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

[Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>]

On Wed, Oct 18, 2017 at 01:33:46AM +0800, Xin Long wrote:
> On Wed, Oct 18, 2017 at 1:27 AM, Marcelo Ricardo Leitner
> <marcelo.leitner@gmail.com> wrote:
> > On Tue, Oct 17, 2017 at 10:20:58AM -0700, Eric Dumazet wrote:
> >> On Tue, Oct 17, 2017 at 10:01 AM, Marcelo Ricardo Leitner
> >> <marcelo.leitner@gmail.com> wrote:
> >> > On Tue, Oct 17, 2017 at 09:44:10AM -0700, Eric Dumazet wrote:
> >> >> On Tue, Oct 17, 2017 at 9:28 AM, Marcelo Ricardo Leitner
> >> >> <marcelo.leitner@gmail.com> wrote:
> >> >> > On Tue, Oct 17, 2017 at 11:31:30PM +0800, Xin Long wrote:
> >> >> >> 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.
> >> >> >
> >> >> > I checked some and sctp_transport_dst_check() should be fine because
> >> >> > by then we are holding a reference on dst. Same goes to
> >> >> > sctp_transport_pmtu_check().
> >> >>
> >> >> Really ?
> >> >>
> >> >
> >> > Yes,
> >> >
> >> >> What about sctp_v4_err() -> sctp_icmp_redirect() -> sctp_transport_dst_check()
> >> >>
> >> >> It seems quite possible that the BH handler can access it, while
> >> >> socket is owned by user.
> >> >
> >> > hidden here:
> >> > sctp_v4_err() {
> >> > ...
> >> >         sk = sctp_err_lookup(net, AF_INET, skb, sctp_hdr(skb), &asoc,
> >> >         &transport);
> >> > ...
> >> > out_unlock:
> >> >         sctp_err_finish(sk, transport);
> >> > }
> >> >
> >> > sctp_err_lookup() {
> >> > ...
> >> >         bh_lock_sock(sk);
> >> >
> >> >         /* If too many ICMPs get dropped on busy
> >> >          * servers this needs to be solved differently.
> >> >          */
> >> >         if (sock_owned_by_user(sk))            [A]
> >> >                 __NET_INC_STATS(net, LINUX_MIB_LOCKDROPPEDICMPS);
> >> >
> >> >         *app = asoc;
> >> >         *tpp = transport;
> >> >         return sk;
> >> > ...
> >> > }
> >> >
> >> > Though that if() on [A] should be bailing out without returning
> >> > nothing. That's a bug. More like:
> >> >
> >> >         if (sock_owned_by_user(sk)) {
> >> >                 __NET_INC_STATS(net, LINUX_MIB_LOCKDROPPEDICMPS);
> >> >                 goto out;
> >> >         }
> >> >
> >>
> >> So why sctp_v4_err() is doing this test ?
> >>
> >> if (!sock_owned_by_user(sk) && inet->recverr) {
> >>
> >> It looks like socket can be owned by the user, and [A] check only
> >> increments an SNMP counter,
> >> that wont help to solve the tp->dst use after free.
> >
> > Hah, missed that. Though the semantics on that counter still looks
> > confusing. It may be incremented when we actually handled the icmp.
> > The other icmp handling in there will postpone in case the socket is
> > locked by the user, and so will the timer callbacks too.
> Maybe that check should be done in sctp_icmp_redirect(), as
> in sctp_icmp_frag_needed(), as well as in tcp_v4_err().
> 
> @@ -421,7 +421,7 @@ void sctp_icmp_redirect(struct sock *sk, struct
> sctp_transport *t,
>  {
>         struct dst_entry *dst;
> 
> -       if (!t)
> +       if (sock_owned_by_user(sk) || !t)
>                 return;

Looks like it.