netdev - Re: [PATCH net-next] net/sched: cls_flower: verify root pointer before dereferncing it

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Message-ID: <CAM_iQpU=t-APBrPuW1K1FdRW8J9UydkbD9Y4h1eD_SU3yK=wvw@mail.gmail.com>
Date:   Thu, 24 Nov 2016 22:23:54 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     David Miller <davem@...emloft.net>, Jiri Pirko <jiri@...nulli.us>,
        Roi Dayan <roid@...lanox.com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Jiri Pirko <jiri@...lanox.com>,
        Or Gerlitz <ogerlitz@...lanox.com>,
        Cong Wang <cwang@...pensource.com>,
        John Fastabend <john.fastabend@...il.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
Subject: Re: [PATCH net-next] net/sched: cls_flower: verify root pointer
 before dereferncing it

On Thu, Nov 24, 2016 at 4:17 PM, Daniel Borkmann <daniel@...earbox.net> wrote:
>
>
> I'm not sure if setting a dummy object for each affected classifier is
> making things better. Are you having this in mind as a target for -net?
>
> We do kfree_rcu() the head (tp->root) and likewise do we kfree_rcu() the
> tp immediately after the callback. The head object's lifetime for such
> classifiers is thus always bound to the tp lifetime. And besides that,
> nothing apart from get() checks whether it's actually really NULL (and
> that check in get() is odd anyway; some cls do it, some don't).
>

Excellent point.

I thought we should exclude any parallel readers when we call destroy(),
you are taking a different approach by observing we only have to exclude
readers when we really free them, this seems fine to me after a second
thought, because the RCU API should take care of races with readers so
as long as we free everything in RCU callback we are good. Hmm...

But I may miss something since I am not an RCU expert.

[...]
>
> (Btw, matchall is still broken besides this fix. mall_delete() sets the
>  RCU_INIT_POINTER(head->filter, NULL), so that a mall_delete() plus
>  mall_destroy() combo doesn't free head->filter twice, but doing that is
>  racy with mall_classify() where head->filter is dereferenced unchecked.
>  Requires additional fix.)

This seems due to matchall only has one filter per tp. But you don't need
to worry since readers never read a freed pointer, right?