media: fix: media: pci: meye: validate offset to avoid arbitrary access

Motion eye video4linux driver for Sony Vaio PictureBook desn't validate user-controlled parameter
'vma->vm_pgoff', a malicious process might access all of kernel memory from user space by trying
pass different arbitrary address.
Discussion: http://www.openwall.com/lists/oss-security/2018/07/06/1

Signed-off-by: Lao Wei <zrlw@qq.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>

drivers/media/pci/meye/meye.c

@@ -1460,7 +1460,7 @@ static int meye_mmap(struct file *file, struct vm_area_struct *vma)
 	unsigned long page, pos;
 
 	mutex_lock(&meye.lock);
-	if (size > gbuffers * gbufsize) {
+	if (size > gbuffers * gbufsize || offset > gbuffers * gbufsize - size) {
 		mutex_unlock(&meye.lock);
 		return -EINVAL;
 	}