tty: link tty and port before configuring it as console

BugLink: https://bugs.launchpad.net/bugs/1860131
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>
Cc: stable <stable@vger.kernel.org>
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: Paolo Pisati <paolo.pisati@canonical.com>

diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
index b0a6eb106edb83d330e484ce9777d4b96fc350da..7c278278573634df5e0e82833ad6ff827ef6a13f 100644 (file)
--- 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 044c3cbdcfa40664497d13bd00e607584eff99c7..5023c85ebc6e0002d9984ad8e24090b8531a7c8b 100644 (file)
--- 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);