tty: link tty and port before configuring it as console

commit fb2b90014d782d80d7ebf663e50f96d8c507a73c upstream.

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@suse.com>
Signed-off-by: Sudip Mukherjee <sudipm.mukherjee@gmail.com>
Link: https://lore.kernel.org/r/20191212131602.29504-1-sudipm.mukherjee@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index 6b71032b81234de2ab3164d353a2c57477a2d93e..1726bab49e4078c581c1662c8ff70b753ba9f879 100644 (file)
--- a/drivers/tty/serial/serial_core.c
+++ b/drivers/tty/serial/serial_core.c
@@ -2639,6 +2639,7 @@ int uart_add_one_port(struct uart_driver *drv, struct uart_port *uport)
                lockdep_set_class(&uport->lock, &port_lock_key);
        }
 
+       tty_port_link_device(port, drv->tty_driver, uport->line);
        uart_configure_port(drv, state, uport);
 
        /*

diff --git a/drivers/tty/tty_port.c b/drivers/tty/tty_port.c
index 3f746c8eb0dd410f36f48c29de2f4835f801c05b..bfdd93ab247215ee754781010596cd6c31c87e20 100644 (file)
--- a/drivers/tty/tty_port.c
+++ b/drivers/tty/tty_port.c
@@ -49,7 +49,8 @@ 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);