Re: omapdss: Division by zero in kernel

[Peter Teoh]

On Fri, Jul 24, 2015 at 9:03 AM, Pali RohÃr <pali.rohar@xxxxxxxxx> 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 RohÃr
> pali.rohar@xxxxxxxxx



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
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