netdev - Re: [regression] TCP_MD5SIG on established sockets

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Date:   Tue, 30 Jun 2020 20:36:51 -0700
From:   Eric Dumazet <edumazet@...gle.com>
To:     Herbert Xu <herbert@...dor.apana.org.au>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        David Miller <davem@...emloft.net>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        LKML <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 7:59 PM Herbert Xu <herbert@...dor.apana.org.au> wrote:
>
> On Tue, Jun 30, 2020 at 07:30:43PM -0700, Eric Dumazet wrote:
> >
> > 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...
>
> I thought you were adding a new pair of smp_rmb/smp_wmb.  If they
> already exist in the code then I agree it's not a big deal.  But
> adding a new pair of bogus smp_Xmb's is bad for maintenance.
>

If I knew so many people were excited about TCP / MD5, I would have
posted all my patches on lkml ;)

Without the smp_wmb() we would still need something to prevent KMSAN
from detecting that we read uninitialized bytes,
if key->keylen is increased.  (initial content of key->key[] is garbage)

Something like this :

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index f111660453241692a17c881dd6dc2910a1236263..c3af8180c7049d5c4987bf5c67e4aff2ed6967c9
100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -4033,11 +4033,9 @@ EXPORT_SYMBOL(tcp_md5_hash_skb_data);

 int tcp_md5_hash_key(struct tcp_md5sig_pool *hp, const struct
tcp_md5sig_key *key)
 {
-       u8 keylen = key->keylen;
+       u8 keylen = READ_ONCE(key->keylen); /* paired with
WRITE_ONCE() in tcp_md5_do_add */
        struct scatterlist sg;

-       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);
diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
index 99916fcc15ca0be12c2c133ff40516f79e6fdf7f..0d08e0134335a21d23702e6a5c24a0f2b3c61c6f
100644
--- a/net/ipv4/tcp_ipv4.c
+++ b/net/ipv4/tcp_ipv4.c
@@ -1114,9 +1114,13 @@ int tcp_md5_do_add(struct sock *sk, const union
tcp_md5_addr *addr,
                /* Pre-existing entry - just update that one. */
                memcpy(key->key, newkey, newkeylen);

-               smp_wmb(); /* pairs with smp_rmb() in tcp_md5_hash_key() */
+               /* Pairs with READ_ONCE() in tcp_md5_hash_key().
+                * Also note that a reader could catch new key->keylen value
+                * but old key->key[], this is the reason we use __GFP_ZERO
+                * at sock_kmalloc() time below these lines.
+                */
+               WRITE_ONCE(key->keylen, newkeylen);

-               key->keylen = newkeylen;
                return 0;
        }

@@ -1132,7 +1136,7 @@ int tcp_md5_do_add(struct sock *sk, const union
tcp_md5_addr *addr,
                rcu_assign_pointer(tp->md5sig_info, md5sig);
        }

-       key = sock_kmalloc(sk, sizeof(*key), gfp);
+       key = sock_kmalloc(sk, sizeof(*key), gfp | __GFP_ZERO);
        if (!key)
                return -ENOMEM;
        if (!tcp_alloc_md5sig_pool()) {