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

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

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.

...

>  	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;

Don't move the variable definition (by a location) without need. I do not
see any need of doing it like that. Moreover this change will add a (style)
regression, id est unneeded blank line. So, just drop the assignment and
leave the definition as it's now.

> +		if (check_mul_overflow(new_info->baud_base, 16, &uartclk) ||
> +		    uartclk == 0)
>  			return -EINVAL;
>  	}

-- 
With Best Regards,
Andy Shevchenko