Subject: Re: [PATCH net 1/3] sctp: fix the transport dead race check by using atomic_add_unless on refcnt

From: Vlad Yasevich <[email-masked]>
Date: Fri, 22 Jan 2016 11:50:20 -0500

On 01/21/2016 12:49 PM, Xin Long wrote:
> Now when __sctp_lookup_association is running in BH, it will try to
> check if t->dead is set, but meanwhile other CPUs may be freeing this
> transport and this assoc and if it happens that
> __sctp_lookup_association checked t->dead a bit too early, it may think
> that the association is still good while it was already freed.
> 
> So we fix this race by using atomic_add_unless in sctp_transport_hold.
> After we get one transport from hashtable, we will hold it only when
> this transport's refcnt is not 0, so that we can make sure t->asoc
> cannot be freed before we hold the asoc again.

atomic_add_unless() uses atomic_read() to check the value.  Since there
don't appear to be any barriers, what guarantees that the value
read will not have been modified in another thread under a proper lock?

> 
> Note that sctp association is not freed using RCU so we can't use
> atomic_add_unless() with it as it may just be too late for that either.
> 
> Fixes: 4f0087812648 ("sctp: apply rhashtable api to send/recv path")
> Reported-by: Vlad Yasevich <[email-masked]>
> Signed-off-by: Xin Long <[email-masked]>
> ---
>  include/net/sctp/structs.h |  2 +-
>  net/sctp/input.c           | 17 +++++++++++------
>  net/sctp/transport.c       |  4 ++--
>  3 files changed, 14 insertions(+), 9 deletions(-)
> 
> diff --git a/include/net/sctp/structs.h b/include/net/sctp/structs.h
> index 20e7212..344da04 100644
> --- a/include/net/sctp/structs.h
> +++ b/include/net/sctp/structs.h
> @@ -955,7 +955,7 @@ void sctp_transport_route(struct sctp_transport *, union sctp_addr *,
>  void sctp_transport_pmtu(struct sctp_transport *, struct sock *sk);
>  void sctp_transport_free(struct sctp_transport *);
>  void sctp_transport_reset_timers(struct sctp_transport *);
> -void sctp_transport_hold(struct sctp_transport *);
> +int sctp_transport_hold(struct sctp_transport *);
>  void sctp_transport_put(struct sctp_transport *);
>  void sctp_transport_update_rto(struct sctp_transport *, __u32);
>  void sctp_transport_raise_cwnd(struct sctp_transport *, __u32, __u32);
> diff --git a/net/sctp/input.c b/net/sctp/input.c
> index bf61dfb..49d2cc7 100644
> --- a/net/sctp/input.c
> +++ b/net/sctp/input.c
> @@ -935,15 +935,22 @@ static struct sctp_association *__sctp_lookup_association(
>  					struct sctp_transport **pt)
>  {
>  	struct sctp_transport *t;
> +	struct sctp_association *asoc = NULL;
>  
> +	rcu_read_lock();
>  	t = sctp_addrs_lookup_transport(net, local, peer);
> -	if (!t || t->dead)
> -		return NULL;
> +	if (!t || !sctp_transport_hold(t))
> +		goto out;
>  
> -	sctp_association_hold(t->asoc);
> +	asoc = t->asoc;
> +	sctp_association_hold(asoc);

I don't think you can modify the reference count on a transport, let alone
the association outside of a lock.

-vlad

>  	*pt = t;
>  
> -	return t->asoc;
> +	sctp_transport_put(t);
> +
> +out:
> +	rcu_read_unlock();
> +	return asoc;
>  }
>  
>  /* Look up an association. protected by RCU read lock */
> @@ -955,9 +962,7 @@ struct sctp_association *sctp_lookup_association(struct net *net,
>  {
>  	struct sctp_association *asoc;
>  
> -	rcu_read_lock();
>  	asoc = __sctp_lookup_association(net, laddr, paddr, transportp);
> -	rcu_read_unlock();
>  
>  	return asoc;
>  }
> diff --git a/net/sctp/transport.c b/net/sctp/transport.c
> index aab9e3f..69f3799 100644
> --- a/net/sctp/transport.c
> +++ b/net/sctp/transport.c
> @@ -296,9 +296,9 @@ void sctp_transport_route(struct sctp_transport *transport,
>  }
>  
>  /* Hold a reference to a transport.  */
> -void sctp_transport_hold(struct sctp_transport *transport)
> +int sctp_transport_hold(struct sctp_transport *transport)
>  {
> -	atomic_inc(&transport->refcnt);
> +	return atomic_add_unless(&transport->refcnt, 1, 0);
>  }
>  
>  /* Release a reference to a transport and clean up
>