Re: [PATCH bpf-next v8 08/11] landlock: Add ptrace restrictions

[Andy Lutomirski <luto@kernel.org>]

On Tue, Feb 27, 2018 at 11:02 PM, Andy Lutomirski <luto@kernel.org> wrote:
> On Tue, Feb 27, 2018 at 10:14 PM, Mickaël Salaün <mic@digikod.net> wrote:
>>
>> On 27/02/2018 06:01, Andy Lutomirski wrote:
>>>
>>>
>>>> On Feb 26, 2018, at 8:17 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>>>>
>>>>> On Tue, Feb 27, 2018 at 12:41 AM, Mickaël Salaün <mic@digikod.net> wrote:
>>>>> A landlocked process has less privileges than a non-landlocked process
>>>>> and must then be subject to additional restrictions when manipulating
>>>>> processes. To be allowed to use ptrace(2) and related syscalls on a
>>>>> target process, a landlocked process must have a subset of the target
>>>>> process' rules.
>>>>>
>>>>> 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: David S. Miller <davem@davemloft.net>
>>>>> Cc: James Morris <james.l.morris@oracle.com>
>>>>> Cc: Kees Cook <keescook@chromium.org>
>>>>> Cc: Serge E. Hallyn <serge@hallyn.com>
>>>>> ---
>>>>>
>>>>> Changes since v6:
>>>>> * factor out ptrace check
>>>>> * constify pointers
>>>>> * cleanup headers
>>>>> * use the new security_add_hooks()
>>>>> ---
>>>>> security/landlock/Makefile       |   2 +-
>>>>> security/landlock/hooks_ptrace.c | 124 +++++++++++++++++++++++++++++++++++++++
>>>>> security/landlock/hooks_ptrace.h |  11 ++++
>>>>> security/landlock/init.c         |   2 +
>>>>> 4 files changed, 138 insertions(+), 1 deletion(-)
>>>>> create mode 100644 security/landlock/hooks_ptrace.c
>>>>> create mode 100644 security/landlock/hooks_ptrace.h
>>>>>
>>>>> diff --git a/security/landlock/Makefile b/security/landlock/Makefile
>>>>> index d0f532a93b4e..605504d852d3 100644
>>>>> --- a/security/landlock/Makefile
>>>>> +++ b/security/landlock/Makefile
>>>>> @@ -3,4 +3,4 @@ obj-$(CONFIG_SECURITY_LANDLOCK) := landlock.o
>>>>> landlock-y := init.o chain.o task.o \
>>>>>        tag.o tag_fs.o \
>>>>>        enforce.o enforce_seccomp.o \
>>>>> -       hooks.o hooks_cred.o hooks_fs.o
>>>>> +       hooks.o hooks_cred.o hooks_fs.o hooks_ptrace.o
>>>>> diff --git a/security/landlock/hooks_ptrace.c b/security/landlock/hooks_ptrace.c
>>>>> new file mode 100644
>>>>> index 000000000000..f1b977b9c808
>>>>> --- /dev/null
>>>>> +++ b/security/landlock/hooks_ptrace.c
>>>>> @@ -0,0 +1,124 @@
>>>>> +/*
>>>>> + * Landlock LSM - ptrace hooks
>>>>> + *
>>>>> + * Copyright © 2017 Mickaël Salaün <mic@digikod.net>
>>>>> + *
>>>>> + * This program is free software; you can redistribute it and/or modify
>>>>> + * it under the terms of the GNU General Public License version 2, as
>>>>> + * published by the Free Software Foundation.
>>>>> + */
>>>>> +
>>>>> +#include <asm/current.h>
>>>>> +#include <linux/errno.h>
>>>>> +#include <linux/kernel.h> /* ARRAY_SIZE */
>>>>> +#include <linux/lsm_hooks.h>
>>>>> +#include <linux/sched.h> /* struct task_struct */
>>>>> +#include <linux/seccomp.h>
>>>>> +
>>>>> +#include "common.h" /* struct landlock_prog_set */
>>>>> +#include "hooks.h" /* landlocked() */
>>>>> +#include "hooks_ptrace.h"
>>>>> +
>>>>> +static bool progs_are_subset(const struct landlock_prog_set *parent,
>>>>> +               const struct landlock_prog_set *child)
>>>>> +{
>>>>> +       size_t i;
>>>>> +
>>>>> +       if (!parent || !child)
>>>>> +               return false;
>>>>> +       if (parent == child)
>>>>> +               return true;
>>>>> +
>>>>> +       for (i = 0; i < ARRAY_SIZE(child->programs); i++) {
>>>>
>>>> ARRAY_SIZE(child->programs) seems misleading.  Is there no define
