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

[George Kennedy]

Hi Geert,

On 10/26/2021 4:30 AM, Geert Uytterhoeven wrote:
> Hi George,
>
> 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>
> > > > ---
> > > >    drivers/video/fbdev/cirrusfb.c | 3 +++
> > > >    1 file changed, 3 insertions(+)
> > > >
> > > > diff --git a/drivers/video/fbdev/cirrusfb.c b/drivers/video/fbdev/cirrusfb.c
> > > > index 93802ab..099ddcb 100644
> > > > --- 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

We can plug in a default value, but I believe it is just covering up the 
fact that an incorrect value has been copied in.

I would think we would want to keep this driver consistent with the 
other fb drivers that return failure with the incorrect value.

Thank you,
George
> > > Shouldn't you be checking further up the call chain where this got set
> > > to 0?
> > The same pixclock check is done in these fb drivers:
> >
> > arch/arm/mach-rpc/include/mach/acornfb.h:    if (!var->pixclock)
> > drivers/video/fbdev/asiliantfb.c:    if (!var->pixclock)
> > drivers/video/fbdev/clps711x-fb.c:    if (!var->pixclock)
> > drivers/video/fbdev/core/fbmon.c:    if (!var->pixclock)
> > drivers/video/fbdev/core/modedb.c:    if (!var->pixclock)
> > drivers/video/fbdev/cirrusfb.c:    if (!var->pixclock)
> > drivers/video/fbdev/kyro/fbdev.c:    if (!var->pixclock)
> > drivers/video/fbdev/riva/fbdev.c:    if (!var->pixclock)
> > drivers/video/fbdev/uvesafb.c:    if (!var->pixclock)
> >
> > > What logic allows this to be a valid value?  What about all other fb
> > > drivers?
> > The "check_var" function, which is set into the ".fb_check_var" element
> > of the fb_ops struct, should do the check, but in the case of cirrusfb,
> > that is not being done.
> >
> > All this patch does is add the same pixclock check that the other above
> > fb drivers do.
> Indeed, several drivers are not following the rounding rules.
>
> 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