selftests/bpf: Verify that rebinding to port < 1024 from BPF works

Return 3 to indicate that permission check for port 111
should be skipped.
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20210127193140.3170382-2-sdf@google.com

tools/testing/selftests/bpf/prog_tests/bind_perm.c

+// SPDX-License-Identifier: GPL-2.0
+#include <test_progs.h>
+#include "bind_perm.skel.h"
+
+#include <sys/types.h>
+#include <sys/socket.h>
+#include <sys/capability.h>
+
+static int duration;
+
+void try_bind(int family, int port, int expected_errno)
+{
+	struct sockaddr_storage addr = {};
+	struct sockaddr_in6 *sin6;
+	struct sockaddr_in *sin;
+	int fd = -1;
+
+	fd = socket(family, SOCK_STREAM, 0);
+	if (CHECK(fd < 0, "fd", "errno %d", errno))
+		goto close_socket;
+
+	if (family == AF_INET) {
+		sin = (struct sockaddr_in *)&addr;
+		sin->sin_family = family;
+		sin->sin_port = htons(port);
+	} else {
+		sin6 = (struct sockaddr_in6 *)&addr;
+		sin6->sin6_family = family;
+		sin6->sin6_port = htons(port);
+	}
+
+	errno = 0;
+	bind(fd, (struct sockaddr *)&addr, sizeof(addr));
+	ASSERT_EQ(errno, expected_errno, "bind");
+
+close_socket:
+	if (fd >= 0)
+		close(fd);
+}
+
+bool cap_net_bind_service(cap_flag_value_t flag)
+{
+	const cap_value_t cap_net_bind_service = CAP_NET_BIND_SERVICE;
+	cap_flag_value_t original_value;
+	bool was_effective = false;
+	cap_t caps;
+
+	caps = cap_get_proc();
+	if (CHECK(!caps, "cap_get_proc", "errno %d", errno))
+		goto free_caps;
+
+	if (CHECK(cap_get_flag(caps, CAP_NET_BIND_SERVICE, CAP_EFFECTIVE,
+			       &original_value),
+		  "cap_get_flag", "errno %d", errno))
+		goto free_caps;
+
+	was_effective = (original_value == CAP_SET);
+
+	if (CHECK(cap_set_flag(caps, CAP_EFFECTIVE, 1, &cap_net_bind_service,
+			       flag),
+		  "cap_set_flag", "errno %d", errno))
+		goto free_caps;
+
+	if (CHECK(cap_set_proc(caps), "cap_set_proc", "errno %d", errno))
+		goto free_caps;
+
+free_caps:
+	CHECK(cap_free(caps), "cap_free", "errno %d", errno);
+	return was_effective;
+}
+
+void test_bind_perm(void)
+{
+	bool cap_was_effective;
+	struct bind_perm *skel;
+	int cgroup_fd;
+
+	cgroup_fd = test__join_cgroup("/bind_perm");
+	if (CHECK(cgroup_fd < 0, "cg-join", "errno %d", errno))
+		return;
+
+	skel = bind_perm__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "skel"))
+		goto close_cgroup_fd;
+
+	skel->links.bind_v4_prog = bpf_program__attach_cgroup(skel->progs.bind_v4_prog, cgroup_fd);
+	if (!ASSERT_OK_PTR(skel, "bind_v4_prog"))
+		goto close_skeleton;
+
+	skel->links.bind_v6_prog = bpf_program__attach_cgroup(skel->progs.bind_v6_prog, cgroup_fd);
+	if (!ASSERT_OK_PTR(skel, "bind_v6_prog"))
+		goto close_skeleton;
+
+	cap_was_effective = cap_net_bind_service(CAP_CLEAR);
+
+	try_bind(AF_INET, 110, EACCES);
+	try_bind(AF_INET6, 110, EACCES);
+
+	try_bind(AF_INET, 111, 0);
+	try_bind(AF_INET6, 111, 0);
+
+	if (cap_was_effective)
+		cap_net_bind_service(CAP_SET);
+
+close_skeleton:
+	bind_perm__destroy(skel);
+close_cgroup_fd:
+	close(cgroup_fd);
+}

tools/testing/selftests/bpf/progs/bind_perm.c

+// SPDX-License-Identifier: GPL-2.0
+
+#include <linux/stddef.h>
+#include <linux/bpf.h>
+#include <sys/types.h>
+#include <sys/socket.h>
+#include <bpf/bpf_helpers.h>
+#include <bpf/bpf_endian.h>
+
+static __always_inline int bind_prog(struct bpf_sock_addr *ctx, int family)
+{
+	struct bpf_sock *sk;
+
+	sk = ctx->sk;
+	if (!sk)
+		return 0;
+
+	if (sk->family != family)
+		return 0;
+
+	if (ctx->type != SOCK_STREAM)
+		return 0;
+
+	/* Return 1 OR'ed with the first bit set to indicate
+	 * that CAP_NET_BIND_SERVICE should be bypassed.
+	 */
+	if (ctx->user_port == bpf_htons(111))
+		return (1 | 2);
+
+	return 1;
+}
+
+SEC("cgroup/bind4")
+int bind_v4_prog(struct bpf_sock_addr *ctx)
+{
+	return bind_prog(ctx, AF_INET);
+}
+
+SEC("cgroup/bind6")
+int bind_v6_prog(struct bpf_sock_addr *ctx)
+{
+	return bind_prog(ctx, AF_INET6);
+}
+
+char _license[] SEC("license") = "GPL";