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

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Message-ID: <20171017162826.GB5357@localhost.localdomain>
Date:   Tue, 17 Oct 2017 14:28:26 -0200
From:   Marcelo Ricardo Leitner <marcelo.leitner@...il.com>
To:     Xin Long <lucien.xin@...il.com>
Cc:     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>,
        Eric Dumazet <edumazet@...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 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().  It's not possible that these would trip
on the update going on on sctp_packet_config() because the socket is
locked. We may not need (much) more than the example patch, I think.

A more thorough check is certainly welcomed, indeed.

  Marcelo