Re: [PATCH bpf-next 2/2] selftests/bpf: verify that rebinding to port < 1024 from BPF works

[Martin KaFai Lau <kafai@fb.com>]

On Wed, Jan 20, 2021 at 05:22:41PM -0800, Stanislav Fomichev wrote:
> BPF rewrites from 111 to 111, but it still should mark the port as
> "changed".
> We also verify that if port isn't touched by BPF, it's still prohibited.
> 
> Signed-off-by: Stanislav Fomichev <sdf@google.com>
> ---
>  .../selftests/bpf/prog_tests/bind_perm.c      | 88 +++++++++++++++++++
>  tools/testing/selftests/bpf/progs/bind_perm.c | 36 ++++++++
>  2 files changed, 124 insertions(+)
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/bind_perm.c
>  create mode 100644 tools/testing/selftests/bpf/progs/bind_perm.c
> 
> diff --git a/tools/testing/selftests/bpf/prog_tests/bind_perm.c b/tools/testing/selftests/bpf/prog_tests/bind_perm.c
> new file mode 100644
> index 000000000000..840a04ac9042
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/prog_tests/bind_perm.c
> @@ -0,0 +1,88 @@
> +// 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 port, int expected_errno)
> +{
> +	struct sockaddr_in sin = {};
> +	int fd = -1;
> +
> +	fd = socket(AF_INET, SOCK_STREAM, 0);
> +	if (CHECK(fd < 0, "fd", "errno %d", errno))
> +		goto close_socket;
> +
> +	sin.sin_family = AF_INET;
> +	sin.sin_port = htons(port);
> +
> +	errno = 0;
> +	bind(fd, (struct sockaddr *)&sin, sizeof(sin));
> +	CHECK(errno != expected_errno, "bind", "errno %d, expected %d",
> +	      errno, expected_errno);
> +
> +close_socket:
> +	if (fd >= 0)
> +		close(fd);
> +}
> +
> +void cap_net_bind_service(cap_flag_value_t flag)
> +{
> +	const cap_value_t cap_net_bind_service = CAP_NET_BIND_SERVICE;
> +	cap_t caps;
> +
> +	caps = cap_get_proc();
> +	if (CHECK(!caps, "cap_get_proc", "errno %d", errno))
> +		goto free_caps;
> +
> +	if (CHECK(cap_set_flag(caps, CAP_EFFECTIVE, 1, &cap_net_bind_service,
> +			       CAP_CLEAR),
> +		  "cap_set_flag", "errno %d", errno))
> +		goto free_caps;
> +
> +	if (CHECK(cap_set_flag(caps, CAP_EFFECTIVE, 1, &cap_net_bind_service,
> +			       CAP_CLEAR),
> +		  "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:
> +	if (CHECK(cap_free(caps), "cap_free", "errno %d", errno))
> +		goto free_caps;
> +}
> +
> +void test_bind_perm(void)
> +{
> +	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 (CHECK(!skel, "skel-load", "errno %d", errno))
> +		goto close_cgroup_fd;
> +
> +	skel->links.bind_v4_prog = bpf_program__attach_cgroup(skel->progs.bind_v4_prog, cgroup_fd);
> +	if (CHECK(IS_ERR(skel->links.bind_v4_prog),
> +		  "cg-attach", "bind4 %ld",
> +		  PTR_ERR(skel->links.bind_v4_prog)))
> +		goto close_skeleton;
> +
> +	cap_net_bind_service(CAP_CLEAR);
> +	try_bind(110, EACCES);
> +	try_bind(111, 0);
> +	cap_net_bind_service(CAP_SET);
> +
> +close_skeleton:
> +	bind_perm__destroy(skel);
> +close_cgroup_fd:
> +	close(cgroup_fd);
> +}
> diff --git a/tools/testing/selftests/bpf/progs/bind_perm.c b/tools/testing/selftests/bpf/progs/bind_perm.c
> new file mode 100644
> index 000000000000..2194587ec806
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/bind_perm.c
> @@ -0,0 +1,36 @@
> +// 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>
> +
> +SEC("cgroup/bind4")
> +int bind_v4_prog(struct bpf_sock_addr *ctx)
> +{
> +	struct bpf_sock *sk;
> +	__u32 user_ip4;
> +	__u16 user_port;
> +
> +	sk = ctx->sk;
> +	if (!sk)
> +		return 0;
> +
> +	if (sk->family != AF_INET)
> +		return 0;
> +
> +	if (ctx->type != SOCK_STREAM)
> +		return 0;
> +
> +	/* Rewriting to the same value should still cause
> +	 * permission check to be bypassed.
> +	 */
> +	if (ctx->user_port == bpf_htons(111))
> +		ctx->user_port = bpf_htons(111);
iiuc, this overwrite is essentially the way to ensure the bind
will succeed (override CAP_NET_BIND_SERVICE in this particular case?).

It seems to be okay if we consider most of the use cases is rewriting
to a different port.

However, it is quite un-intuitive to the bpf prog to overwrite with
the same user_port just to ensure this port can be binded successfully
later.

Is user_port the only case? How about other fields in bpf_sock_addr?

> +
> +	return 1;
> +}
> +
> +char _license[] SEC("license") = "GPL";
> -- 
> 2.30.0.284.gd98b1dd5eaa7-goog
>