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@xxxxxxxxxx>
> ---
> .../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
>
