Hi, On Mon, Aug 1, 2022 at 12:35 PM Helge Deller <deller@xxxxxx> wrote: > > * Zheyu Ma <zheyuma97@xxxxxxxxx>: > > I found a bug in the arkfb driver in the latest kernel, which may cause DoS. > > > > The reason for this bug is that the user controls some input to ioctl, > > making 'mode' 0x7 on line 704, which causes hdiv = 1, hmul = 2, and if > > the pixclock is controlled to be 1, it will cause a division error in > > the function ark_set_pixclock(). > > You are right. > I see in: > drivers/video/fbdev/arkfb.c:784: ark_set_pixclock(info, (hdiv * info->var.pixclock) / hmul); > with hdiv=1, pixclock=1 and hmul=2 you end up with (1*1)/2 = (int) 0. > and then in > drivers/video/fbdev/arkfb.c:504: rv = dac_set_freq(par->dac, 0, 1000000000 / pixclock); > you'll get a division-by-zero. > > > The easiest patch is to check the value of the argument 'pixclock' in > > the ark_set_pixclock function, but this is perhaps too late, should we > > do this check earlier? I'm not sure, so I'll report this bug to you. > > Yes, I think it should be done earlier. > > Geert always mentioned that an invalid pixclock from userspace should be > rounded up to the next valid pixclock. > But since I don't have that hardware, I'm not sure how this can be done > best for this driver. > > Do you have the hardware to test? > If so, could you check the patch below? Thanks for your patch, it works for me :) > It should at least prevent the division-by-zero. > If it works, I'm happy if you could send a final patch... I've sent a patch to the mailing list, thanks again for your reminder. regards, Zheyu Ma
