netdev - Re: net: do not allow changing SO_REUSEADDR/SO

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Date:   Mon, 11 Jun 2018 11:57:49 -0700
From:   Andrei Vagin <avagin@...tuozzo.com>
To:     Maciej Żenczykowski <zenczykowski@...il.com>
Cc:     Maciej Żenczykowski <maze@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        Eric Dumazet <edumazet@...gle.com>, netdev@...r.kernel.org
Subject: Re: net: do not allow changing SO_REUSEADDR/SO_REUSEPORT on bound
 sockets

On Sun, Jun 03, 2018 at 10:47:05AM -0700, Maciej Żenczykowski wrote:
> From: Maciej Żenczykowski <maze@...gle.com>
> 
> It is not safe to do so because such sockets are already in the
> hash tables and changing these options can result in invalidating
> the tb->fastreuse(port) caching.
> 
> This can have later far reaching consequences wrt. bind conflict checks
> which rely on these caches (for optimization purposes).
> 
> Not to mention that you can currently end up with two identical
> non-reuseport listening sockets bound to the same local ip:port
> by clearing reuseport on them after they've already both been bound.
> 
> There is unfortunately no EISBOUND error or anything similar,
> and EISCONN seems to be misleading for a bound-but-not-connected
> socket, so use EUCLEAN 'Structure needs cleaning' which AFAICT
> is the closest you can get to meaning 'socket in bad state'.
> (although perhaps EINVAL wouldn't be a bad choice either?)
> 
> This does unfortunately run the risk of breaking buggy
> userspace programs...
> 
> Signed-off-by: Maciej Żenczykowski <maze@...gle.com>
> Cc: Eric Dumazet <edumazet@...gle.com>
> 
> Change-Id: I77c2b3429b2fdf42671eee0fa7a8ba721c94963b
> Reviewed-by: Eric Dumazet <edumazet@...gle.com>
> ---
>  net/core/sock.c | 15 ++++++++++++++-
>  1 file changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/net/core/sock.c b/net/core/sock.c
> index 435a0ba85e52..feca4c98f8a0 100644
> --- a/net/core/sock.c
> +++ b/net/core/sock.c
> @@ -728,9 +728,22 @@ int sock_setsockopt(struct socket *sock, int level, int optname,
>  			sock_valbool_flag(sk, SOCK_DBG, valbool);
>  		break;
>  	case SO_REUSEADDR:
> -		sk->sk_reuse = (valbool ? SK_CAN_REUSE : SK_NO_REUSE);
> +		val = (valbool ? SK_CAN_REUSE : SK_NO_REUSE);
> +		if ((sk->sk_family == PF_INET || sk->sk_family == PF_INET6) &&
> +		    inet_sk(sk)->inet_num &&
> +		    (sk->sk_reuse != val)) {
> +			ret = (sk->sk_state == TCP_ESTABLISHED) ? -EISCONN : -EUCLEAN;

There are a few more states like TCP_LAST_ACK, TCP_CLOSE_WAIT which
means that a socket is connected.

Actually, I don't see any reasons to return two different values here.

> +			break;
> +		}
> +		sk->sk_reuse = val;
>  		break;
>  	case SO_REUSEPORT:
> +		if ((sk->sk_family == PF_INET || sk->sk_family == PF_INET6) &&
> +		    inet_sk(sk)->inet_num &&
> +		    (sk->sk_reuseport != valbool)) {
> +			ret = (sk->sk_state == TCP_ESTABLISHED) ? -EISCONN : -EUCLEAN;
> +			break;
> +		}
>  		sk->sk_reuseport = valbool;
>  		break;
>  	case SO_TYPE: