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

[Sam Ravnborg]

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.

	Sam