tipc: fix the skb_unshare() in tipc_buf_append()

skb_unshare() drops a reference count on the old skb unconditionally,
so in the failure case, we end up freeing the skb twice here.
And because the skb is allocated in fclone and cloned by caller
tipc_msg_reassemble(), the consequence is actually freeing the
original skb too, thus triggered the UAF by syzbot.

Fix this by replacing this skb_unshare() with skb_cloned()+skb_copy().

Fixes: ff48b6222e65 ("tipc: use skb_unshare() instead in tipc_buf_append()")
Reported-and-tested-by: syzbot+e96a7ba46281824cc46a@syzkaller.appspotmail.com
Cc: Jon Maloy <jmaloy@redhat.com>
Cc: Ying Xue <ying.xue@windriver.com>
Signed-off-by: Cong Wang <xiyou.wangcong@gmail.com>
Reviewed-by: Xin Long <lucien.xin@gmail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

diff --git a/net/tipc/msg.c b/net/tipc/msg.c
index 52e93ba4d8e2c5eeb545af24ebb9f6c5c9f4a971..6812244018714ac2876ffb5f942616a7c9c80965 100644 (file)
--- a/net/tipc/msg.c
+++ b/net/tipc/msg.c
@@ -150,7 +150,8 @@ int tipc_buf_append(struct sk_buff **headbuf, struct sk_buff **buf)
        if (fragid == FIRST_FRAGMENT) {
                if (unlikely(head))
                        goto err;
-               frag = skb_unshare(frag, GFP_ATOMIC);
+               if (skb_cloned(frag))
+                       frag = skb_copy(frag, GFP_ATOMIC);
                if (unlikely(!frag))
                        goto err;
                head = *headbuf = frag;