Re: [PATCH] net: revert "align SO_RCVMARK required privileges with SO_MARK"

[Paolo Abeni <pabeni@redhat.com>]

On Mon, 2023-06-05 at 01:12 -0700, Maciej Żenczykowski wrote:
> This reverts:
>     commit 1f86123b97491cc2b5071d7f9933f0e91890c976
>     net: align SO_RCVMARK required privileges with SO_MARK
> 
>     The commit referenced in the "Fixes" tag added the SO_RCVMARK socket
>     option for receiving the skb mark in the ancillary data.
> 
>     Since this is a new capability, and exposes admin configured details
>     regarding the underlying network setup to sockets, let's align the
>     needed capabilities with those of SO_MARK.
> 
> This reasoning is not really correct:
>   SO_RCVMARK is used for 'reading' incoming skb mark (via cmsg), as such
>   it is more equivalent to 'getsockopt(SO_MARK)' which has no priv check
>   and retrieves the socket mark, rather than 'setsockopt(SO_MARK) which
>   sets the socket mark and does require privs.
> 
>   Additionally incoming skb->mark may already be visible if
>   sysctl_fwmark_reflect and/or sysctl_tcp_fwmark_accept are enabled.
> 
>   Furthermore, it is easier to block the getsockopt via bpf
>   (either cgroup setsockopt hook, or via syscall filters)
>   then to unblock it if it requires CAP_NET_RAW/ADMIN.
> 
> On Android the socket mark is (among other things) used to store
> the network identifier a socket is bound to.  Setting it is privileged,
> but retrieving it is not.  We'd like unprivileged userspace to be able
> to read the network id of incoming packets (where mark is set via iptables
> [to be moved to bpf])...
> 
> An alternative would be to add another sysctl to control whether
> setting SO_RCVMARK is privilged or not.
> (or even a MASK of which bits in the mark can be exposed)
> But this seems like over-engineering...
> 
> Note: This is a non-trivial revert, due to later merged:
>   commit e42c7beee71d0d84a6193357e3525d0cf2a3e168
>   bpf: net: Consider has_current_bpf_ctx() when testing capable() in sk_setsockopt()

When you repost, please additionally change the above with the usual
commit reference, e.g. commit <12# hash> ("<title>")

> which changed both 'ns_capable' into 'sockopt_ns_capable' calls.
> 
> Fixes: 1f86123b9749 ("align SO_RCVMARK required privileges with SO_MARK")
> Cc: Eyal Birger <eyal.birger@gmail.com>
> Cc: Jakub Kicinski <kuba@kernel.org>
> Cc: Eric Dumazet <edumazet@google.com>
> Cc: Patrick Rohr <prohr@google.com>
> Signed-off-by: Maciej Żenczykowski <maze@google.com>
> ---
>  net/core/sock.c | 6 ------
>  1 file changed, 6 deletions(-)
> 
> diff --git a/net/core/sock.c b/net/core/sock.c
> index 24f2761bdb1d..6e5662ca00fe 100644
> --- a/net/core/sock.c
> +++ b/net/core/sock.c
> @@ -1362,12 +1362,6 @@ int sk_setsockopt(struct sock *sk, int level, int optname,
>  		__sock_set_mark(sk, val);
>  		break;
>  	case SO_RCVMARK:
> -		if (!sockopt_ns_capable(sock_net(sk)->user_ns, CAP_NET_RAW) &&
> -		    !sockopt_ns_capable(sock_net(sk)->user_ns, CAP_NET_ADMIN)) {
> -			ret = -EPERM;
> -			break;
> -		}
> -
>  		sock_valbool_flag(sk, SOCK_RCVMARK, valbool);
>  		break;
>