Re: [PATCH v5 net] tcp: avoid the lookup process failing to get sk in ehash table

[Kuniyuki Iwashima]

From:   Jason Xing <kerneljasonxing@xxxxxxxxx>
Date:   Mon, 16 Jan 2023 18:33:41 +0800
> From: Jason Xing <kernelxing@xxxxxxxxxxx>
> 
> 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.
> 
> Besides, Eric pointed out there's one more race condition where it
> handles tw socket hashdance. Only by adding to the tail of the list
> before deleting the old one can we avoid the race if the reader has
> already begun the bucket traversal and it would possibly miss the head.
> 
> Many thanks to Eric for great help from beginning to end.
> 
> Fixes: 5e0724d027f0 ("tcp/dccp: fix hashdance race for passive sessions")
> Suggested-by: Eric Dumazet <edumazet@xxxxxxxxxx>
> Signed-off-by: Jason Xing <kernelxing@xxxxxxxxxxx>
> Link: https://lore.kernel.org/lkml/20230112065336.41034-1-kerneljasonxing@xxxxxxxxx/

I guess there could be regression if a workload has many long-lived
connections, but the change itself looks good.  I left a minor comment
below.

Reviewed-by: Kuniyuki Iwashima <kuniyu@xxxxxxxxxx>


> ---
> v5:
> 1) adjust the style once more.
> 
> v4:
> 1) adjust the code style and make it easier to read.
> 
> v3:
> 1) get rid of else-if statement.
> 
> v2:
> 1) adding the sk node into the tail of list to prevent the race.
> 2) fix the race condition when handling time-wait socket hashdance.
> ---
>  net/ipv4/inet_hashtables.c    | 17 +++++++++++++++--
>  net/ipv4/inet_timewait_sock.c |  6 +++---
>  2 files changed, 18 insertions(+), 5 deletions(-)
> 
> diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
> index 24a38b56fab9..f58d73888638 100644
> --- a/net/ipv4/inet_hashtables.c
> +++ b/net/ipv4/inet_hashtables.c
> @@ -650,8 +650,20 @@ 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);
> -		ret = sk_nulls_del_node_init_rcu(osk);
> -	} else if (found_dup_sk) {
> +		ret = sk_hashed(osk);
> +		if (ret) {
> +			/* 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_tail_rcu(sk, list);
> +			sk_nulls_del_node_init_rcu(osk);
> +		}
> +		goto unlock;
> +	}
> +	if (found_dup_sk) {
>  		*found_dup_sk = inet_ehash_lookup_by_sk(sk, list);
>  		if (*found_dup_sk)
>  			ret = false;
> @@ -660,6 +672,7 @@ bool inet_ehash_insert(struct sock *sk, struct sock *osk, bool *found_dup_sk)
>  	if (ret)
>  		__sk_nulls_add_node_rcu(sk, list);
>  
> +unlock:
>  	spin_unlock(lock);
>  
>  	return ret;
> diff --git a/net/ipv4/inet_timewait_sock.c b/net/ipv4/inet_timewait_sock.c
> index 1d77d992e6e7..6d681ef52bb2 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)

nit: Please indent here.


>  {
> -	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))
> -- 
> 2.37.3