Re: [PATCH bpf-next v7 11/11] selftests/bpf: verify lsm_cgroup struct sock access

[Stanislav Fomichev <sdf@google.com>]

On Mon, May 23, 2022 at 4:33 PM Andrii Nakryiko
<andrii.nakryiko@gmail.com> wrote:
>
> On Wed, May 18, 2022 at 3:56 PM Stanislav Fomichev <sdf@google.com> wrote:
> >
> > sk_priority & sk_mark are writable, the rest is readonly.
> >
> > One interesting thing here is that the verifier doesn't
> > really force me to add NULL checks anywhere :-/
> >
> > Signed-off-by: Stanislav Fomichev <sdf@google.com>
> > ---
> >  .../selftests/bpf/prog_tests/lsm_cgroup.c     | 69 +++++++++++++++++++
> >  1 file changed, 69 insertions(+)
> >
> > diff --git a/tools/testing/selftests/bpf/prog_tests/lsm_cgroup.c b/tools/testing/selftests/bpf/prog_tests/lsm_cgroup.c
> > index 29292ec40343..64b6830e03f5 100644
> > --- a/tools/testing/selftests/bpf/prog_tests/lsm_cgroup.c
> > +++ b/tools/testing/selftests/bpf/prog_tests/lsm_cgroup.c
> > @@ -270,8 +270,77 @@ static void test_lsm_cgroup_functional(void)
> >         lsm_cgroup__destroy(skel);
> >  }
> >
> > +static int field_offset(const char *type, const char *field)
> > +{
> > +       const struct btf_member *memb;
> > +       const struct btf_type *tp;
> > +       const char *name;
> > +       struct btf *btf;
> > +       int btf_id;
> > +       int i;
> > +
> > +       btf = btf__load_vmlinux_btf();
> > +       if (!btf)
> > +               return -1;
> > +
> > +       btf_id = btf__find_by_name_kind(btf, type, BTF_KIND_STRUCT);
> > +       if (btf_id < 0)
> > +               return -1;
> > +
> > +       tp = btf__type_by_id(btf, btf_id);
> > +       memb = btf_members(tp);
> > +
> > +       for (i = 0; i < btf_vlen(tp); i++) {
> > +               name = btf__name_by_offset(btf,
> > +                                          memb->name_off);
> > +               if (strcmp(field, name) == 0)
> > +                       return memb->offset / 8;
> > +               memb++;
> > +       }
> > +
> > +       return -1;
> > +}
> > +
> > +static bool sk_writable_field(const char *type, const char *field, int size)
> > +{
> > +       LIBBPF_OPTS(bpf_prog_load_opts, opts,
> > +                   .expected_attach_type = BPF_LSM_CGROUP);
> > +       struct bpf_insn insns[] = {
> > +               /* r1 = *(u64 *)(r1 + 0) */
> > +               BPF_LDX_MEM(BPF_DW, BPF_REG_1, BPF_REG_1, 0),
> > +               /* r1 = *(u64 *)(r1 + offsetof(struct socket, sk)) */
> > +               BPF_LDX_MEM(BPF_DW, BPF_REG_1, BPF_REG_1, field_offset("socket", "sk")),
> > +               /* r2 = *(u64 *)(r1 + offsetof(struct sock, <field>)) */
> > +               BPF_LDX_MEM(size, BPF_REG_2, BPF_REG_1, field_offset(type, field)),
> > +               /* *(u64 *)(r1 + offsetof(struct sock, <field>)) = r2 */
> > +               BPF_STX_MEM(size, BPF_REG_1, BPF_REG_2, field_offset(type, field)),
> > +               BPF_MOV64_IMM(BPF_REG_0, 1),
> > +               BPF_EXIT_INSN(),
> > +       };
> > +       int fd;
>
> This is really not much better than test_verifier assembly. What I had
> in mind when I was suggesting to use test_progs was that you'd have a
> normal C source code for BPF part, something like this:
>
> __u64 tmp;
>
> SEC("?lsm_cgroup/socket_bind")
> int BPF_PROG(access1_bad, struct socket *sock, struct sockaddr
> *address, int addrlen)
> {
>     *(volatile u16 *)(sock->sk.skc_family) = *(volatile u16
> *)sock->sk.skc_family;
>     return 0;
> }
>
>
> SEC("?lsm_cgroup/socket_bind")
> int BPF_PROG(access2_bad, struct socket *sock, struct sockaddr
> *address, int addrlen)
> {
>     *(volatile u64 *)(sock->sk.sk_sndtimeo) = *(volatile u64
> *)sock->sk.sk_sndtimeo;
>     return 0;
> }
>
> and so on. From user-space you'd be loading just one of those
> accessX_bad programs at a time (note SEC("?"))
>
>
> But having said that, what you did is pretty self-contained, so not
> too bad. It's just not what I was suggesting :)

Yeah, that's what I suggested I was gonna try in:
https://lore.kernel.org/bpf/CAKH8qBuHU7OAjTMk-6GU08Nmwnn6J7Cw1TzP6GwCEq0x1Wwd9w@mail.gmail.com/

I don't really want to separate the program from the test, it seems
like keeping everything in one file is easier to read.
So unless you strongly dislike this new self-contained version, I'd
keep it as is.



> > +
> > +       opts.attach_btf_id = libbpf_find_vmlinux_btf_id("socket_post_create",
> > +                                                       opts.expected_attach_type);
> > +
> > +       fd = bpf_prog_load(BPF_PROG_TYPE_LSM, NULL, "GPL", insns, ARRAY_SIZE(insns), &opts);
> > +       if (fd >= 0)
> > +               close(fd);
> > +       return fd >= 0;
> > +}
> > +
> > +static void test_lsm_cgroup_access(void)
> > +{
> > +       ASSERT_FALSE(sk_writable_field("sock_common", "skc_family", BPF_H), "skc_family");
> > +       ASSERT_FALSE(sk_writable_field("sock", "sk_sndtimeo", BPF_DW), "sk_sndtimeo");
> > +       ASSERT_TRUE(sk_writable_field("sock", "sk_priority", BPF_W), "sk_priority");
> > +       ASSERT_TRUE(sk_writable_field("sock", "sk_mark", BPF_W), "sk_mark");
> > +       ASSERT_FALSE(sk_writable_field("sock", "sk_pacing_rate", BPF_DW), "sk_pacing_rate");
> > +}
> > +
> >  void test_lsm_cgroup(void)
> >  {
> >         if (test__start_subtest("functional"))
> >                 test_lsm_cgroup_functional();
> > +       if (test__start_subtest("access"))
> > +               test_lsm_cgroup_access();
> >  }
> > --
> > 2.36.1.124.g0e6072fb45-goog
> >