On 2021/08/30 9:24, Randy Dunlap wrote:
> Note that yres_virtual is set to 0x10000000. Is there no practical limit
> (hence limit check) that can be used here?
>
> Also, in vga16fb_check_var(), beginning at line 404:
>
> 404 if (yres > vyres)
> 405 vyres = yres;
> 406 if (vxres * vyres > maxmem) {
> 407 vyres = maxmem / vxres;
> 408 if (vyres < yres)
> 409 return -ENOMEM;
> 410 }
>
> At line 406, the product of vxres * vyres overflows 32 bits (is 0 in this
> case/example), so any protection from this block is lost.
OK. Then, we can check overflow like below.
diff --git a/drivers/video/fbdev/vga16fb.c b/drivers/video/fbdev/vga16fb.c
index e2757ff1c23d..e483a3f5fd47 100644
--- a/drivers/video/fbdev/vga16fb.c
+++ b/drivers/video/fbdev/vga16fb.c
@@ -403,7 +403,7 @@ static int vga16fb_check_var(struct fb_var_screeninfo *var,
if (yres > vyres)
vyres = yres;
- if (vxres * vyres > maxmem) {
+ if ((u64) vxres * vyres > (u64) maxmem) {
vyres = maxmem / vxres;
if (vyres < yres)
return -ENOMEM;
But I think we can check overflow in the common code like below. (Both patch fixed the oops.)
diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
index 1c855145711b..8899679bbc46 100644
--- a/drivers/video/fbdev/core/fbmem.c
+++ b/drivers/video/fbdev/core/fbmem.c
@@ -1008,6 +1008,11 @@ fb_set_var(struct fb_info *info, struct fb_var_screeninfo *var)
if (var->xres < 8 || var->yres < 8)
return -EINVAL;
+ /* Don't allow u32 * u32 to overflow. */
+ if ((u64) var->xres * var->yres > (u64) UINT_MAX ||
+ (u64) var->xres_virtual * var->yres_virtual > (u64) UINT_MAX)
+ return -EINVAL;
+
ret = info->fbops->fb_check_var(var, info);
if (ret)
>
> But even if yres_virtual (aka vyres) is "only" 0x01000000, so no
> multiplication overflow occurs, the resulting value of vyres "seems"
> to still be too large and can cause an error [I'm not sure about this
> last part -- I need to use a new gcc so that KASAN will work.]
