Re: omapdss: Division by zero in kernel

Peter Teoh
2015-08-18T14:11:31Z

On Fri, Jul 24, 2015 at 9:03 AM, Pali Rohár <pali.rohar@gmail.com> wrote:
>
> Hello,
>
> when on N900 (real HW or qemu) I run this command
>
> / # echo 0 > /sys/devices/platform/omapdss/overlay0/enabled && echo 0 > /sys/class/graphics/fb0/size
>
> then kernel crash with this error message
>
> / # [ 29.904113] Division by zero in kernel.
> ** 3375 printk messages dropped ** [ 29.963836] [<c01e0008>] (__aeabi_uidivmod) from [<c022071c>]
> (cfb_imageblit+0xac/0x464)
> ** 8426 printk messages dropped ** [ 30.111083] [<c02187a4>] (fbcon_redraw.isra.12) from [<c0218ed0>]
> (fbcon_scroll+0x6a0/0xcbc)
> ** 8380 printk messages dropped ** [ 30.258209] [<c02187a4>] (fbcon_redraw.isra.12) from [<c0218ed0>]
> (fbcon_scroll+0x6a0/0xcbc)
> ** 7813 printk messages dropped ** [ 30.400054] [<c02187a4>] (fbcon_redraw.isra.12) from [<c0218ed0>]
> (fbcon_scroll+0x6a0/0xcbc)
> ** 7666 printk messages dropped ** [ 30.538391] [<c02187a4>] (fbcon_redraw.isra.12) from [<c0218ed0>]
> (fbcon_scroll+0x6a0/0xcbc)
> ** 7687 printk messages dropped ** [ 30.676544] [<c02187a4>] (fbcon_redraw.isra.12) from [<c0218ed0>]
> (fbcon_scroll+0x6a0/0xcbc)
> ** 7960 printk messages dropped ** [ 30.819915] [<c02187a4>] (fbcon_redraw.isra.12) from [<c0218ed0>]
> (fbcon_scroll+0x6a0/0xcbc)
> ** 8317 printk messages dropped ** [ 30.966979] [<c02187a4>] (fbcon_redraw.isra.12) from [<c0218ed0>]
> (fbcon_scroll+0x6a0/0xcbc)
> ** 8590 printk messages dropped ** [ 31.122528] [<c02187a4>] (fbcon_redraw.isra.12) from [<c0218ed0>]
> (fbcon_scroll+0x6a0/0xcbc)
> ** 8885 printk messages dropped ** [ 31.287658] [<c0218ed0>] (fbcon_scroll) from [<c025af90>] (scrup+0x60/0x128)
> ** 9408 printk messages dropped ** [ 31.461425] [<c02187a4>] (fbcon_redraw.isra.12) from [<c0218ed0>]
> (fbcon_scroll+0x6a0/0xcbc)
> ** 9787 printk messages dropped ** [ 31.644287] [<c02187e8>] (fbcon_redraw.isra.12) from [<c0218ed0>]
> (fbcon_scroll+0x6a0/0xcbc)
> ** 10081 printk messages dropped ** [ 31.833984] [<c02187a4>] (fbcon_redraw.isra.12) from [<c0218ed0>]
> (fbcon_scroll+0x6a0/0xcbc)
> ** 10501 printk messages dropped ** [ 32.031066] [<c02187a4>] (fbcon_redraw.isra.12) from [<c0218ed0>]
> (fbcon_scroll+0x6a0/0xcbc)
> ** 10816 printk messages dropped ** [ 32.233001] [<c02187a4>] (fbcon_redraw.isra.12) from [<c0218ed0>]
> (fbcon_scroll+0x6a0/0xcbc)
> ** 10900 printk messages dropped ** [ 32.440490] [<c02187a4>] (fbcon_redraw.isra.12) from [<c0218ed0>]
> (fbcon_scroll+0x6a0/0xcbc)
> ** 10837 printk messages dropped ** [ 32.645233] [<c02187a4>] (fbcon_redraw.isra.12) from [<c0218ed0>]
> (fbcon_scroll+0x6a0/0xcbc)
> ** 10837 printk messages dropped ** [ 32.848999] [<c02187a4>] (fbcon_redraw.isra.12) from [<c0218ed0>]
> (fbcon_scroll+0x6a0/0xcbc)
> ** 10837 printk messages dropped ** [ 33.053833] [<c02187a4>] (fbcon_redraw.isra.12) from [<c0218ed0>]
> (fbcon_scroll+0x6a0/0xcbc)
> ** 10838 printk messages dropped ** [ 33.258361] [<c0218ed0>] (fbcon_scroll) from [<c025af90>] (scrup+0x60/0x128)
>
> I suspect that problem is in omapdss.
>
> I do not know if size 0 make sense, but Maemo userspace is calling above
> commands and on Nokia's 2.6.28 kernel there is no crash or error message.
>
> IMHO Division by zero in kernel should not be there even if userspace
> call "incorrect" command.
>
> --
> Pali Rohár
> pali.rohar@gmail.com

Not sure if my analysis is correct. According to the recent pull
(4.2.0-rc4+), the code is still vulnerable to this division by zero:

Inside drivers/video/fbdev/core/cfbimgblt.c:

void cfb_imageblit(struct fb_info *p, const struct fb_image *image)
{
u32 fgcolor, bgcolor, start_index, bitstart, pitch_index = 0;
u32 bpl = sizeof(u32), bpp = p->var.bits_per_pixel;
u32 width = image->width;
u32 dx = image->dx, dy = image->dy;
u8 __iomem *dst1;

if (p->state != FBINFO_STATE_RUNNING)
return;

bitstart = (dy * p->fix.line_length * 8) + (dx * bpp);
start_index = bitstart & (32 - 1);
pitch_index = (p->fix.line_length & (bpl - 1)) * 8;

bitstart /= 8;
bitstart &= ~(bpl - 1);
dst1 = p->screen_base + bitstart;

if (p->fbops->fb_sync)
p->fbops->fb_sync(p);

if (image->depth == 1) {
if (p->fix.visual == FB_VISUAL_TRUECOLOR ||
p->fix.visual == FB_VISUAL_DIRECTCOLOR) {
fgcolor = ((u32*)(p->pseudo_palette))[image->fg_color];
bgcolor = ((u32*)(p->pseudo_palette))[image->bg_color];
} else {
fgcolor = image->fg_color;
bgcolor = image->bg_color;
}

if (32 % bpp == 0 && !start_index && !pitch_index &&
((width & (32/bpp-1)) == 0) &&
bpp >= 8 && bpp <= 32)
fast_imageblit(image, p, dst1, fgcolor, bgcolor);
else
slow_imageblit(image, p, dst1, fgcolor, bgcolor,
start_index, pitch_index);
} else
color_imageblit(image, p, dst1, start_index, pitch_i

Notice that bpp is not checked for zero, and thus bpp==0 is totally
feasible? resulting in 32/bpp crashing the kernel?

--
Regards,
Peter Teoh