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

[Paul Moore]

On Wed, Apr 12, 2023 at 9:43 PM Andrii Nakryiko
<andrii.nakryiko at gmail.com> wrote:
> 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.

As discussed in the cover letter thread, I'm opposed to using a LSM
hook to skip/bypass/circumvent/etc. existing capability checks.

-- 
paul-moore.com