Re: [regression] TCP_MD5SIG on established sockets

[Eric Dumazet <edumazet@google.com>]

On Tue, Jun 30, 2020 at 7:02 PM Herbert Xu <herbert@gondor.apana.org.au> wrote:
>
> Eric Dumazet <edumazet@google.com> wrote:
> >
> > diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
> > index 810cc164f795f8e1e8ca747ed5df51bb20fec8a2..ecc0e3fabce8b03bef823cbfc5c1b0a9e24df124
> > 100644
> > --- a/net/ipv4/tcp.c
> > +++ b/net/ipv4/tcp.c
> > @@ -4034,9 +4034,12 @@ EXPORT_SYMBOL(tcp_md5_hash_skb_data);
> > int tcp_md5_hash_key(struct tcp_md5sig_pool *hp, const struct
> > tcp_md5sig_key *key)
> > {
> >        struct scatterlist sg;
> > +       u8 keylen = key->keylen;
> >
> > -       sg_init_one(&sg, key->key, key->keylen);
> > -       ahash_request_set_crypt(hp->md5_req, &sg, NULL, key->keylen);
> > +       smp_rmb(); /* paired with smp_wmb() in tcp_md5_do_add() */
> > +
> > +       sg_init_one(&sg, key->key, keylen);
> > +       ahash_request_set_crypt(hp->md5_req, &sg, NULL, keylen);
> >        return crypto_ahash_update(hp->md5_req);
> > }
> > EXPORT_SYMBOL(tcp_md5_hash_key);
> > diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
> > index ad6435ba6d72ffd8caf783bb25cad7ec151d6909..99916fcc15ca0be12c2c133ff40516f79e6fdf7f
> > 100644
> > --- a/net/ipv4/tcp_ipv4.c
> > +++ b/net/ipv4/tcp_ipv4.c
> > @@ -1113,6 +1113,9 @@ int tcp_md5_do_add(struct sock *sk, const union
> > tcp_md5_addr *addr,
> >        if (key) {
> >                /* Pre-existing entry - just update that one. */
> >                memcpy(key->key, newkey, newkeylen);
> > +
> > +               smp_wmb(); /* pairs with smp_rmb() in tcp_md5_hash_key() */
> > +
> >                key->keylen = newkeylen;
> >                return 0;
> >        }
>
> This doesn't make sense.  Your smp_rmb only guarantees that you
> see a version of key->key that's newer than keylen.  What if the
> key got changed twice? You could still read more than what's in
> the key (if that's what you're trying to protect against).

The worst that can happen is a dropped packet.

Which is anyway going to happen anyway eventually if an application
changes a TCP MD5 key on a live flow.

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.