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

[Zheyu Ma]

On Tue, Oct 26, 2021 at 5:45 AM Sam Ravnborg <sam at ravnborg.org> wrote:
>
> Hi George,
>
> On Mon, Oct 25, 2021 at 03:33:43PM -0400, George Kennedy 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 at oracle.com>
> > > > ---
> > > >   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;
> > > Shouldn't you be checking further up the call chain where this got set
> > > to 0?
> >
> I recall we had a similar discussion when Zheyu Ma <zheyuma97 at gmail.com>
> added the same checks to at least 3 fbdev drivers.
> The conclusion was that the check belongs in the individual check
> functions - IIRC.
>
> Maybe Zheyu or google can help find the original thread.

Related discussions are here:
https://lore.kernel.org/all/1627179054-29903-1-git-send-email-zheyuma97@gmail.com/
<https://lore.kernel.org/all/1627179054-29903-1-git-send-email-zheyuma97@gmail.com/#t>
https://lore.kernel.org/all/1626871424-27708-1-git-send-email-zheyuma97@gmail.com/
https://lore.kernel.org/all/1627293835-17441-1-git-send-email-zheyuma97@gmail.com/

Regards,
Zheyu Ma
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