On Fri, Oct 9, 2020 at 1:45 AM Cong Wang <xiyou.wangc...@gmail.com> wrote: > > On Thu, Oct 8, 2020 at 1:45 AM Xin Long <lucien....@gmail.com> wrote: > > > > On Thu, Oct 8, 2020 at 12:12 PM Cong Wang <xiyou.wangc...@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. Didn't notice kfree_skb 'buf' on the err path.
Reviewed-by: Xin Long <lucien....@gmail.com>
