netdev - Re: [PATCH] net: sched: crash on blocks with goto chain action

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Date:   Tue, 21 Nov 2017 11:31:49 -0800
From:   Cong Wang <xiyou.wangcong@...il.com>
To:     Roman Kapl <code@...pl.cz>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH] net: sched: crash on blocks with goto chain action

On Mon, Nov 20, 2017 at 1:41 PM, Roman Kapl <code@...pl.cz> wrote:
> On 11/20/2017 06:54 PM, Cong Wang wrote:
>>
>> On Sun, Nov 19, 2017 at 8:17 AM, Roman Kapl <code@...pl.cz> wrote:
>>>
>>> tcf_block_put_ext has assumed that all filters (and thus their goto
>>> actions) are destroyed in RCU callback and thus can not race with our
>>> list iteration. However, that is not true during netns cleanup (see
>>> tcf_exts_get_net comment).
>>>
>>> Prevent the user after free by holding the current list element we are
>>> iterating over (foreach_safe is not enough).
>>
>> Hmm...
>>
>> Looks like we need to restore the trick we used previously, that is
>> holding refcnt for all list entries before this list iteration.
>
>
> Was there a reason to hold all list entries in that trick? I thought that
> holding just the current element will be enough, but maybe not.
>

Yes, let me quote Jiri's explanation:

"
The reason for the hold above was to avoid use after free in this loop.
Consider following example:

chain1
  1 filter with action goto_chain 2
chain2
  empty

Now in your list_for_each_entry_safe loop, chain1 is flushed, action is
removed and chain is put:
tcf_action_goto_chain_fini->tcf_chain_put(2)

Given the fact chain2 is empty, this put would lead to tcf_chain_destroy(2)

Then in another iteration of list_for_each_entry_safe you are using
already freed chain.
"