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    SubjectRe: [PATCH 4.9 01/42] fbdev: fb_pm2fb: Avoid potential divide by zero error

    > On 09/13/2022 4:07 PM CEST Greg Kroah-Hartman <gregkh@linuxfoundation.org> wrote:
    >
    >
    > 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>
    > Signed-off-by: Sasha Levin <sashal@kernel.org>
    > ---
    > drivers/video/fbdev/pm2fb.c | 5 +++++
    > 1 file changed, 5 insertions(+)
    >
    > 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));
    > --
    > 2.35.1

    This is a duplicate, the same patch has already been applied in 4.9.327 (0f1174f4972ea9fad6becf8881d71adca8e9ca91), so the above snippet of code is now in there twice.

    Doesn't make a difference in functionality in this case, I just happened to notice it when reviewing backports from 4.9 for the CIP 4.4-stable tree.

    CU
    Uli

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