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

[Kees Cook]

On Wed, Sep 14, 2016 at 12:24 AM, MickaÃl SalaÃn <mic@xxxxxxxxxxx> 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: MickaÃl SalaÃn <mic@xxxxxxxxxxx>
> Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
> Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> Cc: Daniel Mack <daniel@xxxxxxxxxx>
> Cc: David S. Miller <davem@xxxxxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Cc: Tejun Heo <tj@xxxxxxxxxx>
> Link: https://lkml.kernel.org/r/20160826021432.GA8291@xxxxxxxxxxxxxxxxxxxxxxx
> Link: https://lkml.kernel.org/r/20160827204307.GA43714@xxxxxxxxxxxxxxxxxxxxxxx
> ---
>  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