130: uart_add_one_port
6.663849: register_console
6.664846: tty_open
6.674391: tty_init_dev
6.675456: tty_port_link_device

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().

Further look into the code and tty_port_link_device() is done by
uart_add_one_port(). 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.

Call add tty_port_link_device() before uart_configure_port() is done and
add a check in tty_port_link_device() so that it only links the port if
it has not been done yet.

Suggested-by: Jiri Slaby <jslaby@xxxxxxxx>
Signed-off-by: Sudip Mukherjee <sudipm.mukherjee@xxxxxxxxx>
---

This actually should be v3. The previous discussions are at:
https://lore.kernel.org/lkml/20191120151709.14148-2-sudipm.mukherjee@xxxxxxxxx/
https://lore.kernel.org/lkml/20191121152239.28405-1-sudipm.mukherjee@xxxxxxxxx/

 drivers/tty/serial/serial_core.c | 1 +
 drivers/tty/tty_port.c           | 3 ++-
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index b0a6eb106edb..7c2782785736 100644
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -2834,6 +2834,7 @@ 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 044c3cbdcfa4..5023c85ebc6e 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);
 
-- 
2.11.0