Re: [PATCH bpf-next v5 4/7] bpf: lsm: Implement attach, detach and execution

Stephen Smalley
2020-03-24T17:48:26Z

On Tue, Mar 24, 2020 at 12:25 PM Casey Schaufler <casey@schaufler-ca.com> wrote:
>
> On 3/24/2020 7:58 AM, Stephen Smalley wrote:
> > On Tue, Mar 24, 2020 at 10:50 AM KP Singh <kpsingh@chromium.org> wrote:
> >> On 24-Mär 10:35, Stephen Smalley wrote:
> >>> On Mon, Mar 23, 2020 at 12:46 PM KP Singh <kpsingh@chromium.org> wrote:
> >>>> From: KP Singh <kpsingh@google.com>
> >>>> diff --git a/kernel/bpf/bpf_lsm.c b/kernel/bpf/bpf_lsm.c
> >>>> index 530d137f7a84..2a8131b640b8 100644
> >>>> --- a/kernel/bpf/bpf_lsm.c
> >>>> +++ b/kernel/bpf/bpf_lsm.c
> >>>> @@ -9,6 +9,9 @@
> >>>> #include <linux/btf.h>
> >>>> #include <linux/lsm_hooks.h>
> >>>> #include <linux/bpf_lsm.h>
> >>>> +#include <linux/jump_label.h>
> >>>> +#include <linux/kallsyms.h>
> >>>> +#include <linux/bpf_verifier.h>
> >>>>
> >>>> /* For every LSM hook that allows attachment of BPF programs, declare a NOP
> >>>> * function where a BPF program can be attached as an fexit trampoline.
> >>>> @@ -27,6 +30,32 @@ noinline __weak void bpf_lsm_##NAME(__VA_ARGS__) {}
> >>>> #include <linux/lsm_hook_names.h>
> >>>> #undef LSM_HOOK
> >>>>
> >>>> +#define BPF_LSM_SYM_PREFX "bpf_lsm_"
> >>>> +
> >>>> +int bpf_lsm_verify_prog(struct bpf_verifier_log *vlog,
> >>>> + const struct bpf_prog *prog)
> >>>> +{
> >>>> + /* Only CAP_MAC_ADMIN users are allowed to make changes to LSM hooks
> >>>> + */
> >>>> + if (!capable(CAP_MAC_ADMIN))
> >>>> + return -EPERM;
> >>> I had asked before, and will ask again: please provide an explicit LSM
> >>> hook for mediating whether one can make changes to the LSM hooks.
> >>> Neither CAP_MAC_ADMIN nor CAP_SYS_ADMIN suffices to check this for SELinux.
> >> What do you think about:
> >>
> >> int security_check_mutable_hooks(void)
> >>
> >> Do you have any suggestions on the signature of this hook? Does this
> >> hook need to be BPF specific?
> > I'd do something like int security_bpf_prog_attach_security(const
> > struct bpf_prog *prog) or similar.
> > Then the security module can do a check based on the current task
> > and/or the prog. We already have some bpf-specific hooks.
>
> I *strongly* disagree with Stephen on this. KRSI and SELinux are peers.
> Just as Yama policy is independent of SELinux policy so KRSI policy should
> be independent of SELinux policy. I understand the argument that BDF programs
> ought to be constrained by SELinux, but I don't think it's right. Further,
> we've got unholy layering when security modules call security_ functions.
> I'm not saying there is no case where it would be appropriate, but this is not
> one of them.

I explained this previously. The difference is that the BPF programs
are loaded from a userspace
process, not a kernel-resident module. They already recognize there
is a difference here or
they wouldn't have the CAP_MAC_ADMIN check above in their patch. The
problem with that
check is just that CAP_MAC_ADMIN doesn't necessarily mean fully
privileged with respect to
SELinux, which is why I want an explicit hook. This gets a NAK from
me until there is such a hook.