Re: A data race between tty_port_open() and uart_do_autoconfig()

[Greg KH <greg@kroah.com>]

On Wed, Apr 14, 2021 at 12:17:54AM +0000, Gong, Sishuai wrote:
> Hi,
> 
> We found a data race between two tty functions tty_port_open() and uart_do_autoconfig() in linux-5.12-rc3 and we are able to reproduce it under x86. In general, when tty_port_open() and uart_do_autoconfig() are running in parallel, uart_do_autoconfig() may fetch an out-of-date value of port->count and enter into a different execution path, as shown below.
> 
> Currently, we haven’t found any explicit errors due to this data race but we noticed the developer has used lock to read port->count, so we want to point out this data race in case this is unexpected.
> 
> ------------------------------------------
> Execution interleaving
> 
> Thread 1					Thread 2
> tty_port_open()			uart_do_autoconfig()
> 
> spin_lock_irq(&port->lock);
> 						if (mutex_lock_interruptible(&port->mutex))
> 						…
> 						if (tty_port_users(port) == 1) {
> 							uart_shutdown(tty, state);
> 
> ++port->count;
> spin_unlock_irq(&port->lock);
> 
> 
> 

Can you send a proposed patch for this to fix the issue as you sem to
have a reproducer for this that you can test if the change resolves the
issue or not?

thanks,

greg k-h