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    Subject[PATCH 4.9 087/175] fbmem: dont allow too huge resolutions
    Date
    From: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>

    commit 8c28051cdcbe9dfcec6bd0a4709d67a09df6edae upstream.

    syzbot is reporting page fault at vga16fb_fillrect() [1], for
    vga16fb_check_var() is failing to detect multiplication overflow.

    if (vxres * vyres > maxmem) {
    vyres = maxmem / vxres;
    if (vyres < yres)
    return -ENOMEM;
    }

    Since no module would accept too huge resolutions where multiplication
    overflow happens, let's reject in the common path.

    Link: https://syzkaller.appspot.com/bug?extid=04168c8063cfdde1db5e [1]
    Reported-by: syzbot <syzbot+04168c8063cfdde1db5e@syzkaller.appspotmail.com>
    Debugged-by: Randy Dunlap <rdunlap@infradead.org>
    Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
    Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
    Cc: stable@vger.kernel.org
    Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
    Link: https://patchwork.freedesktop.org/patch/msgid/185175d6-227a-7b55-433d-b070929b262c@i-love.sakura.ne.jp
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    drivers/video/fbdev/core/fbmem.c | 7 +++++++
    1 file changed, 7 insertions(+)

    --- a/drivers/video/fbdev/core/fbmem.c
    +++ b/drivers/video/fbdev/core/fbmem.c
    @@ -32,6 +32,7 @@
    #include <linux/device.h>
    #include <linux/efi.h>
    #include <linux/fb.h>
    +#include <linux/overflow.h>

    #include <asm/fb.h>

    @@ -981,6 +982,7 @@ fb_set_var(struct fb_info *info, struct
    if ((var->activate & FB_ACTIVATE_FORCE) ||
    memcmp(&info->var, var, sizeof(struct fb_var_screeninfo))) {
    u32 activate = var->activate;
    + u32 unused;

    /* When using FOURCC mode, make sure the red, green, blue and
    * transp fields are set to 0.
    @@ -1005,6 +1007,11 @@ fb_set_var(struct fb_info *info, struct
    if (var->xres < 8 || var->yres < 8)
    return -EINVAL;

    + /* Too huge resolution causes multiplication overflow. */
    + if (check_mul_overflow(var->xres, var->yres, &unused) ||
    + check_mul_overflow(var->xres_virtual, var->yres_virtual, &unused))
    + return -EINVAL;
    +
    ret = info->fbops->fb_check_var(var, info);

    if (ret)

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