[PATCH bpf-next 7/8] bpf, lsm: implement bpf_btf_load_security LSM hook

[Andrii Nakryiko]

On Wed, Apr 12, 2023 at 9:53 AM Paul Moore <paul at paul-moore.com> wrote:
>
> On Wed, Apr 12, 2023 at 12:33 AM Andrii Nakryiko <andrii at kernel.org> wrote:
> >
> > Add new LSM hook, bpf_btf_load_security, that allows custom LSM security
> > policies controlling BTF data loading permissions (BPF_BTF_LOAD command
> > of bpf() syscall) granularly and precisely.
> >
> > This complements bpf_map_create_security LSM hook added earlier and
> > follow the same semantics: 0 means perform standard kernel capabilities-based
> > checks, negative error rejects BTF object load, while positive one skips
> > CAP_BPF check and allows BTF data object creation.
> >
> > With this hook, together with bpf_map_create_security, we now can also allow
> > trusted unprivileged process to create BPF maps that require BTF, which
> > we take advantaged in the next patch to improve the coverage of added
> > BPF selftest.
> >
> > Signed-off-by: Andrii Nakryiko <andrii at kernel.org>
> > ---
> >  include/linux/lsm_hook_defs.h |  1 +
> >  include/linux/lsm_hooks.h     | 13 +++++++++++++
> >  include/linux/security.h      |  6 ++++++
> >  kernel/bpf/bpf_lsm.c          |  1 +
> >  kernel/bpf/syscall.c          | 10 ++++++++++
> >  security/security.c           |  4 ++++
> >  6 files changed, 35 insertions(+)
>
> ...
>
> > diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> > index 42d8473237ab..bbf70bddc770 100644
> > --- a/kernel/bpf/syscall.c
> > +++ b/kernel/bpf/syscall.c
> > @@ -4449,12 +4449,22 @@ static int bpf_obj_get_info_by_fd(const union bpf_attr *attr,
> >
> >  static int bpf_btf_load(const union bpf_attr *attr, bpfptr_t uattr, __u32 uattr_size)
> >  {
> > +       int err;
> > +
> >         if (CHECK_ATTR(BPF_BTF_LOAD))
> >                 return -EINVAL;
> >
> > +       /* security checks */
> > +       err = security_bpf_btf_load(attr);
> > +       if (err < 0)
> > +               return err;
> > +       if (err > 0)
> > +               goto skip_priv_checks;
> > +
> >         if (!bpf_capable())
> >                 return -EPERM;
> >
> > +skip_priv_checks:
> >         return btf_new_fd(attr, uattr, uattr_size);
> >  }
>
> Beyond the objection I brought up in the patchset cover letter, I
> believe the work of the security_bpf_btf_load() hook presented here
> could be done by the existing security_bpf() LSM hook.  If you believe
> that not to be the case, please let me know.

security_bpf() could prevent BTF object loading only, but
security_bpf_btf_load() can *also* allow *trusted* (according to LSM
policy) unprivileged process to proceed. So it doesn't seem like they
are interchangeable.


>
> --
> paul-moore.com