netdev - Re: [PATCH bpf-next v3 1/2] bpf: allow rewriting to ports under ip_unprivileged_port

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Date:   Tue, 26 Jan 2021 10:01:11 -0800
From:   Martin KaFai Lau <kafai@...com>
To:     Stanislav Fomichev <sdf@...gle.com>
CC:     <netdev@...r.kernel.org>, <bpf@...r.kernel.org>, <ast@...nel.org>,
        <daniel@...earbox.net>, Andrey Ignatov <rdna@...com>
Subject: Re: [PATCH bpf-next v3 1/2] bpf: allow rewriting to ports under
 ip_unprivileged_port_start

On Tue, Jan 26, 2021 at 08:51:03AM -0800, Stanislav Fomichev wrote:
> At the moment, BPF_CGROUP_INET{4,6}_BIND hooks can rewrite user_port
> to the privileged ones (< ip_unprivileged_port_start), but it will
> be rejected later on in the __inet_bind or __inet6_bind.
> 
> Let's add another return value to indicate that CAP_NET_BIND_SERVICE
> check should be ignored. Use the same idea as we currently use
> in cgroup/egress where bit #1 indicates CN. Instead, for
> cgroup/bind{4,6}, bit #1 indicates that CAP_NET_BIND_SERVICE should
> be bypassed.
> 
> v3:
> - Update description (Martin KaFai Lau)
> - Fix capability restore in selftest (Martin KaFai Lau)
> 
> v2:
> - Switch to explicit return code (Martin KaFai Lau)
> 

[ ... ]

> @@ -499,7 +501,8 @@ int __inet_bind(struct sock *sk, struct sockaddr *uaddr, int addr_len,
>  
>  	snum = ntohs(addr->sin_port);
>  	err = -EACCES;
> -	if (snum && inet_port_requires_bind_service(net, snum) &&
> +	if (!(flags & BIND_NO_CAP_NET_BIND_SERVICE) &&
> +	    snum && inet_port_requires_bind_service(net, snum) &&
The same change needs to be done on __inet6_bind()
and also adds a test for IPv6 in patch 2.

>  	    !ns_capable(net->user_ns, CAP_NET_BIND_SERVICE))
>  		goto out;
>