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

[Qiujun Huang]

On Sat, Mar 21, 2020 at 9:02 AM Marcelo Ricardo Leitner
<marcelo.leitner@xxxxxxxxx> wrote:
>
> On Sat, Mar 21, 2020 at 07:53:29AM +0800, Qiujun Huang wrote:
> ...
> > > > 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
> > >
> > > The real fix is to fix the migration to the new socket, though the
> > > situation on which it is happening is still not clear.
> > >
> > > The 2nd sendto() call on the reproducer is sending 212992 bytes on a
> > > single call. That's usually the whole sndbuf size, and will cause
> > > fragmentation to happen. That means the datamsg will contain several
> > > skbs. But still, the sacked chunks should be freed if needed while the
> > > remaining ones will be left on the queues that they are.
> >
> > in sctp_sendmsg_to_asoc
> > datamsg holds his chunk result in that the sacked chunks can't be freed
>
> Right! Now I see it, thanks.
> In the end, it's not a locking race condition. It's just not iterating
> on the lists properly.
>
> >
> > list_for_each_entry(chunk, &datamsg->chunks, frag_list) {
> > sctp_chunk_hold(chunk);
> > sctp_set_owner_w(chunk);
> > chunk->transport = transport;
> > }
> >
> > any ideas to handle it?
>
> sctp_for_each_tx_datachunk() needs to be aware of this situation.
> Instead of iterating directly/only over the chunk list, it should
> iterate over the datamsgs instead. Something like the below (just
> compile tested).
>
> Then, the old socket will be free to die regardless of the new one.
> Otherwise, if this association gets stuck on retransmissions or so,
> the old socket would not be freed till then.
>
> diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> index fed26a1e9518..85c742310d26 100644
> --- a/net/sctp/socket.c
> +++ b/net/sctp/socket.c
> @@ -151,9 +151,10 @@ static void sctp_for_each_tx_datachunk(struct sctp_association *asoc,
>                                        void (*cb)(struct sctp_chunk *))
>
>  {
> +       struct sctp_datamsg *msg, *prev_msg = NULL;
>         struct sctp_outq *q = &asoc->outqueue;
>         struct sctp_transport *t;
> -       struct sctp_chunk *chunk;
> +       struct sctp_chunk *chunk, *c;
>
>         list_for_each_entry(t, &asoc->peer.transport_addr_list, transports)
>                 list_for_each_entry(chunk, &t->transmitted, transmitted_list)
> @@ -162,8 +163,14 @@ static void sctp_for_each_tx_datachunk(struct sctp_association *asoc,
>         list_for_each_entry(chunk, &q->retransmit, transmitted_list)
>                 cb(chunk);
>
> -       list_for_each_entry(chunk, &q->sacked, transmitted_list)
> -               cb(chunk);
> +       list_for_each_entry(chunk, &q->sacked, transmitted_list) {
> +               msg = chunk->msg;
> +               if (msg == prev_msg)
> +                       continue;
> +               list_for_each_entry(c, &msg->chunks, frag_list)
> +                       cb(c);
> +               prev_msg = msg;
> +       }

great. I'll trigger a syzbot test. Thanks.

>
>         list_for_each_entry(chunk, &q->abandoned, transmitted_list)
>                 cb(chunk);