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    Subject[PATCH AUTOSEL 5.16 16/59] video: fbdev: cirrusfb: check pixclock to avoid divide by zero
    Date
    From: George Kennedy <george.kennedy@oracle.com>

    [ Upstream commit 5c6f402bdcf9e7239c6bc7087eda71ac99b31379 ]

    Do a sanity check on pixclock value to avoid divide by zero.

    If the pixclock value is zero, the cirrusfb driver will round up
    pixclock to get the derived frequency as close to maxclock as
    possible.

    Syzkaller reported a divide error in cirrusfb_check_pixclock.

    divide error: 0000 [#1] SMP KASAN PTI
    CPU: 0 PID: 14938 Comm: cirrusfb_test Not tainted 5.15.0-rc6 #1
    Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.11.0-2
    RIP: 0010:cirrusfb_check_var+0x6f1/0x1260

    Call Trace:
    fb_set_var+0x398/0xf90
    do_fb_ioctl+0x4b8/0x6f0
    fb_ioctl+0xeb/0x130
    __x64_sys_ioctl+0x19d/0x220
    do_syscall_64+0x3a/0x80
    entry_SYSCALL_64_after_hwframe+0x44/0xae

    Signed-off-by: George Kennedy <george.kennedy@oracle.com>
    Reviewed-by: Geert Uytterhoeven <geert@linux-m68k.org>
    Signed-off-by: Helge Deller <deller@gmx.de>
    Signed-off-by: Sasha Levin <sashal@kernel.org>
    ---
    drivers/video/fbdev/cirrusfb.c | 16 ++++++++--------
    1 file changed, 8 insertions(+), 8 deletions(-)

    diff --git a/drivers/video/fbdev/cirrusfb.c b/drivers/video/fbdev/cirrusfb.c
    index 93802abbbc72..3d47c347b897 100644
    --- a/drivers/video/fbdev/cirrusfb.c
    +++ b/drivers/video/fbdev/cirrusfb.c
    @@ -469,7 +469,7 @@ static int cirrusfb_check_mclk(struct fb_info *info, long freq)
    return 0;
    }

    -static int cirrusfb_check_pixclock(const struct fb_var_screeninfo *var,
    +static int cirrusfb_check_pixclock(struct fb_var_screeninfo *var,
    struct fb_info *info)
    {
    long freq;
    @@ -478,9 +478,7 @@ static int cirrusfb_check_pixclock(const struct fb_var_screeninfo *var,
    unsigned maxclockidx = var->bits_per_pixel >> 3;

    /* convert from ps to kHz */
    - freq = PICOS2KHZ(var->pixclock);
    -
    - dev_dbg(info->device, "desired pixclock: %ld kHz\n", freq);
    + freq = PICOS2KHZ(var->pixclock ? : 1);

    maxclock = cirrusfb_board_info[cinfo->btype].maxclock[maxclockidx];
    cinfo->multiplexing = 0;
    @@ -488,11 +486,13 @@ static int cirrusfb_check_pixclock(const struct fb_var_screeninfo *var,
    /* If the frequency is greater than we can support, we might be able
    * to use multiplexing for the video mode */
    if (freq > maxclock) {
    - dev_err(info->device,
    - "Frequency greater than maxclock (%ld kHz)\n",
    - maxclock);
    - return -EINVAL;
    + var->pixclock = KHZ2PICOS(maxclock);
    +
    + while ((freq = PICOS2KHZ(var->pixclock)) > maxclock)
    + var->pixclock++;
    }
    + dev_dbg(info->device, "desired pixclock: %ld kHz\n", freq);
    +
    /*
    * Additional constraint: 8bpp uses DAC clock doubling to allow maximum
    * pixel clock
    --
    2.34.1
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