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 <vyasev...@gmail.com>
> Signed-off-by: Xin Long <lucien....@gmail.com>
> ---
> 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
>
