Re: [PATCH] serial: core: Fix atomicity violation in uart_tiocmget

[John Ogness]

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>