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Date:   Fri, 7 Aug 2020 14:52:04 -0700
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Johan Knöös <jknoos@...gle.com>
Cc:     Gregory Rose <gvrose8192@...il.com>, bugs <bugs@...nvswitch.org>,
        Tonghao Zhang <xiangxia.m.yue@...il.com>,
        Netdev <netdev@...r.kernel.org>, joel@...lfernandes.org
Subject: Re: [ovs-discuss] Double free in recent kernels after memleak fix

On Fri, Aug 7, 2020 at 8:33 AM Johan Knöös <jknoos@...gle.com> wrote:
>
> On Tue, Aug 4, 2020 at 8:52 AM Gregory Rose <gvrose8192@...il.com> wrote:
> >
> >
> >
> > On 8/3/2020 12:01 PM, Johan Knöös via discuss wrote:
> > > Hi Open vSwitch contributors,
> > >
> > > We have found openvswitch is causing double-freeing of memory. The
> > > issue was not present in kernel version 5.5.17 but is present in
> > > 5.6.14 and newer kernels.
> > >
> > > After reverting the RCU commits below for debugging, enabling
> > > slub_debug, lockdep, and KASAN, we see the warnings at the end of this
> > > email in the kernel log (the last one shows the double-free). When I
> > > revert 50b0e61b32ee890a75b4377d5fbe770a86d6a4c1 ("net: openvswitch:
> > > fix possible memleak on destroy flow-table"), the symptoms disappear.
> > > While I have a reliable way to reproduce the issue, I unfortunately
> > > don't yet have a process that's amenable to sharing. Please take a
> > > look.
> > >
> > > 189a6883dcf7 rcu: Remove kfree_call_rcu_nobatch()
> > > 77a40f97030b rcu: Remove kfree_rcu() special casing and lazy-callback handling
> > > e99637becb2e rcu: Add support for debug_objects debugging for kfree_rcu()
> > > 0392bebebf26 rcu: Add multiple in-flight batches of kfree_rcu() work
> > > 569d767087ef rcu: Make kfree_rcu() use a non-atomic ->monitor_todo
> > > a35d16905efc rcu: Add basic support for kfree_rcu() batching
> > >
> > > Thanks,
> > > Johan Knöös
> >
> > Let's add the author of the patch you reverted and the Linux netdev
> > mailing list.
> >
> > - Greg
>
> I found we also sometimes get warnings from
> https://elixir.bootlin.com/linux/v5.5.17/source/kernel/rcu/tree.c#L2239
> under similar conditions even on kernel 5.5.17, which I believe may be
> related. However, it's much rarer and I don't have a reliable way of
> reproducing it. Perhaps 50b0e61b32ee890a75b4377d5fbe770a86d6a4c1 only
> increases the frequency of a pre-existing bug.

It seems clear we have a double free on table->mask_array when
the reallocation is triggered on the destroy path.

Are you able to test the attached patch (compile tested only)?
Also note: it is generated against the latest net tree, it may not be
applied cleanly to any earlier stable release.

Thanks!

View attachment "openvswitch.diff" of type "text/x-patch" (2227 bytes)

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