linux-kernel - Re: [regression] TCP_MD5SIG on established sockets

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Date:   Tue, 30 Jun 2020 15:38:31 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     "David S. Miller" <davem@...emloft.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        netdev <netdev@...r.kernel.org>,
        Yuchung Cheng <ycheng@...gle.com>,
        Jonathan Rajotte-Julien <joraj@...icios.com>
Subject: Re: [regression] TCP_MD5SIG on established sockets

On Tue, Jun 30, 2020 at 3:07 PM Eric Dumazet <edumazet@...gle.com> wrote:
>
> On Tue, Jun 30, 2020 at 2:54 PM Eric Dumazet <edumazet@...gle.com> wrote:
> >
> > On Tue, Jun 30, 2020 at 2:23 PM Eric Dumazet <edumazet@...gle.com> wrote:
> > >
> > > On Tue, Jun 30, 2020 at 2:17 PM Mathieu Desnoyers
> > > <mathieu.desnoyers@...icios.com> wrote:
> > > >
> > > > ----- On Jun 30, 2020, at 4:56 PM, Eric Dumazet edumazet@...gle.com wrote:
> > > >
> > > > > On Tue, Jun 30, 2020 at 1:44 PM David Miller <davem@...emloft.net> wrote:
> > > > >>
> > > > >> From: Eric Dumazet <edumazet@...gle.com>
> > > > >> Date: Tue, 30 Jun 2020 13:39:27 -0700
> > > > >>
> > > > >> > The (C) & (B) case are certainly doable.
> > > > >> >
> > > > >> > A) case is more complex, I have no idea of breakages of various TCP
> > > > >> > stacks if a flow got SACK
> > > > >> > at some point (in 3WHS) but suddenly becomes Reno.
> > > > >>
> > > > >> I agree that C and B are the easiest to implement without having to
> > > > >> add complicated code to handle various negotiated TCP option
> > > > >> scenerios.
> > > > >>
> > > > >> It does seem to be that some entities do A, or did I misread your
> > > > >> behavioral analysis of various implementations Mathieu?
> > > > >>
> > > > >> Thanks.
> > > > >
> > > > > Yes, another question about Mathieu cases is do determine the behavior
> > > > > of all these stacks vs :
> > > > > SACK option
> > > > > TCP TS option.
> > > >
> > > > I will ask my customer's networking team to investigate these behaviors,
> > > > which will allow me to prepare a thorough reply to the questions raised
> > > > by Eric and David. I expect to have an answer within 2-3 weeks at most.
> > > >
> > > > Thank you!
> > >
> > >
> > > Great, I am working on adding back support for (B) & (C) by the end of
> > > this week.
> >
> > Note that the security issue (of sending uninit bytes to the wire) has
> > been independently fixed with [1]
> >
> > This means syzbot was able to have MD5+TS+SACK  ~6 months ago.
> >
> > It seems we (linux) do not enable this combination for PASSIVE flows,
> > (according to tcp_synack_options()),
> > but  for ACTIVE flows we do nothing special.
> >
> > So maybe code in tcp_synack_options() should be mirrored to
> > tcp_syn_options() for consistency.
> > (disabling TS if  both MD5 and SACK are enabled)
>
> Oh well, tcp_syn_options() is supposed to have the same logic.
>
> Maybe we have an issue with SYNCOOKIES (with MD5 + TS + SACK)
>
> Nice can of worms.
>

For updates of keys, it seems existing code lacks some RCU care.

MD5 keys use RCU for lookups/hashes, but the replacement of a key does
not allocate a new piece of memory.

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;
        }