Hi George,
On Tue, Oct 26, 2021 at 5:48 PM George Kennedy george.kennedy@oracle.com wrote:
On 10/26/2021 10:11 AM, Geert Uytterhoeven wrote:
On Tue, Oct 26, 2021 at 3:38 PM George Kennedy george.kennedy@oracle.com wrote:
On 10/26/2021 4:30 AM, Geert Uytterhoeven wrote:
On Mon, Oct 25, 2021 at 9:37 PM George Kennedy george.kennedy@oracle.com 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@oracle.com > --- 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()).
Gr{oetje,eeting}s,
Geert
-- Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org
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