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

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Date: Wed, 8 Nov 2023 21:58:07 +0100
From: Eric Dumazet <edumazet@...gle.com>
To: Stanislav Fomichev <sdf@...gle.com>
Cc: netdev@...r.kernel.org, davem@...emloft.net, kuba@...nel.org, 
	pabeni@...hat.com
Subject: Re: [PATCH net] net: set SOCK_RCU_FREE before inserting socket into hashtable

On Wed, Nov 8, 2023 at 9:28 PM Stanislav Fomichev <sdf@...gle.com> wrote:
>
> 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
>
> I'm not 100% what is causing them. It might be some kernel change or
> new code path in the bpf program. But looking at the code,
> I'm assuming the issue has been there for a while.
>
> __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 (so not 100% sure whether it should
> wait until net-next instead).
>
> For the fixes tag, I'm using the original commit which added the flag.

When this commit added the flag, precise location of the
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 tested only at dismantle time.

Back then BPF was not able yet to perform lookups, and double check if
SOCK_RCU_FREE
was set or not.

Checking SOCK_RCU_FREE _after_ the lookup to infer if a refcount has
been taken came
with commit 6acc9b432e67 ("bpf: Add helper to retrieve socket in BPF")

I think we can be more precise and help future debugging, in case more problems
need investigations.

Can you augment the changelog and use a different Fixes: tag ?

With that,

Reviewed-by: Eric Dumazet <edumazet@...gle.com>

>
> Fixes: 3b24d854cb35 ("tcp/dccp: do not touch listener sk_refcnt under synflood")
> Signed-off-by: Stanislav Fomichev <sdf@...gle.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
>