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

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

On Wed, Nov 21, 2018 at 9:46 AM Marcelo Ricardo Leitner
<marcelo.leitner@gmail.com> wrote:
>
> On Tue, Nov 20, 2018 at 07:52:48AM -0500, Neil Horman wrote:
> > On Tue, Nov 20, 2018 at 07:09:16PM +0800, Xin Long wrote:
> > > In sctp_hash_transport, it dereferences a transport's asoc only under
> > > rcu_read_lock. Without holding the transport, its asoc could be freed
> > > already, which leads to a use-after-free panic.
> > >
> > > A similar fix as Commit bab1be79a516 ("sctp: hold transport before
> > > accessing its asoc in sctp_transport_get_next") is needed to hold
> > > the transport before accessing its asoc in sctp_hash_transport.
> > >
> > > Fixes: cd2b70875058 ("sctp: check duplicate node before inserting a new transport")
> > > Reported-by: syzbot+0b05d8aa7cb185107483@syzkaller.appspotmail.com
> > > Signed-off-by: Xin Long <lucien.xin@gmail.com>
> > > ---
> > >  net/sctp/input.c | 7 ++++++-
> > >  1 file changed, 6 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/net/sctp/input.c b/net/sctp/input.c
> > > index 5c36a99..69584e9 100644
> > > --- a/net/sctp/input.c
> > > +++ b/net/sctp/input.c
> > > @@ -896,11 +896,16 @@ int sctp_hash_transport(struct sctp_transport *t)
> > >     list = rhltable_lookup(&sctp_transport_hashtable, &arg,
> > >                            sctp_hash_params);
> > >
> > > -   rhl_for_each_entry_rcu(transport, tmp, list, node)
> > > +   rhl_for_each_entry_rcu(transport, tmp, list, node) {
> > > +           if (!sctp_transport_hold(transport))
> > > +                   continue;
> > >             if (transport->asoc->ep == t->asoc->ep) {
> > > +                   sctp_transport_put(transport);
> > >                     rcu_read_unlock();
> > >                     return -EEXIST;
> > >             }
> > > +           sctp_transport_put(transport);
> > > +   }
> > >     rcu_read_unlock();
> > >
> > >     err = rhltable_insert_key(&sctp_transport_hashtable, &arg,
> > > --
> > > 2.1.0
> > >
> > >
> >
> > something doesn't feel at all right about this.  If we are inserting a transport
> > to an association, it would seem to me that we should have at least one user of
> > the association (i.e. non-zero refcount).  As such it seems something is wrong
> > with the association refcount here.  At the very least, if there is a case where
> > an association is being removed while a transport is being added, the better
> > solution would be to ensure that sctp_association_destroy goes through a
> > quiescent point prior to unhashing transports from the list, to ensure that
> > there is no conflict with the add operation above.
Changing to do call_rcu(&transport->rcu, sctp_association_destroy) can
work for this case.
But it means asoc and socket (taking the port) will have to wait for a
grace period, which is not expected. We seemed to have talked about
this before, Marcelo?

>
> Consider that the rhl_for_each_entry_rcu() is traversing the global
> rhashtable, and that it may operate on unrelated transports/asocs.
> E.g., transport->asoc in the for() is potentially different from the
> asoc under socket lock.
>
> The core of the fix is at:
> +               if (!sctp_transport_hold(transport))
> +                       continue;
> If we can get a hold, the asoc will be available for dereferencing in
> subsequent lines. Otherwise, move on.
>
> With that, the patch makes sense to me.
>
> Although I would prefer if we come up with a better way to do this
> jump, or even avoid the jump. We are only comparing pointers here and,
> if we had asoc->ep cached on sctp_transport itself, we could avoid the
> atomics here.
Right, but it's another u64.

>
> This change, in the next patch on sctp_epaddr_lookup_transport, will
> hurt performance as that is called in datapath. Rhashtable will help
> on keeping entry lists to a size, but still.
This loop is not long normally, will only a few atomic operations hurt
a noticeable performance?