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

On Tue, Jan 26, 2021 at 10:01 AM Martin KaFai Lau [off-list ref] wrote:

On Tue, Jan 26, 2021 at 08:51:03AM -0800, Stanislav Fomichev wrote:

quoted

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)

[ ... ]

quoted

@@ -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.

Oh, damn, I did add the flag but forgot to handle it. Thanks for
catching this, will do!