Re: [RFC] sctp: suspicious rcu_read_lock() in sctp_packet_config()

[Xin Long <lucien.xin@gmail.com>]

On Tue, Oct 17, 2017 at 9:45 PM, Eric Dumazet <eric.dumazet@gmail.com> wrote:
> SCTP experts.
>
> syszkaller reported a few crashes in sctp_packet_config() with invalid
> access to a deleted dst.
>
> The rcu_read_lock() in sctp_packet_config() is suspect.
>
> It does not protect anything at the moment.
>
> If we expect tp->dst to be manipulated/changed by another cpu/thread,
> then we need proper rcu protection.
>
> Following patch to show what would be a minimal change (but obviously
> bigger changes are needed, like sctp_transport_pmtu_check() and
> sctp_transport_dst_check(), and proper sparse annotations)
will check all places accessing tp->dst in sctp.

>
>
> BTW, sparse throws a lot of errors, any volunteer to clean this mess ?
will do it.

Thanks for reporting this.

>
> make C=2 M=net/sctp
>
> Thanks.
>
> diff --git a/net/sctp/output.c b/net/sctp/output.c
> index 4a865cd06d76cd5b2aa417de618da3203f7b53e4..d7f320f5acc271189ec9474795b6ececed7ad2b9 100644
> --- a/net/sctp/output.c
> +++ b/net/sctp/output.c
> @@ -86,6 +86,7 @@ void sctp_packet_config(struct sctp_packet *packet, __u32 vtag,
>  {
>         struct sctp_transport *tp = packet->transport;
>         struct sctp_association *asoc = tp->asoc;
> +       struct dst_entry *dst;
>         struct sock *sk;
>
>         pr_debug("%s: packet:%p vtag:0x%x\n", __func__, packet, vtag);
> @@ -121,17 +122,15 @@ void sctp_packet_config(struct sctp_packet *packet, __u32 vtag,
>                         sctp_packet_append_chunk(packet, chunk);
>         }
>
> -       if (!tp->dst)
> -               return;
> -
>         /* set packet max_size with gso_max_size if gso is enabled*/
>         rcu_read_lock();
> -       if (__sk_dst_get(sk) != tp->dst) {
> -               dst_hold(tp->dst);
> -               sk_setup_caps(sk, tp->dst);
> +       dst = rcu_dereference(tp->dst);
> +       if (dst) {
> +               if (__sk_dst_get(sk) != dst && dst_hold_safe(dst))
> +                       sk_setup_caps(sk, dst);
> +               packet->max_size = sk_can_gso(sk) ? dst->dev->gso_max_size
> +                                                 : asoc->pathmtu;
>         }
> -       packet->max_size = sk_can_gso(sk) ? tp->dst->dev->gso_max_size
> -                                         : asoc->pathmtu;
>         rcu_read_unlock();
>  }
>
>
>