Re: [PATCH 1/1] video: fbdev: fix divide error in fbcon_switch

[Saeed Mirzamohammadi <saeed.mirzamohammadi@xxxxxxxxxx>]

Thanks, adding stable.

On Oct 22, 2020, at 12:34 AM, Thomas Zimmermann <tzimmermann@xxxxxxx> wrote:

Hi

On 22.10.20 01:57, saeed.mirzamohammadi@xxxxxxxxxx wrote:

From: Saeed Mirzamohammadi <saeed.mirzamohammadi@xxxxxxxxxx>

This patch fixes the issue due to:

[   89.572883] divide_error: 0000 [#1] SMP KASAN PTI
[   89.572897] CPU: 3 PID: 16083 Comm: repro Not tainted 5.9.0-rc7.20200930.rc1.allarch-19-g3e32d0d.syzk #5
[   89.572902] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 0.5.1 01/01/2011
[   89.572934] RIP: 0010:cirrusfb_check_var+0x84/0x1260

BTW, if you run qemu with cirrus, there's also a DRM driver named
cirrus.ko. Might be a better choice than the old fbdev driver. If you
just care about qemu, but not the actual graphics device, take a look at

 https://urldefense.com/v3/__https://www.kraxel.org/blog/2014/10/qemu-using-cirrus-considered-harmful/__;!!GqivPVa7Brio!LmgeM-pVBVH80uVELF1P1nBGAbAlhvnxKKE_ZrEc9d76AznvAAgP1FAp3_zNa2frKaIUZteK$ 

Anyway, thanks for your patch.

Best regards
Thomas

The error happens when the pixels value is calculated before performing the sanity checks on bits_per_pixel.
A bits_per_pixel set to zero causes divide by zero error.

This patch moves the calculation after the sanity check.

Signed-off-by: Saeed Mirzamohammadi <saeed.mirzamohammadi@xxxxxxxxxx>
Tested-by: Saeed Mirzamohammadi <saeed.mirzamohammadi@xxxxxxxxxx>
---
drivers/video/fbdev/cirrusfb.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/video/fbdev/cirrusfb.c b/drivers/video/fbdev/cirrusfb.c
index 15a9ee7cd734..a7749101b094 100644
--- a/drivers/video/fbdev/cirrusfb.c
+++ b/drivers/video/fbdev/cirrusfb.c
@@ -531,7 +531,7 @@ static int cirrusfb_check_var(struct fb_var_screeninfo *var,
{
int yres;
/* memory size in pixels */
- unsigned pixels = info->screen_size * 8 / var->bits_per_pixel;
+ unsigned int pixels;
struct cirrusfb_info *cinfo = info->par;

switch (var->bits_per_pixel) {
@@ -573,6 +573,7 @@ static int cirrusfb_check_var(struct fb_var_screeninfo *var,
return -EINVAL;
}

+ pixels = info->screen_size * 8 / var->bits_per_pixel;
if (var->xres_virtual < var->xres)
var->xres_virtual = var->xres;
/* use highest possible virtual resolution */

-- 
Thomas Zimmermann
Graphics Driver Developer
SUSE Software Solutions Germany GmbH
Maxfeldstr. 5, 90409 Nürnberg, Germany
(HRB 36809, AG Nürnberg)
Geschäftsführer: Felix Imendörffer

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