fbdev: fbmem: Fix the implicit type casting

In function do_fb_ioctl(), the "arg" is the type of unsigned long,
and in "case FBIOBLANK:" this argument is casted into an int before
passig to fb_blank(). In fb_blank(), the comparision
if (blank > FB_BLANK_POWERDOWN) would be bypass if the original
"arg" is a large number, which is possible because it comes from
the user input. Fix this by adding the check before the function
call.

Reviewed-by: Guenter Roeck <[email protected]>
Reviewed-by: Sam Ravnborg <[email protected]>
Acked-by: Helge Deller <[email protected]>
Signed-off-by: Yizhuo Zhai <[email protected]>
Signed-off-by: Daniel Vetter <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]

Author: Yizhuo Zhai

drivers/video/fbdev/core/fbmem.c

@@ -1160,6 +1160,8 @@ static long do_fb_ioctl(struct fb_info *info, unsigned int cmd,
 		ret = fbcon_set_con2fb_map_ioctl(argp);
 		break;
 	case FBIOBLANK:
+		if (arg > FB_BLANK_POWERDOWN)
+			return -EINVAL;
 		console_lock();
 		lock_fb_info(info);
 		ret = fb_blank(info, arg);