[PATCH] fbdev: fbmem: Fix the implicit type casting

[Sam Ravnborg]

Hi Helge,

On Tue, Feb 01, 2022 at 04:02:40PM +0100, Helge Deller wrote:
> On 1/31/22 07:57, Yizhuo Zhai wrote:
> > In function do_fb_ioctl(), the "arg" is the type of unsigned long,
> 
> yes, because it comes from the ioctl framework...
> 
> > and in "case FBIOBLANK:" this argument is casted into an int before
> > passig to fb_blank().
> 
> which makes sense IMHO.
> 
> > In fb_blank(), the comparision if (blank > FB_BLANK_POWERDOWN) would
> > be bypass if the original "arg" is a large number, which is possible
> > because it comes from the user input.
> 
> The main problem I see with your patch is that you change the behaviour.
> Let's assume someone passes in -1UL.
> With your patch applied, this means the -1 (which is e.g. 0xffffffff on 32bit)
> is converted to a positive integer and will be capped to FB_BLANK_POWERDOWN.
> Since most blank functions just check and react on specific values, you changed
> the behaviour that the screen now gets blanked at -1, while it wasn't before.
> 
> One could now argue, that it's undefined behaviour if people
> pass in wrong values, but anyway, it's different now.

We should just plug this hole and in case an illegal value is passed
then return -EINVAL.

Acceptable values are FB_BLANK_UNBLANK..FB_BLANK_POWERDOWN,
anything less than or greater than should result in -EINVAL.

We miss this kind or early checks in many places, and we see the effect
of this in some drivers where they do it locally for no good.

	Sam