netdev - Re: [RFC] sctp: suspicious rcu_read_lock() in sctp_packet

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Date:   Tue, 17 Oct 2017 09:44:10 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
Cc:     Xin Long <lucien.xin@...il.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Vlad Yasevich <vyasevich@...il.com>,
        Neil Horman <nhorman@...driver.com>,
        netdev <netdev@...r.kernel.org>, Wei Wang <weiwan@...gle.com>,
        linux-sctp@...r.kernel.org
Subject: Re: [RFC] sctp: suspicious rcu_read_lock() in sctp_packet_config()

On Tue, Oct 17, 2017 at 9:28 AM, Marcelo Ricardo Leitner
<marcelo.leitner@...il.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@...il.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 ?

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.