On Fri, 12 Jan 2024, John Ogness wrote: > On 2024-01-12, Gui-Dong Han <2045gemini@xxxxxxxxx> wrote: > > In uart_tiocmget(): > > result = uport->mctrl; > > uart_port_lock_irq(uport); > > result |= uport->ops->get_mctrl(uport); > > uart_port_unlock_irq(uport); > > ... > > return result; > > > > In uart_update_mctrl(): > > uart_port_lock_irqsave(port, &flags); > > ... > > port->mctrl = (old & ~clear) | set; > > ... > > uart_port_unlock_irqrestore(port, flags); > > > > An atomicity violation is identified due to the concurrent execution of > > uart_tiocmget() and uart_update_mctrl(). After assigning > > result = uport->mctrl, the mctrl value may change in uart_update_mctrl(), > > leading to a mismatch between the value returned by > > uport->ops->get_mctrl(uport) and the mctrl value previously read. > > This can result in uart_tiocmget() returning an incorrect value. > > > > This possible bug is found by an experimental static analysis tool > > developed by our team, BassCheck[1]. This tool analyzes the locking APIs > > to extract function pairs that can be concurrently executed, and then > > analyzes the instructions in the paired functions to identify possible > > concurrency bugs including data races and atomicity violations. The above > > possible bug is reported when our tool analyzes the source code of > > Linux 5.17. > > > > To address this issue, it is suggested to move the line > > result = uport->mctrl inside the uart_port_lock block to ensure atomicity > > and prevent the mctrl value from being altered during the execution of > > uart_tiocmget(). With this patch applied, our tool no longer reports the > > bug, with the kernel configuration allyesconfig for x86_64. Due to the > > absence of the requisite hardware, we are unable to conduct runtime > > testing of the patch. Therefore, our verification is solely based on code > > logic analysis. > > > > [1] https://sites.google.com/view/basscheck/ > > > > Fixes: 559c7ff4e324 ("serial: core: Use port lock wrappers") > > It fixes c5f4644e6c8b ("[PATCH] Serial: Adjust serial locking"). That commit only extracted the locks from ->get_mctrl() into the caller but this assignment was outside both pre and post that commit (the issue goes all the way back into history.git domain into 33c0d1b0c3eb ("[PATCH] Serial driver stuff") which introduced ->mctrl). -- i. > > Cc: stable@xxxxxxxxxxxxxxx > > Signed-off-by: Gui-Dong Han <2045gemini@xxxxxxxxx> > > --- > > drivers/tty/serial/serial_core.c | 2 +- > > 1 file changed, 1 insertion(+), 1 deletion(-) > > > > diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c > > index 80085b151b34..a9e39416d877 100644 > > --- a/drivers/tty/serial/serial_core.c > > +++ b/drivers/tty/serial/serial_core.c > > @@ -1085,8 +1085,8 @@ static int uart_tiocmget(struct tty_struct *tty) > > goto out; > > > > if (!tty_io_error(tty)) { > > - result = uport->mctrl; > > uart_port_lock_irq(uport); > > + result = uport->mctrl; > > result |= uport->ops->get_mctrl(uport); > > uart_port_unlock_irq(uport); > > } > > Looking over the RMW accesses to @mctrl, I expect you will also need > this hunk: > > @@ -2242,6 +2242,7 @@ uart_set_options(struct uart_port *port, struct console *co, > { > struct ktermios termios; > static struct ktermios dummy; > + unsigned long flags; > > /* > * Ensure that the serial-console lock is initialised early. > @@ -2279,7 +2280,9 @@ uart_set_options(struct uart_port *port, struct console *co, > * some uarts on other side don't support no flow control. > * So we set * DTR in host uart to make them happy > */ > + uart_port_lock_irqsave(port, &flags); > port->mctrl |= TIOCM_DTR; > + uart_port_unlock_irqrestore(port, flags); > > port->ops->set_termios(port, &termios, &dummy); > /* > > FWIW, > Reviewed-by: John Ogness <john.ogness@xxxxxxxxxxxxx> >
