linux-kernel - Re: [RFC v3 16/22] bpf/cgroup,landlock: Handle Landlock hooks per cgroup

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Message-ID: <CAGXu5jJ-Heb0nnLygYo0xHaJKSrQM5RSVRNYN2NnYddwGHtWkA@mail.gmail.com>
Date:   Mon, 3 Oct 2016 16:43:26 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Mickaël Salaün <mic@...ikod.net>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Andy Lutomirski <luto@...capital.net>,
        Arnd Bergmann <arnd@...db.de>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Daniel Mack <daniel@...que.org>,
        David Drysdale <drysdale@...gle.com>,
        "David S . Miller" <davem@...emloft.net>,
        Elena Reshetova <elena.reshetova@...el.com>,
        "Eric W . Biederman" <ebiederm@...ssion.com>,
        James Morris <james.l.morris@...cle.com>,
        Paul Moore <pmoore@...hat.com>,
        Sargun Dhillon <sargun@...gun.me>,
        "Serge E . Hallyn" <serge@...lyn.com>, Tejun Heo <tj@...nel.org>,
        Will Drewry <wad@...omium.org>,
        "kernel-hardening@...ts.openwall.com" 
        <kernel-hardening@...ts.openwall.com>,
        Linux API <linux-api@...r.kernel.org>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        Network Development <netdev@...r.kernel.org>,
        Cgroups <cgroups@...r.kernel.org>
Subject: Re: [RFC v3 16/22] bpf/cgroup,landlock: Handle Landlock hooks per cgroup

On Wed, Sep 14, 2016 at 12:24 AM, Mickaël Salaün <mic@...ikod.net> wrote:
> This allows to add new eBPF programs to Landlock hooks dedicated to a
> cgroup thanks to the BPF_PROG_ATTACH command. Like for socket eBPF
> programs, the Landlock hooks attached to a cgroup are propagated to the
> nested cgroups. However, when a new Landlock program is attached to one
> of this nested cgroup, this cgroup hierarchy fork the Landlock hooks.
> This design is simple and match the current CONFIG_BPF_CGROUP
> inheritance. The difference lie in the fact that Landlock programs can
> only be stacked but not removed. This match the append-only seccomp
> behavior. Userland is free to handle Landlock hooks attached to a cgroup
> in more complicated ways (e.g. continuous inheritance), but care should
> be taken to properly handle error cases (e.g. memory allocation errors).
>
> Changes since v2:
> * new design based on BPF_PROG_ATTACH (suggested by Alexei Starovoitov)
>
> Signed-off-by: Mickaël Salaün <mic@...ikod.net>
> Cc: Alexei Starovoitov <ast@...nel.org>
> Cc: Andy Lutomirski <luto@...capital.net>
> Cc: Daniel Borkmann <daniel@...earbox.net>
> Cc: Daniel Mack <daniel@...que.org>
> Cc: David S. Miller <davem@...emloft.net>
> Cc: Kees Cook <keescook@...omium.org>
> Cc: Tejun Heo <tj@...nel.org>
> Link: https://lkml.kernel.org/r/20160826021432.GA8291@ast-mbp.thefacebook.com
> Link: https://lkml.kernel.org/r/20160827204307.GA43714@ast-mbp.thefacebook.com
> ---
>  include/linux/bpf-cgroup.h  |  7 +++++++
>  include/linux/cgroup-defs.h |  2 ++
>  include/linux/landlock.h    |  9 +++++++++
>  include/uapi/linux/bpf.h    |  1 +
>  kernel/bpf/cgroup.c         | 33 ++++++++++++++++++++++++++++++---
>  kernel/bpf/syscall.c        | 11 +++++++++++
>  security/landlock/lsm.c     | 40 +++++++++++++++++++++++++++++++++++++++-
>  security/landlock/manager.c | 32 ++++++++++++++++++++++++++++++++
>  8 files changed, 131 insertions(+), 4 deletions(-)
>
> [...]
> diff --git a/kernel/bpf/cgroup.c b/kernel/bpf/cgroup.c
> index 7b75fa692617..1c18fe46958a 100644
> --- a/kernel/bpf/cgroup.c
> +++ b/kernel/bpf/cgroup.c
> @@ -15,6 +15,7 @@
>  #include <linux/bpf.h>
>  #include <linux/bpf-cgroup.h>
>  #include <net/sock.h>
> +#include <linux/landlock.h>
>
>  DEFINE_STATIC_KEY_FALSE(cgroup_bpf_enabled_key);
>  EXPORT_SYMBOL(cgroup_bpf_enabled_key);
> @@ -31,7 +32,15 @@ void cgroup_bpf_put(struct cgroup *cgrp)
>                 union bpf_object pinned = cgrp->bpf.pinned[type];
>
>                 if (pinned.prog) {
> -                       bpf_prog_put(pinned.prog);
> +                       switch (type) {
> +                       case BPF_CGROUP_LANDLOCK:
> +#ifdef CONFIG_SECURITY_LANDLOCK
> +                               put_landlock_hooks(pinned.hooks);
> +                               break;
> +#endif /* CONFIG_SECURITY_LANDLOCK */
> +                       default:
> +                               bpf_prog_put(pinned.prog);
> +                       }
>                         static_branch_dec(&cgroup_bpf_enabled_key);
>                 }
>         }

I get creeped out by type-controlled unions of pointers. :P I don't
have a suggestion to improve this, but I don't like seeing a pointer
type managed separately from the pointer itself as it tends to bypass
a lot of both static and dynamic checking. A union is better than a
cast of void *, but it still worries me. :)

-Kees

-- 
Kees Cook
Nexus Security