[PATCH 5.10 14/37] fbdev: fb_pm2fb: Avoid potential divide by zero error

Greg Kroah-Hartman
2022-09-02T13:02:23Z

From: Letu Ren <fantasquex@gmail.com>

commit 19f953e7435644b81332dd632ba1b2d80b1e37af upstream.

In `do_fb_ioctl()` of fbmem.c, if cmd is FBIOPUT_VSCREENINFO, var will be
copied from user, then go through `fb_set_var()` and
`info->fbops->fb_check_var()` which could may be `pm2fb_check_var()`.
Along the path, `var->pixclock` won't be modified. This function checks
whether reciprocal of `var->pixclock` is too high. If `var->pixclock` is
zero, there will be a divide by zero error. So, it is necessary to check
whether denominator is zero to avoid crash. As this bug is found by
Syzkaller, logs are listed below.

divide error in pm2fb_check_var
Call Trace:
<TASK>
fb_set_var+0x367/0xeb0 drivers/video/fbdev/core/fbmem.c:1015
do_fb_ioctl+0x234/0x670 drivers/video/fbdev/core/fbmem.c:1110
fb_ioctl+0xdd/0x130 drivers/video/fbdev/core/fbmem.c:1189

Reported-by: Zheyu Ma <zheyuma97@gmail.com>
Signed-off-by: Letu Ren <fantasquex@gmail.com>
Signed-off-by: Helge Deller <deller@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
drivers/video/fbdev/pm2fb.c | 5 +++++
1 file changed, 5 insertions(+)

--- a/drivers/video/fbdev/pm2fb.c
+++ b/drivers/video/fbdev/pm2fb.c
@@ -616,6 +616,11 @@ static int pm2fb_check_var(struct fb_var
return -EINVAL;
}

+ if (!var->pixclock) {
+ DPRINTK("pixclock is zero\n");
+ return -EINVAL;
+ }
+
if (PICOS2KHZ(var->pixclock) > PM2_MAX_PIXCLOCK) {
DPRINTK("pixclock too high (%ldKHz)\n",
PICOS2KHZ(var->pixclock));