acpi-video: Fix integer overflow and possible kernel stack trashing

This patch fixes a possible kernel crash through stack trashing triggered by an integer overflow. If count passed from userspace is (size_t)-1lu, the range check will overflow and return false. So the copy_from_user() will end up attempting to copy 0xFFFFFFFF (or 0xFFFFFFFFFFFFFFFF) bytes to the kernel stack. Of course the copy will fail at some point, because we can't allocate a buffer that big. But it will copy as much as it can and then return with an -EFAULT.
This means the userspace process writing to this proc file controls
the kernel stack.

This is probably not useable for a privilege escalation, because the proc file has permissions (S_IFREG | S_IRUGO | S_IWUSR). So only root will be able to crash the machine.

---

This patch is completely untested, because I do not have a machine with acpi-video.

---
 drivers/acpi/video.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

@@ -1185,21 +1185,21 @@ acpi_video_device_write_state(struct fil
 			      const char __user * buffer,
 			      size_t count, loff_t * data)
 {
 	int status;
 	struct seq_file *m = file->private_data;
 	struct acpi_video_device *dev = m->private;
 	char str[12] = { 0 };
 	u32 state = 0;
 
 
-	if (!dev || count + 1 > sizeof str)
+	if (!dev || count >= sizeof str)
 		return -EINVAL;
 
 	if (copy_from_user(str, buffer, count))
 		return -EFAULT;
 
 	str[count] = 0;
 	state = simple_strtoul(str, NULL, 0);
 	state &= ((1ul << 31) | (1ul << 30) | (1ul << 0));
 
 	status = acpi_video_device_set_state(dev, state);

Greetings, Michael.