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From:   Eric Dumazet <edumazet@google.com>
Date:   Tue, 30 Jun 2020 19:30:43 -0700
Message-ID: <CANn89iLKZQAtpejcLHmOu3dsrGf5eyFfHc8JqoMNYisRPWQ8kQ@mail.gmail.com>
Subject: Re: [regression] TCP_MD5SIG on established sockets
To:     Herbert Xu <herbert@gondor.apana.org.au>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        David Miller <davem@davemloft.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        LKML <linux-kernel@vger.kernel.org>,
        netdev <netdev@vger.kernel.org>,
        Yuchung Cheng <ycheng@google.com>,
        Jonathan Rajotte-Julien <joraj@efficios.com>
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On Tue, Jun 30, 2020 at 7:23 PM Herbert Xu <herbert@gondor.apana.org.au> wrote:
>
> On Tue, Jun 30, 2020 at 07:17:46PM -0700, Eric Dumazet wrote:
> >
> > The main issue of the prior code was the double read of key->keylen in
> > tcp_md5_hash_key(), not that few bytes could change under us.
> >
> > I used smp_rmb() to ease backports, since old kernels had no
> > READ_ONCE()/WRITE_ONCE(), but ACCESS_ONCE() instead.
>
> If it's the double-read that you're protecting against, you should
> just use barrier() and the comment should say so too.

I made this clear in the changelog, do we want comments all over the places ?
Do not get me wrong, we had this bug for years and suddenly this is a
big deal...

    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.

    We only want to make sure that in the case key->keylen
    is changed, cpus in tcp_md5_hash_key() wont try to use
    uninitialized data, or crash because key->keylen was
    read twice to feed sg_init_one() and ahash_request_set_crypt()