Re: [PATCH net] sctp: hold transport before accessing its asoc in sctp_transport_get_next

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

On Wed, Aug 29, 2018 at 7:36 PM Neil Horman <nhorman@tuxdriver.com> wrote:
>
> On Wed, Aug 29, 2018 at 12:08:40AM +0800, Xin Long wrote:
> > On Mon, Aug 27, 2018 at 9:08 PM Neil Horman <nhorman@tuxdriver.com> wrote:
> > >
> > > On Mon, Aug 27, 2018 at 06:38:31PM +0800, Xin Long wrote:
> > > > As Marcelo noticed, in sctp_transport_get_next, it is iterating over
> > > > transports but then also accessing the association directly, without
> > > > checking any refcnts before that, which can cause an use-after-free
> > > > Read.
> > > >
> > > > So fix it by holding transport before accessing the association. With
> > > > that, sctp_transport_hold calls can be removed in the later places.
> > > >
> > > > Fixes: 626d16f50f39 ("sctp: export some apis or variables for sctp_diag and reuse some for proc")
> > > > Reported-by: syzbot+fe62a0c9aa6a85c6de16@syzkaller.appspotmail.com
> > > > Signed-off-by: Xin Long <lucien.xin@gmail.com>
> > > > ---
> > > >  net/sctp/proc.c   |  4 ----
> > > >  net/sctp/socket.c | 22 +++++++++++++++-------
> > > >  2 files changed, 15 insertions(+), 11 deletions(-)
> > > >
> > > > diff --git a/net/sctp/proc.c b/net/sctp/proc.c
> > > > index ef5c9a8..4d6f1c8 100644
> > > > --- a/net/sctp/proc.c
> > > > +++ b/net/sctp/proc.c
> > > > @@ -264,8 +264,6 @@ static int sctp_assocs_seq_show(struct seq_file *seq, void *v)
> > > >       }
> > > >
> > > >       transport = (struct sctp_transport *)v;
> > > > -     if (!sctp_transport_hold(transport))
> > > > -             return 0;
> > > >       assoc = transport->asoc;
> > > >       epb = &assoc->base;
> > > >       sk = epb->sk;
> > > > @@ -322,8 +320,6 @@ static int sctp_remaddr_seq_show(struct seq_file *seq, void *v)
> > > >       }
> > > >
> > > >       transport = (struct sctp_transport *)v;
> > > > -     if (!sctp_transport_hold(transport))
> > > > -             return 0;
> > > >       assoc = transport->asoc;
> > > >
> > > >       list_for_each_entry_rcu(tsp, &assoc->peer.transport_addr_list,
> > > > diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> > > > index e96b15a..aa76586 100644
> > > > --- a/net/sctp/socket.c
> > > > +++ b/net/sctp/socket.c
> > > > @@ -5005,9 +5005,14 @@ struct sctp_transport *sctp_transport_get_next(struct net *net,
> > > >                       break;
> > > >               }
> > > >
> > > > +             if (!sctp_transport_hold(t))
> > > > +                     continue;
> > > > +
> > > >               if (net_eq(sock_net(t->asoc->base.sk), net) &&
> > > >                   t->asoc->peer.primary_path = t)
> > > >                       break;
> > > > +
> > > > +             sctp_transport_put(t);
> > > >       }
> > > >
> > > >       return t;
> > > > @@ -5017,13 +5022,18 @@ struct sctp_transport *sctp_transport_get_idx(struct net *net,
> > > >                                             struct rhashtable_iter *iter,
> > > >                                             int pos)
> > > >  {
> > > > -     void *obj = SEQ_START_TOKEN;
> > > > +     struct sctp_transport *t;
> > > >
> > > > -     while (pos && (obj = sctp_transport_get_next(net, iter)) &&
> > > > -            !IS_ERR(obj))
> > > > -             pos--;
> > > > +     if (!pos)
> > > > +             return SEQ_START_TOKEN;
> > > >
> > > > -     return obj;
> > > > +     while ((t = sctp_transport_get_next(net, iter)) && !IS_ERR(t)) {
> > > > +             if (!--pos)
> > > > +                     break;
> > > > +             sctp_transport_put(t);
> > > > +     }
> > > > +
> > > > +     return t;
> > > >  }
> > > >
> > > >  int sctp_for_each_endpoint(int (*cb)(struct sctp_endpoint *, void *),
> > > > @@ -5082,8 +5092,6 @@ int sctp_for_each_transport(int (*cb)(struct sctp_transport *, void *),
> > > >
> > > >       tsp = sctp_transport_get_idx(net, &hti, *pos + 1);
> > > >       for (; !IS_ERR_OR_NULL(tsp); tsp = sctp_transport_get_next(net, &hti)) {
> > > > -             if (!sctp_transport_hold(tsp))
> > > > -                     continue;
> > > >               ret = cb(tsp, p);
> > > >               if (ret)
> > > >                       break;
> > > > --
> > > > 2.1.0
> > > >
> > > >
> > > Acked-by: Neil Horman <nhorman@tuxdriver.com>
> > >
> > > Additionally, its not germaine to this particular fix, but why are we still
> > > using that pos variable in sctp_transport_get_idx?  With the conversion to
> > > rhashtables, it doesn't seem particularly useful anymore.
> > For proc, seems so, hti is saved into seq->private.
> > But for diag, "hti" in sctp_for_each_transport() is a local variable.
> > do you think where we can save it?
> >
> Sorry, wasn't suggesting that it had to be removed from sctp_for_each_trasnport,
> its clearly used as both an input and output there.  All I was sugesting was
> that, in sctp_transport_get_idx, the pos variable might no longer be needed
> there specifically, as sctp_transprt_get_next should terminate the loop on its
> own.  Or is there another purpose for that positional variable I am missing
Yes, Neil, all transports/asocs could not be dumped once as one buffer/rtnl msg
is not big enough for them. when coming into proc/diag again, it needs to start
from the *next* one, and 'pos' is used to save its position.

Without 'pos', it would always start from the first one and dump the duplicate
ones in the next times. Make sense?

>
> Neil
>