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

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

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;

>
> Will look more, thanks.
>