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

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
[off-list ref] wrote:

quoted

On Tue, Oct 17, 2017 at 11:31:30PM +0800, Xin Long wrote:

quoted

On Tue, Oct 17, 2017 at 9:45 PM, Eric Dumazet [off-list ref] wrote:

quoted

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;
	}