netdev - Re: Use after free in __dst_destroy_metrics

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Date:   Sat, 16 Sep 2017 01:38:18 +0300 (EEST)
From:   Julian Anastasov <ja@....bg>
To:     Eric Dumazet <eric.dumazet@...il.com>
cc:     Cong Wang <xiyou.wangcong@...il.com>,
        Subash Abhinov Kasiviswanathan <subashab@...eaurora.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Lorenzo Colitti <lorenzo@...gle.com>
Subject: Re: Use after free in __dst_destroy_metrics_generic


	Hello,

On Fri, 15 Sep 2017, Eric Dumazet wrote:

> On Fri, 2017-09-08 at 09:10 -0700, Cong Wang wrote:
> > On Thu, Sep 7, 2017 at 5:52 PM, Subash Abhinov Kasiviswanathan
> > <subashab@...eaurora.org> wrote:
> > > We are seeing a possible use after free in ip6_dst_destroy.
> > >
> > > It appears as if memory of the __DST_METRICS_PTR(old) was freed in some path
> > > and allocated
> > > to ion driver. ion driver has also freed it. Finally the memory is freed by
> > > the
> > > fib gc and crashes since it is already deallocated.
> > 
> > Does the attach (compile-only) patch help anything?
> > 
> > From my _quick_ glance, it seems we miss the refcnt'ing
> > right in __dst_destroy_metrics_generic().
> > 
> > Thanks!
> 
> 
> Hi Cong
> 
> I believe your patch makes a lot of sense, please submit it formally ?

	Cong's patch is wrong for few reasons:

- it will stop to kfree non-refcounted metrics

- report was for IPV6 and we set DST_METRICS_REFCOUNTED only
for IPv4, for DST_METRICS_READ_ONLY metrics

- __dst_destroy_metrics_generic is called for val without
DST_METRICS_READ_ONLY flag and such metrics are not with
DST_METRICS_REFCOUNTED flag

- ->cow_metrics and dst_cow_metrics_generic are called with 
DST_METRICS_READ_ONLY flag set, there is no chance to write
new value twice, especially to write DST_METRICS_REFCOUNTED flag
and later to see this flag in __dst_destroy_metrics_generic

	So, I'm not sure where exactly is the bug with the
metrics.

	May be I'm missing some posting but I don't see if
the patch was tested successfully.

Regards

--
Julian Anastasov <ja@....bg>