Re: [PATCH] video: fbdev: cirrusfb: check pixclock to avoid divide by zero

[George Kennedy]

On 10/27/2021 2:53 AM, Geert Uytterhoeven wrote:
> Hi George,
>
> On Wed, Oct 27, 2021 at 3:13 AM George Kennedy
> <george.kennedy@xxxxxxxxxx> wrote:
> > On 10/26/2021 1:12 PM, Geert Uytterhoeven wrote:
> > > On Tue, Oct 26, 2021 at 5:48 PM George Kennedy
> > > <george.kennedy@xxxxxxxxxx> wrote:
> > > > On 10/26/2021 10:11 AM, Geert Uytterhoeven wrote:
> > > > > On Tue, Oct 26, 2021 at 3:38 PM George Kennedy
> > > > > <george.kennedy@xxxxxxxxxx> wrote:
> > > > > > On 10/26/2021 4:30 AM, Geert Uytterhoeven wrote:
> > > > > > > On Mon, Oct 25, 2021 at 9:37 PM George Kennedy
> > > > > > > <george.kennedy@xxxxxxxxxx> wrote:
> > > > > > > > On 10/25/2021 3:07 PM, Greg KH wrote:
> > > > > > > > > On Mon, Oct 25, 2021 at 02:01:30PM -0500, George Kennedy wrote:
> > > > > > > > > > Do a sanity check on pixclock value before using it as a divisor.
> > > > > > > > > >
> > > > > > > > > > Syzkaller reported a divide error in cirrusfb_check_pixclock.
> > > > > > > > > >
> > > > > > > > > > divide error: 0000 [#1] SMP KASAN PTI
> > > > > > > > > > CPU: 0 PID: 14938 Comm: cirrusfb_test Not tainted 5.15.0-rc6 #1
> > > > > > > > > > Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.0-2
> > > > > > > > > > RIP: 0010:cirrusfb_check_var+0x6f1/0x1260
> > > > > > > > > >
> > > > > > > > > > Call Trace:
> > > > > > > > > >       fb_set_var+0x398/0xf90
> > > > > > > > > >       do_fb_ioctl+0x4b8/0x6f0
> > > > > > > > > >       fb_ioctl+0xeb/0x130
> > > > > > > > > >       __x64_sys_ioctl+0x19d/0x220
> > > > > > > > > >       do_syscall_64+0x3a/0x80
> > > > > > > > > >       entry_SYSCALL_64_after_hwframe+0x44/0xae
> > > > > > > > > >
> > > > > > > > > > Signed-off-by: George Kennedy <george.kennedy@xxxxxxxxxx>
> > > > > > > > > > --- a/drivers/video/fbdev/cirrusfb.c
> > > > > > > > > > +++ b/drivers/video/fbdev/cirrusfb.c
> > > > > > > > > > @@ -477,6 +477,9 @@ static int cirrusfb_check_pixclock(const struct fb_var_screeninfo *var,
> > > > > > > > > >          struct cirrusfb_info *cinfo = info->par;
> > > > > > > > > >          unsigned maxclockidx = var->bits_per_pixel >> 3;
> > > > > > > > > >
> > > > > > > > > > +    if (!var->pixclock)
> > > > > > > > > > +            return -EINVAL;
> > > > > > > This is not correct: fbdev drivers should round up invalid values,
> > > > > > > and only return an error if rounding up cannot yield a valid value.
> > > > > > What default value would you recommend? Here are examples of some of the
> > > > > > possible cirrusfb pixclock values:
> > > > > > 40000: 25MHz
> > > > > > 20000: 50Mhz
> > > > > > 12500: 80Mhz
> > > > > You should pick the lowest supported value.
> > > > In bestclock() the frequency value ("freq") is not allowed to go below 8000.
> > > >
> > > >            if (freq < 8000)
> > > >                    freq = 8000;
> > > >
> > > > If pixclock is passed in as zero to cirrusfb_check_pixclock(), is it ok
> > > > to then set the value of pixclock to 125000, which will result in "freq"
> > > > being set to 8000 (or adjust the passed in pixclock value to make sure
> > > > "freq" does not get below 8000)?
> > > No, clock rate is the inverse of clock period.
> > > So the smallest clock period (fb_var_screeninfo.pixclock) corresponds
> > > to the largest clock rate (freq in bestclock()).
> > How about this?
> >
> > This gets the frequency derived from pixclock to maxclock or rounds up
> > pixclock to get the frequency as close to maxclock as possible.
> >
> > diff --git a/drivers/video/fbdev/cirrusfb.c b/drivers/video/fbdev/cirrusfb.c
> > index 93802ab..2e8e620 100644
> > --- a/drivers/video/fbdev/cirrusfb.c
> > +++ b/drivers/video/fbdev/cirrusfb.c
> > @@ -620,6 +620,18 @@ static int cirrusfb_check_var(struct
> > fb_var_screeninfo *var,
> >                   return -EINVAL;
> >           }
> >
> > +       if (!var->pixclock) {
> > +               long maxclock;
> > +               unsigned maxclockidx = var->bits_per_pixel >> 3;
> > +
> > +               maxclock =
> > cirrusfb_board_info[cinfo->btype].maxclock[maxclockidx];
> > +
> > +               var->pixclock = KHZ2PICOS(maxclock);
> > +               while (PICOS2KHZ(var->pixclock) > maxclock) {
> > +                       var->pixclock++;
> > +               }
> > +       }
> > +
> >           if (cirrusfb_check_pixclock(var, info))
> >                   return -EINVAL;
> >
> > The work can't be done in cirrusfb_check_pixclock() as var->pixclock is
> > read-only because "var" is "const struct fb_var_screeninfo *var".
> Perhaps the const should be dropped from the var parameter, so the
> rounding can be done in the function where it makes most sense,
> and where most of the above operations are already done?
>
> Then, you can simplify:
>
> -        freq = PICOS2KHZ(var->pixclock);
> +        freq = PICOS2KHZ(var->pixclock ? : 1);
>
> and change the "if (freq > maxclock) return -EINVAL" to use maxclock
> instead.

Thanks Geert,

Will make the suggested changes and send out a v2 review.

George
> Gr{oetje,eeting}s,
>
>                          Geert
>
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx
>
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
>                                  -- Linus Torvalds