()

On Sat, Jan 14, 2023 at 1:06 PM Jason Xing [off-list ref] wrote:

On Sat, Jan 14, 2023 at 5:45 PM Eric Dumazet [off-list ref] wrote:

quoted

On Thu, Jan 12, 2023 at 7:54 AM Jason Xing [off-list ref] wrote:

quoted

From: Jason Xing <kernelxing@tencent.com>

While one cpu is working on looking up the right socket from ehash
table, another cpu is done deleting the request socket and is about
to add (or is adding) the big socket from the table. It means that
we could miss both of them, even though it has little chance.

Let me draw a call trace map of the server side.
   CPU 0                           CPU 1
   -----                           -----
tcp_v4_rcv()                  syn_recv_sock()
                            inet_ehash_insert()
                            -> sk_nulls_del_node_init_rcu(osk)
__inet_lookup_established()
                            -> __sk_nulls_add_node_rcu(sk, list)

Notice that the CPU 0 is receiving the data after the final ack
during 3-way shakehands and CPU 1 is still handling the final ack.

Why could this be a real problem?
This case is happening only when the final ack and the first data
receiving by different CPUs. Then the server receiving data with
ACK flag tries to search one proper established socket from ehash
table, but apparently it fails as my map shows above. After that,
the server fetches a listener socket and then sends a RST because
it finds a ACK flag in the skb (data), which obeys RST definition
in RFC 793.

Many thanks to Eric for great help from beginning to end.

Fixes: 5e0724d027f0 ("tcp/dccp: fix hashdance race for passive sessions")
Signed-off-by: Jason Xing <kernelxing@tencent.com>
---
 net/ipv4/inet_hashtables.c | 10 ++++++++++
 1 file changed, 10 insertions(+)

diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
index 24a38b56fab9..18f88cb4efcb 100644
--- a/net/ipv4/inet_hashtables.c
+++ b/net/ipv4/inet_hashtables.c

@@ -650,7 +650,16 @@ bool inet_ehash_insert(struct sock *sk, struct sock *osk, bool *found_dup_sk)
        spin_lock(lock);
        if (osk) {
                WARN_ON_ONCE(sk->sk_hash != osk->sk_hash);
+               if (sk_hashed(osk))
+                       /* Before deleting the node, we insert a new one to make
+                        * sure that the look-up=sk process would not miss either
+                        * of them and that at least one node would exist in ehash
+                        * table all the time. Otherwise there's a tiny chance
+                        * that lookup process could find nothing in ehash table.
+                        */
+                       __sk_nulls_add_node_rcu(sk, list);

In our private email exchange, I suggested to insert sk at the _tail_
of the hash bucket.

Yes, I noticed that. At that time I kept considering the race
condition of the RCU itself, not the scene you mentioned as below.

quoted

Inserting it at the _head_ would still leave a race condition, because
a concurrent reader might
have already started the bucket traversal, and would not see 'sk'.

Thanks for the detailed explanation. Now I see why. I'll replace it
with __sk_nulls_add_node_tail_rcu() function and send the v2 patch.

By the way, I checked the removal of TIMEWAIT socket which is included
in this patch.
I write down the call-trace:
inet_hash_connect()
    -> __inet_hash_connect()
        -> if (sk_unhashed(sk)) {
                inet_ehash_nolisten(sk, (struct sock *)tw, NULL);
                    -> inet_ehash_insert(sk, osk, found_dup_sk);
Therefore, this patch covers the timewait case.

This is the path handling the TIME_WAIT ---> ESTABLISH case.

I was referring to the more common opposite case which is the case
where a race could possibly happen.

This is inet_twsk_hashdance, and I suspect we want something like:

diff --git a/net/ipv4/inet_timewait_sock.c b/net/ipv4/inet_timewait_sock.c
index 1d77d992e6e77f7d96bd061be6dbb802c2566b3f..6d681ef52bb24b984a9dbda25b19291fc4393914

100644

--- a/net/ipv4/inet_timewait_sock.c
+++ b/net/ipv4/inet_timewait_sock.c

@@ -91,10 +91,10 @@ void inet_twsk_put(struct inet_timewait_sock *tw)
 }
 EXPORT_SYMBOL_GPL(inet_twsk_put);

-static void inet_twsk_add_node_rcu(struct inet_timewait_sock *tw,
+static void inet_twsk_add_node_tail_rcu(struct inet_timewait_sock *tw,
                                   struct hlist_nulls_head *list)
 {
-       hlist_nulls_add_head_rcu(&tw->tw_node, list);
+       hlist_nulls_add_tail_rcu(&tw->tw_node, list);
 }

 static void inet_twsk_add_bind_node(struct inet_timewait_sock *tw,

@@ -147,7 +147,7 @@ void inet_twsk_hashdance(struct inet_timewait_sock

*tw, struct sock *sk,

        spin_lock(lock);

-       inet_twsk_add_node_rcu(tw, &ehead->chain);
+       inet_twsk_add_node_tail_rcu(tw, &ehead->chain);

        /* Step 3: Remove SK from hash chain */
        if (__sk_nulls_del_node_init_rcu(sk))