>>>> NUM_LANDLOCK_PROG_TYPES or similar?
>>>>
>>>>> +               struct landlock_prog_list *walker;
>>>>> +               bool found_parent = false;
>>>>> +
>>>>> +               if (!parent->programs[i])
>>>>> +                       continue;
>>>>> +               for (walker = child->programs[i]; walker;
>>>>> +                               walker = walker->prev) {
>>>>> +                       if (walker == parent->programs[i]) {
>>>>> +                               found_parent = true;
>>>>> +                               break;
>>>>> +                       }
>>>>> +               }
>>>>> +               if (!found_parent)
>>>>> +                       return false;
>>>>> +       }
>>>>> +       return true;
>>>>> +}
>>>>
>>>> If you used seccomp, you'd get this type of check for free, and it
>>>> would be a lot easier to comprehend.  AFAICT the only extra leniency
>>>> you're granting is that you're agnostic to the order in which the
>>>> rules associated with different program types were applied, which
>>>> could easily be added to seccomp.
>>>
>>> On second thought, this is all way too complicated.  I think the correct logic is either "if you are filtered by Landlock, you cannot ptrace anything" or to delete this patch entirely.
>>
>> This does not fit a lot of use cases like running a container
>> constrained with some Landlock programs. We should not deny users the
>> ability to debug their stuff.
>>
>> This patch add the minimal protection which are needed to have
>> meaningful Landlock security policy. Without it, they may be easily
>> bypassable, hence useless.
>>
>
> I think you're wrong here.  Any sane container trying to use Landlock
> like this would also create a PID namespace.  Problem solved.  I still
> think you should drop this patch.
>
>>
>>> If something like Tycho's notifiers goes in, then it's not obvious that, just because you have the same set of filters, you have the same privilege.  Similarly, if a feature that lets a filter query its cgroup goes in (and you proposed this once!) then the logic you implemented here is wrong.
>>
>> I don't get your point. Please take a look at the tests (patch 10).
>
> I don't know what you want me to look at.
>
> What I'm saying is: suppose I write a filter like this:
>
> bool allow_some_action(void)
> {
>   int value_from_container_manager = call_out_to_user_notifier();
>   return value_from_container_manager == 42;
> }
>
> or
>
> bool allow_some_action(void)
> {
>   return my_cgroup_is("/foo/bar");
> }
>
> In both of these cases, your code will do the wrong thing.
>
>>
>>>
>>> Or you could just say that it's the responsibility of a Landlock user to properly filter ptrace() just like it's the responsibility of seccomp users to filter ptrace if needed.
>>
>> A user should be able to enforce a security policy on ptrace as well,
>> but this patch enforce a minimal set of security boundaries. It will be
>> easy to add a new Landlock program type to get this kind of access control.
>
> It sounds like you want Landlock to be a complete container system all
> by itself.  I disagree with that design goal.

Having actually read your series more correctly now (oops!), I still
think that this patch should be dropped.  I can see an argument for
having a flag that one can set when adding a seccomp filter that says
"prevent ptrace of any child that doesn't have this exact stack
installed", but I think that could be added later and should not be
part of an initial submission.  For now, Landlock users can block
ptrace() entirely or use PID namespaces.