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

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