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

[Stephen Smalley]

On Tue, Mar 24, 2020 at 12:25 PM Casey Schaufler <casey@xxxxxxxxxxxxxxxx> wrote:
>
> On 3/24/2020 7:58 AM, Stephen Smalley wrote:
> > On Tue, Mar 24, 2020 at 10:50 AM KP Singh <kpsingh@xxxxxxxxxxxx> wrote:
> >> On 24-MÃr 10:35, Stephen Smalley wrote:
> >>> On Mon, Mar 23, 2020 at 12:46 PM KP Singh <kpsingh@xxxxxxxxxxxx> wrote:
> >>>> From: KP Singh <kpsingh@xxxxxxxxxx>
> >>>> 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.