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

From: Marcelo Ricardo Leitner <[email-masked]>
Date: Tue, 17 Oct 2017 15:01:32 -0200

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
> <[email-masked]> 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 <[email-masked]> 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;
	}