Re: net: use-after-free in neigh_timer_handler/sock_wfree

From	Eric Dumazet <edumazet@google.com>
Newsgroups	linux.kernel
Subject	Re: net: use-after-free in neigh_timer_handler/sock_wfree
Date	2017-03-02 06:50 +0100
Message-ID	<tgrMJ-69z-3@gated-at.bofh.it> (permalink)
References	(2 earlier) <tgkie-DA-11@gated-at.bofh.it> <tglxE-1Kg-9@gated-at.bofh.it> <tglHk-1Og-13@gated-at.bofh.it> <tglQZ-1Rz-7@gated-at.bofh.it> <tgrtn-612-3@gated-at.bofh.it>
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On Wed, Mar 1, 2017 at 9:25 PM, Cong Wang <xiyou.wangcong@gmail.com> wrote:
> On Wed, Mar 1, 2017 at 3:15 PM, Eric Dumazet <edumazet@google.com> wrote:
>> On Wed, Mar 1, 2017 at 3:09 PM, Cong Wang <xiyou.wangcong@gmail.com> wrote:
>>
>>>
>>> But I doubt skb_orphan() is the solution here, shouldn't we just
>>> update sk->sk_wmem_alloc with skb->truesize changes?
>>
>> Is it worth it ? Apart from syszkaller I mean...
>>
>> We started with something that had a real impact on real workloads.
>>
>> 158f323b9868b59967ad96957c4ca388161be321 net: adjust skb->truesize in
>> pskb_expand_head()
>>
>> Note that auditing the stack took me a while.
>
> I don't know how sk refcnt could work correctly without making
> sk_wmem_alloc correctly. We certainly could just call skb_orphan()
> is we don't need skb->sk any more, probably like the frag case,
> but for this case, the neigh one, the skb's sitting in neigh->arp_queue
> are not going to be freed unless in failed case, therefore skb->sk
> should not be orphaned so early.

There is absolutely no issue in arp/nd case.
Many skbs can sit there and it is fine.
Same with skbs sitting a long time in a qdisc.

Of course we try to not call skb_orphan() unless really needed.

tcp_gso_segment() tries very hard to propagate skb ownership to the segments,
but even something apparently easy like that took some patches before
being done right.

(for details : 0d08c42cf9a71530fef5ebcfe368f38f2dd0476f "tcp: gso: fix
truesize tracking")

conntrack reasm is mostly used in forwarding workloads, where skb->sk
is already NULL.

Are you thinking of a real workload where skb->sk _needs_ to be kept
in ipv6 reasm ?

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Re: net: use-after-free in neigh_timer_handler/sock_wfree Cong Wang <xiyou.wangcong@gmail.com> - 2017-03-01 22:40 +0100
  Re: net: use-after-free in neigh_timer_handler/sock_wfree Cong Wang <xiyou.wangcong@gmail.com> - 2017-03-01 22:50 +0100
    Re: net: use-after-free in neigh_timer_handler/sock_wfree Eric Dumazet <edumazet@google.com> - 2017-03-02 00:10 +0100
      Re: net: use-after-free in neigh_timer_handler/sock_wfree Cong Wang <xiyou.wangcong@gmail.com> - 2017-03-02 00:20 +0100
        Re: net: use-after-free in neigh_timer_handler/sock_wfree Eric Dumazet <edumazet@google.com> - 2017-03-02 00:30 +0100
          Re: net: use-after-free in neigh_timer_handler/sock_wfree Cong Wang <xiyou.wangcong@gmail.com> - 2017-03-02 06:30 +0100
            Re: net: use-after-free in neigh_timer_handler/sock_wfree Eric Dumazet <edumazet@google.com> - 2017-03-02 06:50 +0100

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