bpf: Allow rewriting to ports under ip_unprivileged_port_start

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.

v5:
- rename flags to be less confusing (Andrey Ignatov)
- rework BPF_PROG_CGROUP_INET_EGRESS_RUN_ARRAY to work on flags
  and accept BPF_RET_SET_CN (no behavioral changes)

v4:
- Add missing IPv6 support (Martin KaFai Lau)

v3:
- Update description (Martin KaFai Lau)
- Fix capability restore in selftest (Martin KaFai Lau)

v2:
- Switch to explicit return code (Martin KaFai Lau)

Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Reviewed-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Andrey Ignatov <rdna@fb.com>
Link: https://lore.kernel.org/bpf/20210127193140.3170382-1-sdf@google.com

1 files changed, 6 insertions, 3 deletions

diff --git a/net/ipv4/af_inet.c b/net/ipv4/af_inet.c
index 6ba2930ff49b..aaa94bea19c3 100644
--- a/net/ipv4/af_inet.c
+++ b/net/ipv4/af_inet.c
@@ -438,6 +438,7 @@ EXPORT_SYMBOL(inet_release);
 int inet_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
 {
 	struct sock *sk = sock->sk;
+	u32 flags = BIND_WITH_LOCK;
 	int err;
 
 	/* If the socket has its own bind function then use it. (RAW) */
@@ -450,11 +451,12 @@ int inet_bind(struct socket *sock, struct sockaddr *uaddr, int addr_len)
 	/* BPF prog is run before any checks are done so that if the prog
 	 * changes context in a wrong way it will be caught.
 	 */
-	err = BPF_CGROUP_RUN_PROG_INET4_BIND_LOCK(sk, uaddr);
+	err = BPF_CGROUP_RUN_PROG_INET_BIND_LOCK(sk, uaddr,
+						 BPF_CGROUP_INET4_BIND, &flags);
 	if (err)
 		return err;
 
-	return __inet_bind(sk, uaddr, addr_len, BIND_WITH_LOCK);
+	return __inet_bind(sk, uaddr, addr_len, flags);
 }
 EXPORT_SYMBOL(inet_bind);
 
@@ -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) &&
 	    !ns_capable(net->user_ns, CAP_NET_BIND_SERVICE))
 		goto out;