2022-08-03 09:45:59

by Zheyu Ma

Subject: [PATCH] video: fbdev: arkfb: Fix a divide-by-zero bug in ark_set_pixclock()

Since the user can control the arguments of the ioctl() from the user
space, under special arguments that may result in a divide-by-zero bug
in:
drivers/video/fbdev/arkfb.c:784: ark_set_pixclock(info, (hdiv * info->var.pixclock) / hmul);
with hdiv=1, pixclock=1 and hmul=2 you end up with (1*1)/2 = (int) 0.
and then in:
drivers/video/fbdev/arkfb.c:504: rv = dac_set_freq(par->dac, 0, 1000000000 / pixclock);
we'll get a division-by-zero.

The following log can reveal it:

divide error: 0000 [#1] PREEMPT SMP KASAN PTI
RIP: 0010:ark_set_pixclock drivers/video/fbdev/arkfb.c:504 [inline]
RIP: 0010:arkfb_set_par+0x10fc/0x24c0 drivers/video/fbdev/arkfb.c:784
Call Trace:
fb_set_var+0x604/0xeb0 drivers/video/fbdev/core/fbmem.c:1034
do_fb_ioctl+0x234/0x670 drivers/video/fbdev/core/fbmem.c:1110
fb_ioctl+0xdd/0x130 drivers/video/fbdev/core/fbmem.c:1189

Fix this by checking the argument of ark_set_pixclock() first.

Fixes: 681e14730c73 ("arkfb: new framebuffer driver for ARK Logic cards")
Signed-off-by: Zheyu Ma <[email protected]>
---
drivers/video/fbdev/arkfb.c | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/drivers/video/fbdev/arkfb.c b/drivers/video/fbdev/arkfb.c
index eb3e47c58c5f..ed76ddc7df3d 100644
--- a/drivers/video/fbdev/arkfb.c
+++ b/drivers/video/fbdev/arkfb.c
@@ -781,7 +781,12 @@ static int arkfb_set_par(struct fb_info *info)
return -EINVAL;
}

- ark_set_pixclock(info, (hdiv * info->var.pixclock) / hmul);
+ value = (hdiv * info->var.pixclock) / hmul;
+ if (!value) {
+ fb_dbg(info, "invalid pixclock\n");
+ value = 1;
+ }
+ ark_set_pixclock(info, value);
svga_set_timings(par->state.vgabase, &ark_timing_regs, &(info->var), hmul, hdiv,
(info->var.vmode & FB_VMODE_DOUBLE) ? 2 : 1,
(info->var.vmode & FB_VMODE_INTERLACED) ? 2 : 1,
--
2.25.1

2022-08-03 13:40:07

by Helge Deller

Subject: Re: [PATCH] video: fbdev: arkfb: Fix a divide-by-zero bug in ark_set_pixclock()

On 8/3/22 11:23, Zheyu Ma wrote:
> Since the user can control the arguments of the ioctl() from the user
> space, under special arguments that may result in a divide-by-zero bug
> in:
> drivers/video/fbdev/arkfb.c:784: ark_set_pixclock(info, (hdiv * info->var.pixclock) / hmul);
> with hdiv=1, pixclock=1 and hmul=2 you end up with (1*1)/2 = (int) 0.
> and then in:
> drivers/video/fbdev/arkfb.c:504: rv = dac_set_freq(par->dac, 0, 1000000000 / pixclock);
> we'll get a division-by-zero.
>
> The following log can reveal it:
>
> divide error: 0000 [#1] PREEMPT SMP KASAN PTI
> RIP: 0010:ark_set_pixclock drivers/video/fbdev/arkfb.c:504 [inline]
> RIP: 0010:arkfb_set_par+0x10fc/0x24c0 drivers/video/fbdev/arkfb.c:784
> Call Trace:
> fb_set_var+0x604/0xeb0 drivers/video/fbdev/core/fbmem.c:1034
> do_fb_ioctl+0x234/0x670 drivers/video/fbdev/core/fbmem.c:1110
> fb_ioctl+0xdd/0x130 drivers/video/fbdev/core/fbmem.c:1189
>
> Fix this by checking the argument of ark_set_pixclock() first.
>
> Fixes: 681e14730c73 ("arkfb: new framebuffer driver for ARK Logic cards")
> Signed-off-by: Zheyu Ma <[email protected]>

applied to fbdev git tree.

Thanks!
Helge

> ---
> drivers/video/fbdev/arkfb.c | 7 ++++++-
> 1 file changed, 6 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/video/fbdev/arkfb.c b/drivers/video/fbdev/arkfb.c
> index eb3e47c58c5f..ed76ddc7df3d 100644
> --- a/drivers/video/fbdev/arkfb.c
> +++ b/drivers/video/fbdev/arkfb.c
> @@ -781,7 +781,12 @@ static int arkfb_set_par(struct fb_info *info)
> return -EINVAL;
> }
>
> - ark_set_pixclock(info, (hdiv * info->var.pixclock) / hmul);
> + value = (hdiv * info->var.pixclock) / hmul;
> + if (!value) {
> + fb_dbg(info, "invalid pixclock\n");
> + value = 1;
> + }
> + ark_set_pixclock(info, value);
> svga_set_timings(par->state.vgabase, &ark_timing_regs, &(info->var), hmul, hdiv,
> (info->var.vmode & FB_VMODE_DOUBLE) ? 2 : 1,
> (info->var.vmode & FB_VMODE_INTERLACED) ? 2 : 1,