From: KP Singh <kpsingh@chromium.org>
To: Stephen Smalley <stephen.smalley.work@gmail.com>
Cc: Kees Cook <keescook@chromium.org>,
Casey Schaufler <casey@schaufler-ca.com>,
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>, 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 19:27:59 +0100 [thread overview]
Message-ID: <20200324182759.GA5557@chromium.org> (raw)
In-Reply-To: <CAEjxPJ7ebh1FHBjfuoWquFLJi0TguipfRq5ozaSepLVt8+qaMQ@mail.gmail.com>
On 24-Mär 14:21, Stephen Smalley wrote:
> On Tue, Mar 24, 2020 at 2:06 PM KP Singh <kpsingh@chromium.org> wrote:
> >
> > On 24-Mär 11:01, Kees Cook wrote:
> > > On Tue, Mar 24, 2020 at 01:49:34PM -0400, Stephen Smalley wrote:
> > > > On Tue, Mar 24, 2020 at 12:25 PM Casey Schaufler <casey@schaufler-ca.com> wrote:
> > > > >
> > > > > On 3/24/2020 7:58 AM, Stephen Smalley wrote:
> > > > > > On Tue, Mar 24, 2020 at 10:50 AM KP Singh <kpsingh@chromium.org> wrote:
> > > > > >> 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>
> > > > > >>>> 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.
> > >
> > > Doesn't the existing int (*bpf_prog)(struct bpf_prog *prog); cover
> > > SELinux's need here? I.e. it can already examine that a hook is being
> > > created for the LSM (since it has a distinct type, etc)?
> >
> > I was about to say the same, specifically for the BPF use-case, we do
> > have the "bpf_prog" i.e. :
> >
> > "Do a check when the kernel generate and return a file descriptor for
> > eBPF programs."
> >
> > SELinux can implement its policy logic for BPF_PROG_TYPE_LSM by
> > providing a callback for this hook.
>
> Ok. In that case do we really need the capable() check here at all?
We do not have a specific capable check for BPF_PROG_TYPE_LSM programs
now. There is a general check which requires CAP_SYS_ADMIN when
unprivileged BPF is disabled:
in kernel/bpf/sycall.c:
if (sysctl_unprivileged_bpf_disabled && !capable(CAP_SYS_ADMIN))
return -EPERM;
AFAIK, Most distros disable unprivileged eBPF.
Now that I look at this, I think we might need a CAP_MAC_ADMIN check
though as unprivileged BPF being enabled will result in an
unprivileged user being able to load MAC policies.
- KP
next prev parent reply other threads:[~2020-03-24 18:28 UTC|newest]
Thread overview: 59+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-03-23 16:44 [PATCH bpf-next v5 0/8] MAC and Audit policy using eBPF (KRSI) KP Singh
2020-03-23 16:44 ` [PATCH bpf-next v5 1/7] bpf: Introduce BPF_PROG_TYPE_LSM KP Singh
2020-03-23 19:02 ` Yonghong Song
2020-03-23 16:44 ` [PATCH bpf-next v5 2/7] security: Refactor declaration of LSM hooks KP Singh
2020-03-23 19:33 ` Kees Cook
2020-03-23 19:56 ` Andrii Nakryiko
2020-03-24 16:06 ` KP Singh
2020-03-23 16:44 ` [PATCH bpf-next v5 3/7] bpf: lsm: provide attachment points for BPF LSM programs KP Singh
2020-03-23 19:04 ` Yonghong Song
2020-03-23 19:33 ` Kees Cook
2020-03-23 19:59 ` Andrii Nakryiko
2020-03-24 10:39 ` KP Singh
2020-03-24 16:12 ` KP Singh
2020-03-24 21:26 ` Andrii Nakryiko
2020-03-24 22:39 ` KP Singh
2020-03-23 16:44 ` [PATCH bpf-next v5 4/7] bpf: lsm: Implement attach, detach and execution KP Singh
2020-03-23 19:16 ` Yonghong Song
2020-03-23 19:44 ` KP Singh
2020-03-23 20:18 ` Andrii Nakryiko
2020-03-24 19:00 ` KP Singh
2020-03-24 14:35 ` Stephen Smalley
2020-03-24 14:50 ` KP Singh
2020-03-24 14:58 ` Stephen Smalley
2020-03-24 16:25 ` Casey Schaufler
2020-03-24 17:49 ` Stephen Smalley
2020-03-24 18:01 ` Kees Cook
2020-03-24 18:06 ` KP Singh
2020-03-24 18:21 ` Stephen Smalley
2020-03-24 18:27 ` KP Singh [this message]
2020-03-24 18:31 ` KP Singh
2020-03-24 18:34 ` Kees Cook
2020-03-24 18:33 ` Kees Cook
2020-03-23 16:44 ` [PATCH bpf-next v5 5/7] bpf: lsm: Initialize the BPF LSM hooks KP Singh
2020-03-23 19:44 ` Kees Cook
2020-03-23 19:47 ` KP Singh
2020-03-23 20:21 ` Andrii Nakryiko
2020-03-23 20:47 ` Casey Schaufler
2020-03-23 21:44 ` Kees Cook
2020-03-23 21:58 ` Casey Schaufler
2020-03-23 22:12 ` Kees Cook
2020-03-23 23:39 ` Casey Schaufler
2020-03-24 1:53 ` KP Singh
2020-03-25 14:35 ` KP Singh
2020-03-24 1:13 ` Casey Schaufler
2020-03-24 1:52 ` KP Singh
2020-03-24 14:37 ` Stephen Smalley
2020-03-24 14:42 ` KP Singh
2020-03-24 14:51 ` Stephen Smalley
2020-03-24 14:51 ` KP Singh
2020-03-24 17:57 ` Kees Cook
2020-03-23 16:44 ` [PATCH bpf-next v5 6/7] tools/libbpf: Add support for BPF_PROG_TYPE_LSM KP Singh
2020-03-23 19:21 ` Yonghong Song
2020-03-23 20:25 ` Andrii Nakryiko
2020-03-24 1:57 ` KP Singh
2020-03-23 16:44 ` [PATCH bpf-next v5 7/7] bpf: lsm: Add selftests " KP Singh
2020-03-23 20:04 ` Yonghong Song
2020-03-24 20:04 ` KP Singh
2020-03-24 23:54 ` Andrii Nakryiko
2020-03-25 0:36 ` KP Singh
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