On 2024-01-12, Gui-Dong Han <[email protected]> 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: [email protected]
> Signed-off-by: Gui-Dong Han <[email protected]>
> ---
> 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 <[email protected]>
On Fri, 12 Jan 2024, John Ogness wrote:
> On 2024-01-12, Gui-Dong Han <[email protected]> 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: [email protected]
> > Signed-off-by: Gui-Dong Han <[email protected]>
> > ---
> > 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 <[email protected]>
>
Hi,
You are correct about the 'Fixes' tag. It should indeed be c5f4644e6c8b
("[PATCH] Serial: Adjust serial locking"). I will update this in the
patch v2.
Regarding the issue found in Linux 5.17, I mistakenly used git blame
which led to the incorrect identification of commit 559c7ff4e324. The
issue indeed exists in Linux 5.17 and I acknowledge the error in tracing
the commit.
In uart_tiocmget(), the result variable is stable. However, there's a
risk of inconsistency due to the updates in uart_update_mctrl().
Consider a scenario where uart_tiocmget() reads uport->mctrl into result
before entering the critical section. If uart_update_mctrl() updates
port->mctrl and calls set_mctrl concurrently, the subsequent execution
of result |= uport->ops->get_mctrl(uport); in uart_tiocmget() might
yield an inaccurate result. This happens because result contains the old
value of port->mctrl, which no longer matches the updated state
retrieved by get_mctrl.
Thanks,
Han