tipc: fix double-free in tipc_buf_append()

tipc_msg_validate() can potentially reallocate the skb it is validating,
freeing the old one.  In tipc_buf_append(), it was being called with a
pointer to a local variable which was a copy of the caller's skb
pointer.

If the skb was reallocated and validation subsequently failed, the error
handling path would free the original skb pointer, which had already
been freed, leading to double-free.

Fix this by checking if head now points to a newly allocated reassembled
skb.  If it does, reassign *headbuf for later freeing operations.

Fixes: d618d09 ("tipc: enforce valid ratio between skb truesize and contents")
Suggested-by: Tung Nguyen <[email protected]>
Signed-off-by: Lee Jones <[email protected]>
Reviewed-by: Tung Nguyen <[email protected]>
Signed-off-by: Jakub Kicinski <[email protected]>

Author: lag-linaro

net/tipc/msg.c

@@ -177,8 +177,20 @@ int tipc_buf_append(struct sk_buff **headbuf, struct sk_buff **buf)
 
 	if (fragid == LAST_FRAGMENT) {
 		TIPC_SKB_CB(head)->validated = 0;
-		if (unlikely(!tipc_msg_validate(&head)))
+
+		/* If the reassembled skb has been freed in
+		 * tipc_msg_validate() because of an invalid truesize,
+		 * then head will point to a newly allocated reassembled
+		 * skb, while *headbuf points to freed reassembled skb.
+		 * In such cases, correct *headbuf for freeing the newly
+		 * allocated reassembled skb later.
+		 */
+		if (unlikely(!tipc_msg_validate(&head))) {
+			if (head != *headbuf)
+				*headbuf = head;
 			goto err;
+		}
+
 		*buf = head;
 		TIPC_SKB_CB(head)->tail = NULL;
 		*headbuf = NULL;