On Tue, Oct 17, 2017 at 10:01 AM, Marcelo Ricardo Leitner
<marcelo.leitner@xxxxxxxxx> 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@xxxxxxxxx> 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@xxxxxxxxx> 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.
I
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