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

[Kees Cook <keescook@chromium.org>]

On Wed, Oct 5, 2016 at 1:58 PM, Mickaël Salaün <mic@digikod.net> wrote:
>
>
> On 04/10/2016 01:43, Kees Cook wrote:
>> On Wed, Sep 14, 2016 at 12:24 AM, Mickaël Salaün <mic@digikod.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@digikod.net>
>>> Cc: Alexei Starovoitov <ast@kernel.org>
>>> Cc: Andy Lutomirski <luto@amacapital.net>
>>> Cc: Daniel Borkmann <daniel@iogearbox.net>
>>> Cc: Daniel Mack <daniel@zonque.org>
>>> Cc: David S. Miller <davem@davemloft.net>
>>> Cc: Kees Cook <keescook@chromium.org>
>>> Cc: Tejun Heo <tj@kernel.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. :)
>
> This is not fully satisfactory for me neither but the other approach is
> to use two distinct struct fields instead of a union.
> Do you prefer if there is a "type" field in the "pinned" struct to
> select the union?

Since memory usage isn't a huge deal for this, I'd actually prefer
there just be no union at all. Have a type field, and a distinct
pointer field for each type you're expecting to use. That way there
can never be confusion between types and you could even validate that
only a single field type has been populated, etc.

-Kees

-- 
Kees Cook
Nexus Security