[PATCH bpf-next v4 4/7] bpf: Attachment verification for BPF_MODIFY_RETURN

[Alexei Starovoitov]

On Thu, Mar 05, 2020 at 08:43:11AM -0500, Stephen Smalley wrote:
> On Wed, Mar 4, 2020 at 2:20 PM KP Singh <kpsingh at chromium.org> wrote:
> >
> > From: KP Singh <kpsingh at google.com>
> >
> > - Allow BPF_MODIFY_RETURN attachment only to functions that are:
> >
> >     * Whitelisted for error injection by checking
> >       within_error_injection_list. Similar discussions happened for the
> >       bpf_override_return helper.
> >
> >     * security hooks, this is expected to be cleaned up with the LSM
> >       changes after the KRSI patches introduce the LSM_HOOK macro:
> >
> >         https://lore.kernel.org/bpf/20200220175250.10795-1-kpsingh@chromium.org/
> >
> > - The attachment is currently limited to functions that return an int.
> >   This can be extended later other types (e.g. PTR).
> >
> > Signed-off-by: KP Singh <kpsingh at google.com>
> > Acked-by: Andrii Nakryiko <andriin at fb.com>
> > ---
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 2460c8e6b5be..ae32517d4ccd 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -9800,6 +9801,33 @@ static int check_struct_ops_btf_id(struct bpf_verifier_env *env)
> >
> >         return 0;
> >  }
> > +#define SECURITY_PREFIX "security_"
> > +
> > +static int check_attach_modify_return(struct bpf_verifier_env *env)
> > +{
> > +       struct bpf_prog *prog = env->prog;
> > +       unsigned long addr = (unsigned long) prog->aux->trampoline->func.addr;
> > +
> > +       if (within_error_injection_list(addr))
> > +               return 0;
> > +
> > +       /* This is expected to be cleaned up in the future with the KRSI effort
> > +        * introducing the LSM_HOOK macro for cleaning up lsm_hooks.h.
> > +        */
> > +       if (!strncmp(SECURITY_PREFIX, prog->aux->attach_func_name,
> > +                    sizeof(SECURITY_PREFIX) - 1)) {
> > +
> > +               if (!capable(CAP_MAC_ADMIN))
> > +                       return -EPERM;
> 
> CAP_MAC_ADMIN was originally introduced for Smack and is not
> all-powerful wrt SELinux, so this is not a sufficient check for
> SELinux.

I think you're misunderstanding the intent here.
This facility is just a faster version of kprobe based fault injection.
It doesn't care about LSM. Security is not a focus here.
It can fault inject in a lot of places in the kernel: syscalls,
kmalloc, page_alloc, fs internals, etc
I think above capable() check created this confusion and
we should remove it.