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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Tue, 21 Nov 2017 21:02:04 +0100
From:   Roman Kapl <code@...pl.cz>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: [PATCH] net: sched: crash on blocks with goto chain action

On 11/21/2017 08:31 PM, Cong Wang wrote:
> 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
I believe the exact same example is part of the 'how to reproduce' part 
of commit and the patch helped me get rid of that crash.
>
> Now in your list_for_each_entry_safe loop,
Note that list_for_each_entry_safe was replaced by pure 
list_for_each_entry in my proposed patch.
> 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.
> "

No, I believe that the last iteration would simply stop, because at the 
point you reach second iteration, chain->next == head.

But maybe the "hold all chains" approach from 822e86d997 (net_sched: 
remove tcf_block_put_deferred())  is simpler to understand?