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

From: KP Singh <kpsingh@chromium.org>
To: Stephen Smalley <stephen.smalley.work@gmail.com>
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>
Subject: Re: [PATCH bpf-next v5 4/7] bpf: lsm: Implement attach, detach and execution
Date: Tue, 24 Mar 2020 15:50:03 +0100	[thread overview]
Message-ID: <20200324145003.GA2685@chromium.org> (raw)
In-Reply-To: <CAEjxPJ4MukexdmAD=py0r7vkE6vnn6T1LVcybP_GSJYsAdRuxA@mail.gmail.com>

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>
> >
> > JITed BPF programs are dynamically attached to the LSM hooks
> > using BPF trampolines. The trampoline prologue generates code to handle
> > conversion of the signature of the hook to the appropriate BPF context.
> >
> > The allocated trampoline programs are attached to the nop functions
> > initialized as LSM hooks.
> >
> > BPF_PROG_TYPE_LSM programs must have a GPL compatible license and
> > and need CAP_SYS_ADMIN (required for loading eBPF programs).
> >
> > Upon attachment:
> >
> > * A BPF fexit trampoline is used for LSM hooks with a void return type.
> > * A BPF fmod_ret trampoline is used for LSM hooks which return an
> >   int. The attached programs can override the return value of the
> >   bpf LSM hook to indicate a MAC Policy decision.
> >
> > Signed-off-by: KP Singh <kpsingh@google.com>
> > Reviewed-by: Brendan Jackman <jackmanb@google.com>
> > Reviewed-by: Florent Revest <revest@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?

- KP