Re: [PATCH v3] sctp: fix refcount bug in sctp_wfree

[Eric Dumazet <eric.dumazet@gmail.com>]

On 3/20/20 4:09 AM, Qiujun Huang wrote:
> Do accounting for skb's real sk.
> In some case skb->sk != asoc->base.sk:
> 
> for the trouble SKB, it was in outq->transmitted queue
> 
> sctp_outq_sack
> 	sctp_check_transmitted
> 		SKB was moved to outq->sack
> 	then throw away the sack queue
> 		SKB was deleted from outq->sack
> (but the datamsg held SKB at sctp_datamsg_to_asoc
> So, sctp_wfree was not called to destroy SKB)
> 
> then migrate happened
> 
> 	sctp_for_each_tx_datachunk(
> 	sctp_clear_owner_w);
> 	sctp_assoc_migrate();
> 	sctp_for_each_tx_datachunk(
> 	sctp_set_owner_w);
> SKB was not in the outq, and was not changed to newsk
> 
> finally
> 
> __sctp_outq_teardown
> 	sctp_chunk_put (for another skb)
> 		sctp_datamsg_put
> 			__kfree_skb(msg->frag_list)
> 				sctp_wfree (for SKB)
> this case in sctp_wfree SKB->sk was oldsk.
> 
> 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);
>  }
> 

This does not really solve the issue.

Even if the particular syzbot repro is now fine.

Really, having anything _after_ the sock_wfree(skb) is the bug, since the current thread no longer
own a reference on a socket.