[PATCH v2] fbdev: Detect integer underflow at "struct fbcon_ops"->clear_margins.

[Dan Carpenter]

On Wed, Jul 15, 2020 at 11:02:58PM +0900, Tetsuo Handa wrote:
> On 2020/07/15 20:17, Tetsuo Handa wrote:
> > On 2020/07/15 18:48, Dan Carpenter wrote:
> >>> @@ -216,7 +216,7 @@ static void bit_clear_margins(struct vc_data *vc, struct fb_info *info,
> >>>  	region.color = color;
> >>>  	region.rop = ROP_COPY;
> >>>  
> >>> -	if (rw && !bottom_only) {
> >>> +	if ((int) rw > 0 && !bottom_only) {
> >>>  		region.dx = info->var.xoffset + rs;
> >>                             ^^^^^^^^^^^^^^^^^^^^^^
> >>
> >> If you choose a very high positive "rw" then this addition can overflow.
> >> info->var.xoffset comes from the user and I don't think it's checked...
> > 
> > Well, I think it would be checked by "struct fb_ops"->check_var hook.
> > For example, vmw_fb_check_var() has
> > 
> > 	if ((var->xoffset + var->xres) > par->max_width ||
> > 	    (var->yoffset + var->yres) > par->max_height) {
> > 		DRM_ERROR("Requested geom can not fit in framebuffer\n");
> > 		return -EINVAL;
> > 	}
> > 
> > check. Of course, there might be integer overflow in that check...
> > Having sanity check at caller of "struct fb_ops"->check_var might be nice.
> > 
> 
> Well, while
> 
>         const int fd = open("/dev/fb0", O_ACCMODE);
>         struct fb_var_screeninfo var = { };
>         ioctl(fd, FBIOGET_VSCREENINFO, &var);
>         var.xres = var.yres = 4;
>         var.xoffset = 4294967292U;
>         ioctl(fd, FBIOPUT_VSCREENINFO, &var);
> 
> bypassed
> 
>   (var->xoffset + var->xres) > par->max_width
> 
> check in vmw_fb_check_var(),
> 
> ----------
> --- a/drivers/video/fbdev/core/bitblit.c
> +++ b/drivers/video/fbdev/core/bitblit.c
> @@ -216,6 +216,7 @@ static void bit_clear_margins(struct vc_data *vc, struct fb_info *info,
>         region.color = color;
>         region.rop = ROP_COPY;
> 
> +       printk(KERN_INFO "%s info->var.xoffset=%u rs=%u info->var.yoffset=%u bs=%u\n", __func__, info->var.xoffset, rs, info->var.yoffset, bs);
>         if ((int) rw > 0 && !bottom_only) {
>                 region.dx = info->var.xoffset + rs;
>                 region.dy = 0;
> ----------
> 
> says that info->var.xoffset does not come from the user.
> 
> ----------
>  bit_clear_margins info->var.xoffset=0 rs=1024 info->var.yoffset=0 bs=800
> ----------

In fb_set_var() we do:

drivers/video/fbdev/core/fbmem.c
  1055          ret = info->fbops->fb_check_var(var, info);
  1056  
  1057          if (ret)
  1058                  return ret;
  1059  
  1060          if ((var->activate & FB_ACTIVATE_MASK) != FB_ACTIVATE_NOW)
  1061                  return 0;
  1062  
  1063          if (!basic_checks(var))
  1064                  return -EINVAL;
  1065  
  1066          if (info->fbops->fb_get_caps) {
  1067                  ret = fb_check_caps(info, var, var->activate);
  1068  
  1069                  if (ret)
  1070                          return ret;
  1071          }
  1072  
  1073          old_var = info->var;
  1074          info->var = *var;
                ^^^^^^^^^^^^^^^^
This should set "info->var.offset".

  1075  
  1076          if (info->fbops->fb_set_par) {
  1077                  ret = info->fbops->fb_set_par(info);
  1078  
  1079                  if (ret) {
  1080                          info->var = old_var;
  1081                          printk(KERN_WARNING "detected "

I've complained about integer overflows in fbdev for a long time...

What I'd like to see is something like the following maybe.  I don't
know how to get the vc_data in fbmem.c so it doesn't include your checks
for negative.

diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
index caf817bcb05c..5c74181fea5d 100644
--- a/drivers/video/fbdev/core/fbmem.c
+++ b/drivers/video/fbdev/core/fbmem.c
@@ -934,6 +934,54 @@ fb_pan_display(struct fb_info *info, struct fb_var_screeninfo *var)
 }
 EXPORT_SYMBOL(fb_pan_display);
 
+static bool basic_checks(struct fb_var_screeninfo *var)
+{
+	unsigned int v_margins, h_margins;
+
+	/* I think var->height and var->width == UINT_MAX means something. */
+
+	if (var->xres > INT_MAX ||
+	    var->yres > INT_MAX ||
+	    var->xres_virtual > INT_MAX ||
+	    var->yres_virtual > INT_MAX ||
+	    var->xoffset > INT_MAX ||
+	    var->yoffset > INT_MAX ||
+	    var->left_margin > INT_MAX ||
+	    var->right_margin > INT_MAX ||
+	    var->upper_margin > INT_MAX ||
+	    var->lower_margin > INT_MAX ||
+	    var->hsync_len > INT_MAX ||
+	    var->vsync_len > INT_MAX)
+		return false;
+
+	if (var->bits_per_pixel > 128)
+		return false;
+	if (var->rotate > FB_ROTATE_CCW)
+		return false;
+
+	if (var->xoffset > INT_MAX - var->xres)
+		return false;
+	if (var->yoffset > INT_MAX - var->yres)
+		return false;
+
+	if (var->left_margin > INT_MAX - var->right_margin ||
+	    var->upper_margin > INT_MAX - var->lower_margin)
+		return false;
+
+	v_margins = var->left_margin + var->right_margin;
+	h_margins = var->upper_margin + var->lower_margin;
+
+	if (var->xres > INT_MAX - var->hsync_len ||
+	    var->yres > INT_MAX - var->vsync_len)
+		return false;
+
+	if (v_margins > INT_MAX - var->hsync_len - var->xres ||
+	    h_margins > INT_MAX - var->vsync_len - var->yres)
+		return false;
+
+	return true;
+}
+
 static int fb_check_caps(struct fb_info *info, struct fb_var_screeninfo *var,
 			 u32 activate)
 {
@@ -1012,6 +1060,9 @@ fb_set_var(struct fb_info *info, struct fb_var_screeninfo *var)
 	if ((var->activate & FB_ACTIVATE_MASK) != FB_ACTIVATE_NOW)
 		return 0;
 
+	if (!basic_checks(var))
+		return -EINVAL;
+
 	if (info->fbops->fb_get_caps) {
 		ret = fb_check_caps(info, var, var->activate);