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From:   Stephen Smalley <stephen.smalley.work@gmail.com>
Date:   Tue, 24 Mar 2020 10:58:12 -0400
Message-ID: <CAEjxPJ4YnCCeQUTK36Ao550AWProHrkrW1a6K5RKuKYcPcfhyA@mail.gmail.com>
Subject: Re: [PATCH bpf-next v5 4/7] bpf: lsm: Implement attach, detach and execution
To:     KP Singh <kpsingh@chromium.org>
Cc:     linux-kernel@vger.kernel.org, bpf@vger.kernel.org,
        LSM List <linux-security-module@vger.kernel.org>,
        Brendan Jackman <jackmanb@google.com>,
        Florent Revest <revest@google.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        James Morris <jmorris@namei.org>,
        Kees Cook <keescook@chromium.org>,
        Paul Turner <pjt@google.com>, Jann Horn <jannh@google.com>,
        Florent Revest <revest@chromium.org>,
        Brendan Jackman <jackmanb@chromium.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Paul Moore <paul@paul-moore.com>
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On Tue, Mar 24, 2020 at 10:50 AM KP Singh <kpsingh@chromium.org> wrote:
>
> On 24-M=C3=A4r 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, decla=
re a NOP
> > >   * function where a BPF program can be attached as an fexit trampoli=
ne.
> > > @@ -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 LS=
M 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 SELi=
nux.
>
> 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.