On Sat, Sep 10, 2016 at 11:11:23PM +0800, Xin Long wrote:
> Since commit 4f0087812648 ("sctp: apply rhashtable api to send/recv
> path"), sctp uses transport rhashtable with .obj_cmpfn sctp_hash_cmp,
> in which it compares the members of the transport with the rhashtable
> args to check if it's the right transport.
>
> But sctp uses the transport without holding it in sctp_hash_cmp, it can
> cause a use-after-free panic. As after it gets transport from hashtable,
> another CPU may close the sk and free the asoc. In sctp_association_free,
> it frees all the transports, meanwhile, the assoc's refcnt may be reduced
> to 0, assoc can be destroyed by sctp_association_destroy.
>
> So after that, transport->assoc is actually an unavailable memory address
> in sctp_hash_cmp. Although sctp_hash_cmp is under rcu_read_lock, it still
> can not avoid this, as assoc is not freed by RCU.
>
> This patch is to hold the transport before checking it's members with
> sctp_transport_hold, in which it checks the refcnt first, holds it if
> it's not 0.
>
> Fixes: 4f0087812648 ("sctp: apply rhashtable api to send/recv path")
> Signed-off-by: Xin Long <lucien....@gmail.com>
Note that we cannot defer the free of the asoc too because that cause
issues with port re-use (issue already hit and fixed in the past), as
the port would be still in use during the RCU grace period.
Acked-by: Marcelo Ricardo Leitner <marcelo.leit...@gmail.com>
> ---
> net/sctp/input.c | 27 +++++++++++++++++----------
> 1 file changed, 17 insertions(+), 10 deletions(-)
>
> diff --git a/net/sctp/input.c b/net/sctp/input.c
> index 69444d3..1555fb8 100644
> --- a/net/sctp/input.c
> +++ b/net/sctp/input.c
> @@ -796,27 +796,34 @@ struct sctp_hash_cmp_arg {
> static inline int sctp_hash_cmp(struct rhashtable_compare_arg *arg,
> const void *ptr)
> {
> + struct sctp_transport *t = (struct sctp_transport *)ptr;
> const struct sctp_hash_cmp_arg *x = arg->key;
> - const struct sctp_transport *t = ptr;
> - struct sctp_association *asoc = t->asoc;
> - const struct net *net = x->net;
> + struct sctp_association *asoc;
> + int err = 1;
>
> if (!sctp_cmp_addr_exact(&t->ipaddr, x->paddr))
> - return 1;
> - if (!net_eq(sock_net(asoc->base.sk), net))
> - return 1;
> + return err;
> + if (!sctp_transport_hold(t))
> + return err;
> +
> + asoc = t->asoc;
> + if (!net_eq(sock_net(asoc->base.sk), x->net))
> + goto out;
> if (x->ep) {
> if (x->ep != asoc->ep)
> - return 1;
> + goto out;
> } else {
> if (x->laddr->v4.sin_port != htons(asoc->base.bind_addr.port))
> - return 1;
> + goto out;
> if (!sctp_bind_addr_match(&asoc->base.bind_addr,
> x->laddr, sctp_sk(asoc->base.sk)))
> - return 1;
> + goto out;
> }
>
> - return 0;
> + err = 0;
> +out:
> + sctp_transport_put(t);
> + return err;
> }
>
> static inline u32 sctp_hash_obj(const void *data, u32 len, u32 seed)
> --
> 2.1.0
>
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