fbcon: Out-Of-Bounds write in sys_imageblit, add range check

yres and vyres can be controlled by user mode paramaters, and cause p->vrows to become a negative value. While this value be passed to real_y function, the ypos will be out of screen range.
This is an out-of-bounds write bug.

On Mon, Jul 26, 2021 at 11:32:37AM +0000, tcs_kernel(腾讯云内核开发者) wrote:
> yres and vyres can be controlled by user mode paramaters, and cause p->vrows to become a negative value. While this value be passed to real_y function, the ypos will be out of screen range.
> This is an out-of-bounds write bug.
> 
> 
> diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
> index 22bb3892f6bd..0970de46782f 100644
> --- a/drivers/video/fbdev/core/fbcon.c
> +++ b/drivers/video/fbdev/core/fbcon.c
> @@ -1956,11 +1956,12 @@ static void updatescrollmode(struct fbcon_display *p,
>         int yres = FBCON_SWAP(ops->rotate, info->var.yres, info->var.xres);
>         int vyres = FBCON_SWAP(ops->rotate, info->var.yres_virtual,
>                                    info->var.xres_virtual);
> +       int rows = vc->vc_rows;
>  
>         p->vrows = vyres/fh;
> -       if (yres > (fh * (vc->vc_rows + 1)))
> -               p->vrows -= (yres - (fh * vc->vc_rows)) / fh;
> -       if ((yres % fh) && (vyres % fh < yres % fh))
> +       if ((yres > (fh * (rows + 1))) && (vyres >= (yres - (fh * rows))) && p->vrows)
> +               p->vrows -= (yres - (fh * rows)) / fh;
> +       if ((yres % fh) && (vyres % fh < yres % fh) && p->vrows)
>                 p->vrows--;
>  }
> 

Hi,

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yres and vyres can be controlled by user mode paramaters, and cause p->vrows to become a negative value. While this value be passed to real_y function, the ypos will be out of screen range.
This is an out-of-bounds write bug.
I think updatescrollmode is the right place to validate values supplied by a user ioctl, because only here makes --operation,and 0 is a legal value before that.

Signed-off-by: Tencent Cloud System tcs_kernel@tencent.com


diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c
index 22bb3892f6bd..0970de46782f 100644
--- a/drivers/video/fbdev/core/fbcon.c
+++ b/drivers/video/fbdev/core/fbcon.c
@@ -1956,11 +1956,12 @@ static void updatescrollmode(struct fbcon_display *p,
        int yres = FBCON_SWAP(ops->rotate, info->var.yres, info->var.xres);
        int vyres = FBCON_SWAP(ops->rotate, info->var.yres_virtual,
                                   info->var.xres_virtual);
+       int rows = vc->vc_rows;
 
        p->vrows = vyres/fh;
-       if (yres > (fh * (vc->vc_rows + 1)))
-               p->vrows -= (yres - (fh * vc->vc_rows)) / fh;
-       if ((yres % fh) && (vyres % fh < yres % fh))
+       if ((yres > (fh * (rows + 1))) && (vyres >= (yres - (fh * rows))) && p->vrows)
+               p->vrows -= (yres - (fh * rows)) / fh;
+       if ((yres % fh) && (vyres % fh < yres % fh) && p->vrows)
                p->vrows--;
 }

在 2021/7/26 21:45，“Sam Ravnborg”<sam@ravnborg.org> 写入:

    Hi,
    On Mon, Jul 26, 2021 at 11:32:37AM +0000, tcs_kernel(腾讯云内核开发者) wrote:
    > yres and vyres can be controlled by user mode paramaters, and cause p->vrows to become a negative value. While this value be passed to real_y function, the ypos will be out of screen range.

    > This is an out-of-bounds write bug.


    Please investigate if you can validate the user-supplied values for yres
    and vyres earlier so the code never reaches the below statements.
    This would also make it much more explicit what is going on.

    	Sam

    > 

    > 

    > diff --git a/drivers/video/fbdev/core/fbcon.c b/drivers/video/fbdev/core/fbcon.c

    > index 22bb3892f6bd..0970de46782f 100644

    > --- a/drivers/video/fbdev/core/fbcon.c

    > +++ b/drivers/video/fbdev/core/fbcon.c

    > @@ -1956,11 +1956,12 @@ static void updatescrollmode(struct fbcon_display *p,

    >         int yres = FBCON_SWAP(ops->rotate, info->var.yres, info->var.xres);

    >         int vyres = FBCON_SWAP(ops->rotate, info->var.yres_virtual,

    >                                    info->var.xres_virtual);

    > +       int rows = vc->vc_rows;

    >  

    >         p->vrows = vyres/fh;

    > -       if (yres > (fh * (vc->vc_rows + 1)))

    > -               p->vrows -= (yres - (fh * vc->vc_rows)) / fh;

    > -       if ((yres % fh) && (vyres % fh < yres % fh))

    > +       if ((yres > (fh * (rows + 1))) && (vyres >= (yres - (fh * rows))) && p->vrows)

    > +               p->vrows -= (yres - (fh * rows)) / fh;

    > +       if ((yres % fh) && (vyres % fh < yres % fh) && p->vrows)

    >                 p->vrows--;

    >  }

    >