| Message ID | 20240112075732.16730-1-2045gemini@gmail.com |
|---|---|
| State | New |
| Headers | show |
| Series | serial: core: Fix atomicity violation in uart_tiocmget | expand |
On 2024-01-12, Gui-Dong Han <2045gemini@gmail.com> 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@vger.kernel.org > Signed-off-by: Gui-Dong Han <2045gemini@gmail.com> > --- > 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@linutronix.de>
On Fri, 12 Jan 2024, John Ogness wrote: > On 2024-01-12, Gui-Dong Han <2045gemini@gmail.com> 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).
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
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); }
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") Cc: stable@vger.kernel.org Signed-off-by: Gui-Dong Han <2045gemini@gmail.com> --- drivers/tty/serial/serial_core.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)