On Fri, 12 Jan 2024, Gui-Dong Han wrote:
> In uart_throttle() and uart_unthrottle():
> if (port->status & mask) {
> port->ops->throttle/unthrottle(port);
> mask &= ~port->status;
> }
> // Code segment utilizing the mask value to determine UART behavior
>
> In uart_change_line_settings():
> uart_port_lock_irq(uport);
> // Code segment responsible for updating uport->status
> uart_port_unlock_irq(uport);
>
> In the uart_throttle() and uart_unthrottle() functions, there is a double
> fetch issue due to concurrent execution with uart_change_line_settings().
> In uart_throttle() and uart_unthrottle(), the check
> if (port->status & mask) is made, followed by mask &= ~port->status,
> where the relevant bits are cleared. However, port->status may be modified
> in uart_change_line_settings(). The current implementation does not ensure
> atomicity in the access and modification of port->status and mask. This
> can result in mask being updated based on a modified port->status value,
> leading to improper UART actions.
>
> 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 resolve this double fetch, it is suggested to add a uart_port_lock pair
> in uart_throttle() and uart_unthrottle(). 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: 391f93f2ec9f ("serial: core: Rework hw-assisted flow control support")
> Cc: [email protected]
> Signed-off-by: Gui-Dong Han <[email protected]>
> ---
> drivers/tty/serial/serial_core.c | 6 +++++-
> 1 file changed, 5 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
> index 80085b151b34..9d905fdf2843 100644
> --- a/drivers/tty/serial/serial_core.c
> +++ b/drivers/tty/serial/serial_core.c
> @@ -723,11 +723,13 @@ static void uart_throttle(struct tty_struct *tty)
> mask |= UPSTAT_AUTOXOFF;
> if (C_CRTSCTS(tty))
> mask |= UPSTAT_AUTORTS;
> -
> +
> + uart_port_lock_irq(port);
> if (port->status & mask) {
> port->ops->throttle(port);
> mask &= ~port->status;
> }
> + uart_port_unlock_irq(port);
>
> if (mask & UPSTAT_AUTORTS)
> uart_clear_mctrl(port, TIOCM_RTS);
> @@ -753,10 +755,12 @@ static void uart_unthrottle(struct tty_struct *tty)
> if (C_CRTSCTS(tty))
> mask |= UPSTAT_AUTORTS;
>
> + uart_port_lock_irq(port);
> if (port->status & mask) {
> port->ops->unthrottle(port);
> mask &= ~port->status;
> }
> + uart_port_unlock_irq(port);
>
> if (mask & UPSTAT_AUTORTS)
> uart_set_mctrl(port, TIOCM_RTS);
Hi,
This is very bogus "fix". While change to the local variable gets
"protected", uart_change_line_settings() can race after unlock and the
value held in mask is again stale.
If, and it's a big if, this is a real problem, the patch does not fix
anything! It proves your tool is flawed because it doesn't detect the
race with uart_change_line_settings() issue still exists after this
non-fix.
So NAK from me. Please provide a real fix instead if you think there is
a real issue.
Also, don't use vague wording like "leading to improper UART action" but
describe precisely what goes wrong!
--
i.
Hi
I apologize for any issues and lack of clarity in my previous patch.
In patch v2, I've revised the fix to use local variables instead of
locks and improved the description to clearly explain the harm and
potential for concurrency.
The patch was developed and tested on linux-next, not Linux 5.17. My
reference to 5.17 was due to a project I'm working on, which involves
kernel static analysis and a comparison with earlier studies that
support up to Linux 5.17. Therefore, I initially ran my tool on 5.17
to filter potential bugs that are still unaddressed in the upstream.
Then I worked on linux-next to develop and test the patch. I
understand this might have caused misunderstandings, and I'll aim for
clearer communication in future submissions.
Thanks,
Han