Re: [PATCH v7 2/2] serial: core: reject baud_base values that overflow port->uartclk in uart_set_info()

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

On Wed, Sep 30, 2026 at 12:59:01PM +0000, Hui Peng wrote:
> In uart_set_info(), new_info->baud_base is multiplied by 16 and stored in
> uport->uartclk (an unsigned int):
> 
>   uport->uartclk = new_info->baud_base * 16;
> 
> While uart_set_info() checks if (uartclk == 0) and
> if (new_info->baud_base < 9600), when new_info->baud_base exceeds
> UINT_MAX / 16 with low bits set (for example, 0x10000001), multiplying
> by 16 wraps around in 32-bit unsigned arithmetic to a small non-zero value
> (16), bypassing both uartclk == 0 and new_info->baud_base < 9600 and
> setting uport->uartclk = 16 (baud_base = 1, well below the required
> minimum of 9600 * 16).
> 
> Use check_mul_overflow(new_info->baud_base, 16, &uartclk) in
> uart_set_info() to reject overflowing baud_base values with -EINVAL.
> 
> Tested in QEMU against Linux 7.3.0-rc3 by calling ioctl(fd, TIOCSSERIAL,
> &ss) with ss.baud_base = 0x10000001 on /dev/ttyS1: on the unfixed kernel
> TIOCSSERIAL succeeds (ret = 0) and wraps uport->uartclk to 16
> (TIOCGSERIAL reports baud_base = 1), whereas with the fix applied
> TIOCSSERIAL returns -EINVAL and preserves the existing uport->uartclk.
> 
> Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
> Fixes: 6eabce6608d6 ("serial: core: check uartclk for zero to avoid divide by zero")
> Cc: stable@vger.kernel.org
> Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
> Assisted-by: LLM
> Signed-off-by: Hui Peng <benquike@gmail.com>
> ---
> Changes in v7:
> - Use check_mul_overflow() instead of raw UINT_MAX / 16 comparison as
>   suggested by Jiri Slaby.
> 
>  drivers/tty/serial/serial_core.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/tty/serial/serial_core.c b/drivers/tty/serial/serial_core.c
> index 6ed0195e6912..9a8f4c2e1180 100644
> --- a/drivers/tty/serial/serial_core.c
> +++ b/drivers/tty/serial/serial_core.c
> @@ -14,6 +14,7 @@
>  #include <linux/tty_flip.h>
>  #include <linux/serial_core.h>
>  #include <linux/console.h>
> +#include <linux/overflow.h>
>  #include <linux/gpio/consumer.h>
>  #include <linux/kernel.h>
>  #include <linux/of.h>
> @@ -931,9 +932,11 @@ static int uart_set_info(struct tty_struct *tty, struct tty_port *port,
>  	old_custom_divisor = uport->custom_divisor;
> 
>  	if (!(uport->flags & UPF_FIXED_PORT)) {
> -		unsigned int uartclk = new_info->baud_base * 16;
> 
>  		/* check needs to be done here before other settings made */
> -		if (uartclk == 0)
> +		unsigned int uartclk;

As Andy said, this style change is not ok.

Please slow down and be more careful.

thanks,

greg k-h