Re: [PATCH v2] sctp: fix refcount bug in sctp_wfree

[Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>]

Hi,

On Tue, Mar 17, 2020 at 11:55:36PM +0800, Qiujun Huang wrote:
> Do accounting for skb's real sk.
> In some case skb->sk != asoc->base.sk:
> 
> migrate routing        sctp_check_transmitted routing
> ------------                    ---------------
                                 sctp_close();
				   lock_sock(sk2);
				 sctp_primitive_ABORT();
                                 sctp_do_sm();
                                 sctp_cmd_interpreter();
                                 sctp_cmd_process_sack();
                                 sctp_outq_sack();
				 sctp_check_transmitted();

  lock_sock(sk1);
  sctp_getsockopt_peeloff();
  sctp_do_peeloff();
  sctp_sock_migrate();
> lock_sock_nested(sk2);
>                                mv the transmitted skb to
>                                the it's local tlist


How can sctp_do_sm() be called in the 2nd column so that it bypasses
the locks in the left column, allowing this mv to happen?

> 
> sctp_for_each_tx_datachunk(
> sctp_clear_owner_w);
> sctp_assoc_migrate();
> sctp_for_each_tx_datachunk(
> sctp_set_owner_w);
> 
>                                put the skb back to the
>                                assoc lists
> ----------------------------------------------------
> 
> The skbs which held bysctp_check_transmitted were not changed
> to newsk. They were not dealt with by sctp_for_each_tx_datachunk
> (sctp_clear_owner_w/sctp_set_owner_w).

It would make sense but I'm missing one step earlier, I'm not seeing
how the move to local list is allowed/possible in there. It really
shouldn't be possible.

> 
> It looks only trouble here so handling it in sctp_wfree is enough.
> 
> Reported-and-tested-by: syzbot+cea71eec5d6de256d54d@syzkaller.appspotmail.com
> Signed-off-by: Qiujun Huang <hqjagain@gmail.com>
> ---
>  net/sctp/socket.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> index 1b56fc440606..5f5c28b30e25 100644
> --- a/net/sctp/socket.c
> +++ b/net/sctp/socket.c
> @@ -9080,7 +9080,7 @@ static void sctp_wfree(struct sk_buff *skb)
>  {
>  	struct sctp_chunk *chunk = skb_shinfo(skb)->destructor_arg;
>  	struct sctp_association *asoc = chunk->asoc;
> -	struct sock *sk = asoc->base.sk;
> +	struct sock *sk = skb->sk;
>  
>  	sk_mem_uncharge(sk, skb->truesize);
>  	sk->sk_wmem_queued -= skb->truesize + sizeof(struct sctp_chunk);
> @@ -9109,7 +9109,7 @@ static void sctp_wfree(struct sk_buff *skb)
>  	}
>  
>  	sock_wfree(skb);
> -	sctp_wake_up_waiters(sk, asoc);
> +	sctp_wake_up_waiters(asoc->base.sk, asoc);
>  
>  	sctp_association_put(asoc);
>  }
> -- 
> 2.17.1
>