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
