media: uvcvideo: Fix missing check to determine if element is found in list

The list iterator will point to a bogus position containing HEAD if
the list is empty or the element is not found in list. This case
should be checked before any use of the iterator, otherwise it will
lead to a invalid memory access. The missing check here is before
"pin = iterm->id;", just add check here to fix the security bug.

In addition, the list iterator value will *always* be set and non-NULL
by list_for_each_entry(), so it is incorrect to assume that the iterator
value will be NULL if the element is not found in list, considering
the (mis)use here: "if (iterm == NULL".

Use a new value 'it' as the list iterator, while use the old value
'iterm' as a dedicated pointer to point to the found element, which
1. can fix this bug, due to 'iterm' is NULL only if it's not found.
2. do not need to change all the uses of 'iterm' after the loop.
3. can also limit the scope of the list iterator 'it' *only inside*
   the traversal loop by simply declaring 'it' inside the loop in the
   future, as usage of the iterator outside of the list_for_each_entry
   is considered harmful. https://lkml.org/lkml/2022/2/17/1032

Fixes: d5e90b7a ("[media] uvcvideo: Move to video_ioctl2")
Signed-off-by: Xiaomeng Tong <xiam0nd.tong@gmail.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>

drivers/media/usb/uvc/uvc_v4l2.c

@@ -871,29 +871,31 @@ static int uvc_ioctl_enum_input(struct file *file, void *fh,
 	struct uvc_video_chain *chain = handle->chain;
 	const struct uvc_entity *selector = chain->selector;
 	struct uvc_entity *iterm = NULL;
+	struct uvc_entity *it;
 	u32 index = input->index;
-	int pin = 0;
 
 	if (selector == NULL ||
 	    (chain->dev->quirks & UVC_QUIRK_IGNORE_SELECTOR_UNIT)) {
 		if (index != 0)
 			return -EINVAL;
-		list_for_each_entry(iterm, &chain->entities, chain) {
-			if (UVC_ENTITY_IS_ITERM(iterm))
+		list_for_each_entry(it, &chain->entities, chain) {
+			if (UVC_ENTITY_IS_ITERM(it)) {
+				iterm = it;
 				break;
+			}
 		}
-		pin = iterm->id;
 	} else if (index < selector->bNrInPins) {
-		pin = selector->baSourceID[index];
-		list_for_each_entry(iterm, &chain->entities, chain) {
-			if (!UVC_ENTITY_IS_ITERM(iterm))
+		list_for_each_entry(it, &chain->entities, chain) {
+			if (!UVC_ENTITY_IS_ITERM(it))
 				continue;
-			if (iterm->id == pin)
+			if (it->id == selector->baSourceID[index]) {
+				iterm = it;
 				break;
+			}
 		}
 	}
 
-	if (iterm == NULL || iterm->id != pin)
+	if (iterm == NULL)
 		return -EINVAL;
 
 	memset(input, 0, sizeof(*input));