netdev - Re: [PATCH net v2] net: set SOCK_RCU_FREE before inserting socket into hashtable

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Message-ID: <20231108233009.29311-1-kuniyu@amazon.com>
Date: Wed, 8 Nov 2023 15:30:09 -0800
From: Kuniyuki Iwashima <kuniyu@...zon.com>
To: <sdf@...gle.com>
CC: <davem@...emloft.net>, <edumazet@...gle.com>, <kuba@...nel.org>,
	<netdev@...r.kernel.org>, <pabeni@...hat.com>, <kuniyu@...zon.com>
Subject: Re: [PATCH net v2] net: set SOCK_RCU_FREE before inserting socket into hashtable

From: Stanislav Fomichev <sdf@...gle.com>
Date: Wed,  8 Nov 2023 13:13:25 -0800
> We've started to see the following kernel traces:
> 
>  WARNING: CPU: 83 PID: 0 at net/core/filter.c:6641 sk_lookup+0x1bd/0x1d0
> 
>  Call Trace:
>   <IRQ>
>   __bpf_skc_lookup+0x10d/0x120
>   bpf_sk_lookup+0x48/0xd0
>   bpf_sk_lookup_tcp+0x19/0x20
>   bpf_prog_<redacted>+0x37c/0x16a3
>   cls_bpf_classify+0x205/0x2e0
>   tcf_classify+0x92/0x160
>   __netif_receive_skb_core+0xe52/0xf10
>   __netif_receive_skb_list_core+0x96/0x2b0
>   napi_complete_done+0x7b5/0xb70
>   <redacted>_poll+0x94/0xb0
>   net_rx_action+0x163/0x1d70
>   __do_softirq+0xdc/0x32e
>   asm_call_irq_on_stack+0x12/0x20
>   </IRQ>
>   do_softirq_own_stack+0x36/0x50
>   do_softirq+0x44/0x70
> 
> __inet_hash can race with lockless (rcu) readers on the other cpus:
> 
>   __inet_hash
>     __sk_nulls_add_node_rcu
>     <- (bpf triggers here)
>     sock_set_flag(SOCK_RCU_FREE)
> 
> Let's move the SOCK_RCU_FREE part up a bit, before we are inserting
> the socket into hashtables. Note, that the race is really harmless;
> the bpf callers are handling this situation (where listener socket
> doesn't have SOCK_RCU_FREE set) correctly, so the only
> annoyance is a WARN_ONCE.
> 
> More details from Eric regarding SOCK_RCU_FREE timeline:
> 
> Commit 3b24d854cb35 ("tcp/dccp: do not touch listener sk_refcnt under
> synflood") added SOCK_RCU_FREE. At that time, the precise location of
> sock_set_flag(sk, SOCK_RCU_FREE) did not matter, because the thread calling
> __inet_hash() owns a reference on sk. SOCK_RCU_FREE was only tested
> at dismantle time.
> 
> Commit 6acc9b432e67 ("bpf: Add helper to retrieve socket in BPF")
> started checking SOCK_RCU_FREE _after_ the lookup to infer whether
> the refcount has been taken care of.
> 
> Fixes: 6acc9b432e67 ("bpf: Add helper to retrieve socket in BPF")
> Reviewed-by: Eric Dumazet <edumazet@...gle.com>
> Signed-off-by: Stanislav Fomichev <sdf@...gle.com>

Reviewed-by: Kuniyuki Iwashima <kuniyu@...zon.com>


> ---
>  net/ipv4/inet_hashtables.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/ipv4/inet_hashtables.c b/net/ipv4/inet_hashtables.c
> index 598c1b114d2c..a532f749e477 100644
> --- a/net/ipv4/inet_hashtables.c
> +++ b/net/ipv4/inet_hashtables.c
> @@ -751,12 +751,12 @@ int __inet_hash(struct sock *sk, struct sock *osk)
>  		if (err)
>  			goto unlock;
>  	}
> +	sock_set_flag(sk, SOCK_RCU_FREE);
>  	if (IS_ENABLED(CONFIG_IPV6) && sk->sk_reuseport &&
>  		sk->sk_family == AF_INET6)
>  		__sk_nulls_add_node_tail_rcu(sk, &ilb2->nulls_head);
>  	else
>  		__sk_nulls_add_node_rcu(sk, &ilb2->nulls_head);
> -	sock_set_flag(sk, SOCK_RCU_FREE);
>  	sock_prot_inuse_add(sock_net(sk), sk->sk_prot, 1);
>  unlock:
>  	spin_unlock(&ilb2->lock);
> -- 
> 2.42.0.869.gea05f2083d-goog