3.16.83-rc1 review patch. If anyone has any objections, please let me know. ------------------ From: Sudip Mukherjee <sudipm.mukherjee@xxxxxxxxx> 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@xxxxxxxx> Signed-off-by: Sudip Mukherjee <sudipm.mukherjee@xxxxxxxxx> Link: https://lore.kernel.org/r/20191212131602.29504-1-sudipm.mukherjee@xxxxxxxxx Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx> Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx> --- drivers/tty/serial/serial_core.c | 1 + drivers/tty/tty_port.c | 3 ++- 2 files changed, 3 insertions(+), 1 deletion(-) --- 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 lockdep_set_class(&uport->lock, &port_lock_key); } + tty_port_link_device(port, drv->tty_driver, uport->line); uart_configure_port(drv, state, uport); /* --- a/drivers/tty/tty_port.c +++ b/drivers/tty/tty_port.c @@ -49,7 +49,8 @@ void tty_port_link_device(struct tty_por { 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);
