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

[Geert Uytterhoeven]

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.

> > 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