Re: omapdss: Division by zero in kernel

Pali Rohár
2015-08-19T08:11:48Z

On Tuesday 18 August 2015 07:11:27 Peter Teoh wrote:
> 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?
>

Hm... this could really be a problem! But how to patch it? Which branch
should be called (fast_ or slow_ function) if bpp is zero?

And is there some way to force kernel to dump backtrace into dmesg when
division by zero occur?