netdev - Re: [PATCH net-next v2] net: core: enable SO

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Date:   Thu, 2 Apr 2020 11:31:03 -0600
From:   David Ahern <dsahern@...il.com>
To:     Vincent Bernat <vincent@...nat.ch>, netdev@...r.kernel.org,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        Martin KaFai Lau <kafai@...com>
Subject: Re: [PATCH net-next v2] net: core: enable SO_BINDTODEVICE for
 non-root users

On 3/31/20 7:20 AM, Vincent Bernat wrote:
> Currently, SO_BINDTODEVICE requires CAP_NET_RAW. This change allows a
> non-root user to bind a socket to an interface if it is not already
> bound. This is useful to allow an application to bind itself to a
> specific VRF for outgoing or incoming connections. Currently, an
> application wanting to manage connections through several VRF need to
> be privileged.
> 
> Previously, IP_UNICAST_IF and IPV6_UNICAST_IF were added for
> Wine (76e21053b5bf3 and c4062dfc425e9) specifically for use by
> non-root processes. However, they are restricted to sendmsg() and not
> usable with TCP. Allowing SO_BINDTODEVICE would allow TCP clients to
> get the same privilege. As for TCP servers, outside the VRF use case,
> SO_BINDTODEVICE would only further restrict connections a server could
> accept.
> 
> When an application is restricted to a VRF (with `ip vrf exec`), the
> socket is bound to an interface at creation and therefore, a
> non-privileged call to SO_BINDTODEVICE to escape the VRF fails.
> 
> When an application bound a socket to SO_BINDTODEVICE and transmit it
> to a non-privileged process through a Unix socket, a tentative to
> change the bound device also fails.
> 
> Before:
> 
>     >>> import socket
>     >>> s=socket.socket(socket.AF_INET, socket.SOCK_STREAM)
>     >>> s.setsockopt(socket.SOL_SOCKET, socket.SO_BINDTODEVICE, b"dummy0")
>     Traceback (most recent call last):
>       File "<stdin>", line 1, in <module>
>     PermissionError: [Errno 1] Operation not permitted
> 
> After:
> 
>     >>> import socket
>     >>> s=socket.socket(socket.AF_INET, socket.SOCK_STREAM)
>     >>> s.setsockopt(socket.SOL_SOCKET, socket.SO_BINDTODEVICE, b"dummy0")
>     >>> s.setsockopt(socket.SOL_SOCKET, socket.SO_BINDTODEVICE, b"dummy0")
>     Traceback (most recent call last):
>       File "<stdin>", line 1, in <module>
>     PermissionError: [Errno 1] Operation not permitted
> 
> Signed-off-by: Vincent Bernat <vincent@...nat.ch>
> ---
>  net/core/sock.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/net/core/sock.c b/net/core/sock.c
> index da32d9b6d09f..ce1d8dce9b7a 100644
> --- a/net/core/sock.c
> +++ b/net/core/sock.c
> @@ -574,7 +574,7 @@ static int sock_setbindtodevice_locked(struct sock *sk, int ifindex)
>  
>  	/* Sorry... */
>  	ret = -EPERM;
> -	if (!ns_capable(net->user_ns, CAP_NET_RAW))
> +	if (sk->sk_bound_dev_if && !ns_capable(net->user_ns, CAP_NET_RAW))
>  		goto out;
>  
>  	ret = -EINVAL;
> 


Reviewed-by: David Ahern <dsahern@...il.com>