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

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Date:   Tue, 24 Mar 2020 10:58:12 -0400
From:   Stephen Smalley <stephen.smalley.work@...il.com>
To:     KP Singh <kpsingh@...omium.org>
Cc:     linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
        LSM List <linux-security-module@...r.kernel.org>,
        Brendan Jackman <jackmanb@...gle.com>,
        Florent Revest <revest@...gle.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        James Morris <jmorris@...ei.org>,
        Kees Cook <keescook@...omium.org>,
        Paul Turner <pjt@...gle.com>, Jann Horn <jannh@...gle.com>,
        Florent Revest <revest@...omium.org>,
        Brendan Jackman <jackmanb@...omium.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Paul Moore <paul@...l-moore.com>
Subject: Re: [PATCH bpf-next v5 4/7] bpf: lsm: Implement attach, detach and execution

On Tue, Mar 24, 2020 at 10:50 AM KP Singh <kpsingh@...omium.org> wrote:
>
> On 24-Mär 10:35, Stephen Smalley wrote:
> > On Mon, Mar 23, 2020 at 12:46 PM KP Singh <kpsingh@...omium.org> wrote:
> > >
> > > From: KP Singh <kpsingh@...gle.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.