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

From: [email-masked]
Date: Thu, 21 Jan 2021 14:57:44 -0800

On 01/21, Martin KaFai Lau wrote:
> 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 <[email-masked]>
> > ---
> >  .../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?).
Correct. The alternative might be to export ignore_perm_check
via bpf_sock_addr and make it explicit.

> 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.
I'm testing a corner case here when the address is rewritten to the same
value, but the intention is to rewrite X to Y < 1024.

> Is user_port the only case? How about other fields in bpf_sock_addr?
Good question. For our use case only the port matters because
we rewrite both port and address (and never only address).

It does feel like it should also work when BPF rewrites address only
(and port happens to be in the privileged range). I guess I can
apply the same logic to the user_ip4 and user_ip6?