[PATCH V4] fbcon: fix fbcon out-of-bounds write in sys_imageblit

[tcs.kernel@gmail.com]

From: Haimin Zhang <tcs_kernel@tencent.com>

yres and vyres can be controlled by user mode parameters, 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.
some driver will check xres and yres in fb_check_var callback,but some not
so we add a common check after that callback.

Signed-off-by: Haimin Zhang <tcs_kernel@tencent.com>
Signed-off-by: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
---
 drivers/video/fbdev/core/fbmem.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/drivers/video/fbdev/core/fbmem.c b/drivers/video/fbdev/core/fbmem.c
index 1c85514..5599372 100644
--- a/drivers/video/fbdev/core/fbmem.c
+++ b/drivers/video/fbdev/core/fbmem.c
@@ -1013,6 +1013,10 @@ static int fb_check_caps(struct fb_info *info, struct fb_var_screeninfo *var,
 	if (ret)
 		return ret;
 
+	/* virtual resolution cannot be smaller than visible resolution. */
+	if (var->yres_virtual < var->yres || var->xres_virtual < var->xres)
+		return -EINVAL;
+
 	if ((var->activate & FB_ACTIVATE_MASK) != FB_ACTIVATE_NOW)
 		return 0;
 
-- 
1.8.3.1