This is a note to let you know that I've just added the patch titled
tty: link tty and port before configuring it as console
to my tty git tree which can be found at git://git.kernel.org/pub/scm/linux/kernel/git/gregkh/tty.git in the tty-linus branch.
The patch will show up in the next release of the linux-next tree (usually sometime within the next 24 hours during the week.)
The patch will hopefully also be merged in Linus's tree for the next -rc kernel release.
If you have any questions about this process, please let me know.
From fb2b90014d782d80d7ebf663e50f96d8c507a73c Mon Sep 17 00:00:00 2001
From: Sudip Mukherjee sudipm.mukherjee@gmail.com Date: Thu, 12 Dec 2019 13:16:02 +0000 Subject: tty: link tty and port before configuring it as console
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 --- 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);