Patch "fbdev: fb_pm2fb: Avoid potential divide by zero error" has been added to the 4.9-stable tree

[Sasha Levin <sashal@xxxxxxxxxx>]

This is a note to let you know that I've just added the patch titled

    fbdev: fb_pm2fb: Avoid potential divide by zero error

to the 4.9-stable tree which can be found at:
    http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
     fbdev-fb_pm2fb-avoid-potential-divide-by-zero-error.patch
and it can be found in the queue-4.9 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.



commit a60919a7c9b888edd3a17478c9c10affa493aa79
Author: Letu Ren <fantasquex@xxxxxxxxx>
Date:   Thu Aug 18 18:44:24 2022 +0800

    fbdev: fb_pm2fb: Avoid potential divide by zero error
    
    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@xxxxxxxxx>
    Signed-off-by: Letu Ren <fantasquex@xxxxxxxxx>
    Signed-off-by: Helge Deller <deller@xxxxxx>
    Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
    Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>

diff --git a/drivers/video/fbdev/pm2fb.c b/drivers/video/fbdev/pm2fb.c
index 9b32b9fc44a5c..50b569d047b10 100644
--- a/drivers/video/fbdev/pm2fb.c
+++ b/drivers/video/fbdev/pm2fb.c
@@ -619,6 +619,11 @@ static int pm2fb_check_var(struct fb_var_screeninfo *var, struct fb_info *info)
 		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));