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"). > 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>
