Re: [Patch net] tipc: fix the skb_unshare() in tipc_buf_append()

[Cong Wang <xiyou.wangcong@gmail.com>]

On Thu, Oct 8, 2020 at 1:45 AM Xin Long <lucien.xin@gmail.com> wrote:
>
> On Thu, Oct 8, 2020 at 12:12 PM Cong Wang <xiyou.wangcong@gmail.com> wrote:
> >
> > 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.
> Do you mean:
>                 frag = skb_clone(skb, GFP_ATOMIC);
> frag = skb_unshare(frag) will free the 'skb' too?

Yes, more precisely, I mean:

new = skb_clone(old)
kfree_skb(new)
kfree_skb(new)

would free 'old' eventually when 'old' is a fast clone. The skb_clone()
sets ->fclone_ref to 2 and returns the clone, whose skb->fclone is
SKB_FCLONE_CLONE. So, the first call of kfree_skbmem() will
just decrease ->fclone_ref by 1, but the second call will trigger
kmem_cache_free() which frees _both_  skb's.

Thanks.