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

From: [email-masked]
Date: Fri, 22 Jan 2021 08:16:40 -0800

On 01/21, Martin KaFai Lau wrote:
> On Thu, Jan 21, 2021 at 04:30:08PM -0800, [email-masked] wrote:
> > On 01/21, Martin KaFai Lau wrote:
> > > On Thu, Jan 21, 2021 at 02:57:44PM -0800, [email-masked] wrote:
> > > > 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.
> > > An explicit field is one option.
> >
> > > or a different return value (e.g.  
> BPF_PROG_CGROUP_INET_EGRESS_RUN_ARRAY).
> >
> > > Not sure which one (including the one in the current patch) is better
> > > at this point.
> > Same. My reasoning was: if the BPF program rewrites the port, it knows
> > what it's doing, so it doesn't seem like adding another explicit
> > signal makes sense. So I decided to go without external api change.
> >
> > > Also, from patch 1, if one cgrp bpf prog says no-perm-check,
> > > it does not matter what the latter cgrp bpf progs have to say?
> > Right, it doesn't matter. But I think it's fine: if the latter
> > one rewrites the (previously rewritten) address to something
> > new, it still wants that address to be bound to, right?
> >
> > If some program returns EPERM, it also doesn't matter.
> >
> > > > > 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.
> > > It is a legit corner case though.
> >
> > > Also, is it possible that the compiler may optimize this
> > > same-value-assignment out?
> > Yeah, it's a legit case, that's why I tested it. Good point on
> > optimizing (can be "healed" with volatile?),
> hmm... It is too fragile.

> > but it should only matter if
> > the program is installed to bypass the permission checks for some ports
> > (as it does in this selftest). As you mention below, it's not clear  
> what's
> > the 'default' use-case is. Is it rewriting to a different port or just
> > bypassing the cap_net_bind_service for some ports? Feels like rewriting
> > to a different address/port was the reason the hooks were added,
> > so I was targeting this one.
> It sounds like having a bpf to bypass permission only without changing
> the port is not the target but more like a by-product of this change.
Right, we might have a use-case for that as well, but it's not
strictly required. We can convert it to be something like
'rewrite this magic addr+port to this real addr+port'.

> How about only bypass cap_net_bind_service when bpf did change the
> address/port.  Will it become too slow for bind?
But this is what I'm doing already, isn't it? There is just a by-product
of triggering it for the same port = port address. Tracking
the real change will require extra space to keep the original
address and then memcmp to figure out if the change was made.
Assuming the majority of rewrites don't happen for <1024 ports
this seems like a bunch of wasted work (vs setting that ctx->port_changed).