netdev - Re: [PATCH net] net: bridge: validate the NUD_PERMANENT bit when adding an extern

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Message-ID: <c6d91ec7-6081-85d0-9bc8-569838d8f9a4@nvidia.com>
Date:   Tue, 10 Aug 2021 13:15:32 +0300
From:   Nikolay Aleksandrov <nikolay@...dia.com>
To:     Vladimir Oltean <vladimir.oltean@....com>,
        Ido Schimmel <idosch@...sch.org>
Cc:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Jakub Kicinski <kuba@...nel.org>,
        "David S. Miller" <davem@...emloft.net>,
        Jiri Pirko <jiri@...nulli.us>, Roopa Prabhu <roopa@...dia.com>,
        "bridge@...ts.linux-foundation.org" 
        <bridge@...ts.linux-foundation.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        "syzbot+9ba1174359adba5a5b7c@...kaller.appspotmail.com" 
        <syzbot+9ba1174359adba5a5b7c@...kaller.appspotmail.com>
Subject: Re: [PATCH net] net: bridge: validate the NUD_PERMANENT bit when
 adding an extern_learn FDB entry

On 10/08/2021 13:09, Vladimir Oltean wrote:
> On Tue, Aug 10, 2021 at 09:46:34AM +0300, Ido Schimmel wrote:
>> On Mon, Aug 09, 2021 at 04:05:22PM +0000, Vladimir Oltean wrote:
>>> On Mon, Aug 09, 2021 at 03:16:40PM +0300, Ido Schimmel wrote:
>>>> I have at least once selftest where I forgot the 'static' keyword:
>>>>
>>>> bridge fdb add de:ad:be:ef:13:37 dev $swp1 master extern_learn vlan 1
>>>>
>>>> This patch breaks the test when run against both the kernel and hardware
>>>> data paths. I don't mind patching these tests, but we might get more
>>>> reports in the future.
>>>
>>> Is it the 'static' keyword that you forgot, or 'dynamic'? The
>>> tools/testing/selftests/net/forwarding/bridge_vlan_aware.sh selftest
>>> looks to me like it's testing the behavior of an FDB entry which should
>>> roam, and which without the extern_learn flag would be ageable.
>>
>> static - no aging, no roaming
>> dynamic - aging, roaming
>> extern_learn - no aging, roaming
>>
>> So these combinations do not make any sense and the kernel will ignore
>> static/dynamic when extern_learn is specified. It's needed to work
>> around iproute2 behavior of "assume permanent"
> 
> Since NTF_EXT_LEARNED is part of ndm->ndm_flags and NUD_REACHABLE/NUD_NOARP
> are part of ndm->ndm_state, it is not at all clear to me that 'extern_learn'
> belongs to the same class of bridge neighbor attributes as 'static'/'dynamic',
> and that it is invalid to have the full degree of freedom. If it isn't,
> shouldn't the kernel validate that, instead of just ignoring the ndm->ndm_state?
> If it's too late to validate, shouldn't we at least document somewhere
> that the ndm_state is ignored in the presence of ndm_flags & NTF_EXT_LEARNED?
> It is user API after all, easter eggs like this aren't very enjoyable.
> 

It's too late unfortunately, ignoring other flags in that case has been the standard
behaviour for a long time (it has never made sense to specify flags for extern_learn
entries). I'll send a separate patch that adds a comment to document it or if you have
another thing in mind feel free to send a patch.

Thanks,
 Nik