netdev - Re: [PATCH net] tcp: md5: add missing memory barriers in tcp_md5_do_add()/tcp_md5_hash

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Message-ID: <240687586.18247.1593564005555.JavaMail.zimbra@efficios.com>
Date:   Tue, 30 Jun 2020 20:40:05 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>
Subject: Re: [PATCH net] tcp: md5: add missing memory barriers in
 tcp_md5_do_add()/tcp_md5_hash_key()

----- On Jun 30, 2020, at 8:34 PM, Eric Dumazet edumazet@...gle.com wrote:

> On Tue, Jun 30, 2020 at 5:27 PM Mathieu Desnoyers
> <mathieu.desnoyers@...icios.com> wrote:
>>
>> ----- On Jun 30, 2020, at 7:50 PM, Eric Dumazet edumazet@...gle.com wrote:
>>
>> > On Tue, Jun 30, 2020 at 4:47 PM Mathieu Desnoyers
>> > <mathieu.desnoyers@...icios.com> wrote:
>> >>
>> >> ----- On Jun 30, 2020, at 7:41 PM, Eric Dumazet edumazet@...gle.com wrote:
>> >>
>> >> > MD5 keys are read with RCU protection, and tcp_md5_do_add()
>> >> > might update in-place a prior key.
>> >> >
>> >> > Normally, typical RCU updates would allocate a new piece
>> >> > of memory. In this case only key->key and key->keylen might
>> >> > be updated, and we do not care if an incoming packet could
>> >> > see the old key, the new one, or some intermediate value,
>> >> > since changing the key on a live flow is known to be problematic
>> >> > anyway.
>> >>
>> >> What makes it acceptable to observe an intermediate bogus key during the
>> >> transition ?
>> >
>> > If you change a key while packets are in flight, the result is that :
>> >
>> > 1) Either your packet has the correct key and is handled
>> >
>> > 2) Or the key do not match, packet is dropped.
>> >
>> > Sender will retransmit eventually.
>>
>> This train of thoughts seem to apply to incoming traffic, what about outgoing ?
> 
> 
> Outgoing path is protected by the socket lock.
> 
> You can not change the TCP MD5 key while xmit is in progress.

Allright, this is the part I missed, thanks!

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com