netdev - Re: Fw: Linux tcp_xmit_retransmit_queue use after free on 4.8-rc1 / master]

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Date:	Tue, 16 Aug 2016 13:27:59 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Cong Wang <xiyou.wangcong@...il.com>
Cc:	Greg KH <gregkh@...uxfoundation.org>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	Marco Grassi <marco.gra@...il.com>,
	Yuchung Cheng <ycheng@...gle.com>,
	Neal Cardwell <ncardwell@...gle.com>
Subject: Re: Fw: Linux tcp_xmit_retransmit_queue use after free on 4.8-rc1 /
 master]

On Tue, 2016-08-16 at 13:19 -0700, Cong Wang wrote:
> On Tue, Aug 16, 2016 at 11:39 AM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> > On Tue, 2016-08-16 at 08:39 -0700, Eric Dumazet wrote:
> >> On Tue, 2016-08-16 at 12:45 +0200, Greg KH wrote:
> >> > For some reason Marco's emails can't make it to netdev, so I'm
> >> > forwarding this on.  Please cc: him on responses.
> >>
> >> Thanks for the report Greg and Marco.
> >>
> >> My first guess is this is caused by
> >>
> >> d41a69f1d390 tcp: make tcp_sendmsg() aware of socket backlog
> >>
> >> And a combination of funky sendmsg() flags (like FastOpen)
> >>
> >> I will look at this problem today.
> >>
> >
> > No, above commit was innocent ;)
> >
> > It looks like the bug is very old, and following patch would fix it.
> > I will submit it formally after few tests.
> >
> >
> > diff --git a/include/net/tcp.h b/include/net/tcp.h
> > index c00e7d51bb18..7717302cab91 100644
> > --- a/include/net/tcp.h
> > +++ b/include/net/tcp.h
> > @@ -1523,6 +1523,8 @@ static inline void tcp_check_send_head(struct sock *sk, struct sk_buff *skb_unli
> >  {
> >         if (sk->sk_send_head == skb_unlinked)
> >                 sk->sk_send_head = NULL;
> > +       if (tcp_sk(sk)->highest_sack == skb_unlinked)
> > +               tcp_sk(sk)->highest_sack = NULL;
> >  }
> 
> Hmm, but from the stack traces it indicates the skb is freed
> inside tcp_sendmsg(), which must be:
> 
> 
> do_fault:
>         if (!skb->len) {
>                 tcp_unlink_write_queue(skb, sk);
>                 /* It is the one place in all of TCP, except connection
>                  * reset, where we can be unlinking the send_head.
>                  */
>                 tcp_check_send_head(sk, skb);
>                 sk_wmem_free_skb(sk, skb);
>         }
> 
> In this case, skb->len == 0 means it is newly allocated skb by
> sk_stream_alloc_skb(), so it should not have a chance to be
> picked by tp->highest_sack yet b/c the whole function locks
> the sock?
> 
> I must miss something here.


Look at skb_entail() : It calls tcp_add_write_queue_tail()

And tcp_add_write_queue_tail() looks like :


static inline void tcp_add_write_queue_tail(struct sock *sk, struct sk_buff *skb)
{
        __tcp_add_write_queue_tail(sk, skb);

        /* Queue it, remembering where we must start sending. */
        if (sk->sk_send_head == NULL) {
                sk->sk_send_head = skb;

                if (tcp_sk(sk)->highest_sack == NULL)
                        tcp_sk(sk)->highest_sack = skb;
        }
}


So we definitely need to undo what tcp_add_write_queue_tail() did.