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Date:   Sun, 24 May 2020 11:14:41 -0700
From:   Maciej ┼╗enczykowski <zenczykowski@...il.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     "David S . Miller" <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Willem de Bruijn <willemb@...gle.com>
Subject: Re: [PATCH net-next] tcp: allow traceroute -Mtcp for unpriv users

On Sun, May 24, 2020 at 11:00 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> Unpriv users can use traceroute over plain UDP sockets, but not TCP ones.
>
> $ traceroute -Mtcp 8.8.8.8
> You do not have enough privileges to use this traceroute method.
>
> $ traceroute -n -Mudp 8.8.8.8
> traceroute to 8.8.8.8 (8.8.8.8), 30 hops max, 60 byte packets
>  1  192.168.86.1  3.631 ms  3.512 ms  3.405 ms
>  2  10.1.10.1  4.183 ms  4.125 ms  4.072 ms
>  3  96.120.88.125  20.621 ms  19.462 ms  20.553 ms
>  4  96.110.177.65  24.271 ms  25.351 ms  25.250 ms
>  5  69.139.199.197  44.492 ms  43.075 ms  44.346 ms
>  6  68.86.143.93  27.969 ms  25.184 ms  25.092 ms
>  7  96.112.146.18  25.323 ms 96.112.146.22  25.583 ms 96.112.146.26  24.502 ms
>  8  72.14.239.204  24.405 ms 74.125.37.224  16.326 ms  17.194 ms
>  9  209.85.251.9  18.154 ms 209.85.247.55  14.449 ms 209.85.251.9  26.296 ms^C
>
> We can easily support traceroute over TCP, by queueing an error message
> into socket error queue.
>
> Note that applications need to set IP_RECVERR/IPV6_RECVERR option to
> enable this feature, and that the error message is only queued
> while in SYN_SNT state.
>
> socket(AF_INET6, SOCK_STREAM, IPPROTO_IP) = 3
> setsockopt(3, SOL_IPV6, IPV6_RECVERR, [1], 4) = 0
> setsockopt(3, SOL_SOCKET, SO_TIMESTAMP_OLD, [1], 4) = 0
> setsockopt(3, SOL_IPV6, IPV6_UNICAST_HOPS, [5], 4) = 0
> connect(3, {sa_family=AF_INET6, sin6_port=htons(8787), sin6_flowinfo=htonl(0),
>         inet_pton(AF_INET6, "2002:a05:6608:297::", &sin6_addr), sin6_scope_id=0}, 28) = -1 EHOSTUNREACH (No route to host)
> recvmsg(3, {msg_name={sa_family=AF_INET6, sin6_port=htons(8787), sin6_flowinfo=htonl(0),
>         inet_pton(AF_INET6, "2002:a05:6608:297::", &sin6_addr), sin6_scope_id=0},
>         msg_namelen=1024->28, msg_iov=[{iov_base="`\r\337\320\0004\6\1&\7\370\260\200\231\16\27\0\0\0\0\0\0\0\0 \2\n\5f\10\2\227"..., iov_len=1024}],
>         msg_iovlen=1, msg_control=[{cmsg_len=32, cmsg_level=SOL_SOCKET, cmsg_type=SO_TIMESTAMP_OLD, cmsg_data={tv_sec=1590340680, tv_usec=272424}},
>                                    {cmsg_len=60, cmsg_level=SOL_IPV6, cmsg_type=IPV6_RECVERR}],
>         msg_controllen=96, msg_flags=MSG_ERRQUEUE}, MSG_ERRQUEUE) = 144
>
> Suggested-by: Maciej ┼╗enczykowski <maze@...gle.com
> Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> Cc: Willem de Bruijn <willemb@...gle.com>
> ---
>  net/ipv4/tcp_ipv4.c | 2 ++
>  net/ipv6/tcp_ipv6.c | 2 ++
>  2 files changed, 4 insertions(+)
>
> diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c
> index 6c05f1ceb538cbb9981835440163485de2ccf716..900c6d154cbcf04fb09d71f1445d0723bcf3c409 100644
> --- a/net/ipv4/tcp_ipv4.c
> +++ b/net/ipv4/tcp_ipv4.c
> @@ -573,6 +573,8 @@ int tcp_v4_err(struct sk_buff *icmp_skb, u32 info)
>                 if (fastopen && !fastopen->sk)
>                         break;
>
> +               ip_icmp_error(sk, skb, err, th->dest, info, (u8 *)th);
> +
>                 if (!sock_owned_by_user(sk)) {
>                         sk->sk_err = err;
>
> diff --git a/net/ipv6/tcp_ipv6.c b/net/ipv6/tcp_ipv6.c
> index 413b3425ac66bd758bb83562efa955f277da90a5..01a6f5111a77b4397038bf4d40cc09a94e57408c 100644
> --- a/net/ipv6/tcp_ipv6.c
> +++ b/net/ipv6/tcp_ipv6.c
> @@ -463,6 +463,8 @@ static int tcp_v6_err(struct sk_buff *skb, struct inet6_skb_parm *opt,
>                 if (fastopen && !fastopen->sk)
>                         break;
>
> +               ipv6_icmp_error(sk, skb, err, th->dest, ntohl(info), (u8 *)th);
> +
>                 if (!sock_owned_by_user(sk)) {
>                         sk->sk_err = err;
>                         sk->sk_error_report(sk);                /* Wake people up to see the error (see connect in sock.c) */
> --
> 2.27.0.rc0.183.gde8f92d652-goog

Thanks!  I was not expecting it to be this easy...

Reviewed-by: Maciej ┼╗enczykowski <maze@...gle.com>

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