[PATCH] fbcon: Fix boundary checks for fbcon=vc:n1-n2 parameters

[Daniel Vetter]

On Thu, Jun 02, 2022 at 10:06:28PM +0200, Helge Deller wrote:
> The user may use the fbcon=vc:<n1>-<n2> option to tell fbcon to take
> over the given range (n1...n2) of consoles. The value for n1 and n2
> needs to be a positive number and up to (MAX_NR_CONSOLES - 1).
> The given values were not fully checked against those boundaries yet.
> 
> To fix the issue, convert first_fb_vc and last_fb_vc to unsigned
> integers and check them against the upper boundary, and make sure that
> first_fb_vc is smaller than last_fb_vc.
> 
> Signed-off-by: Helge Deller <deller at gmx.de>
> Cc: stable at vger.kernel.org

Reviewed-by: Daniel Vetter <daniel.vetter at ffwll.ch>

> 
> diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
> index 2fc1b80a26ad..c2f9e5711c39 100644
> --- a/drivers/video/fbdev/core/fbcon.c
> +++ b/drivers/video/fbdev/core/fbcon.c
> @@ -115,8 +115,8 @@ static int logo_lines;
>     enums.  */
>  static int logo_shown = FBCON_LOGO_CANSHOW;
>  /* console mappings */
> -static int first_fb_vc;
> -static int last_fb_vc = MAX_NR_CONSOLES - 1;
> +static unsigned int first_fb_vc;
> +static unsigned int last_fb_vc = MAX_NR_CONSOLES - 1;
>  static int fbcon_is_default = 1;
>  static int primary_device = -1;
>  static int fbcon_has_console_bind;
> @@ -464,10 +464,12 @@ static int __init fb_console_setup(char *this_opt)
>  			options += 3;
>  			if (*options)
>  				first_fb_vc = simple_strtoul(options, &options, 10) - 1;
> -			if (first_fb_vc < 0)
> +			if (first_fb_vc >= MAX_NR_CONSOLES)
>  				first_fb_vc = 0;
>  			if (*options++ == '-')
>  				last_fb_vc = simple_strtoul(options, &options, 10) - 1;
> +			if (last_fb_vc < first_fb_vc || last_fb_vc >= MAX_NR_CONSOLES)
> +				last_fb_vc = MAX_NR_CONSOLES - 1;
>  			fbcon_is_default = 0;
>  			continue;
>  		}

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch