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Message-ID: <CALCETrUTiUeV71jgYWup0CZwG8eQ3=W7X5FgJqmgPxnYbFsccw@mail.gmail.com>
Date: Tue, 27 Feb 2018 23:02:16 +0000
From: Andy Lutomirski <luto@...nel.org>
To: Mickaël Salaün <mic@...ikod.net>
Cc: LKML <linux-kernel@...r.kernel.org>, Alexei Starovoitov <ast@...nel.org>,
Arnaldo Carvalho de Melo <acme@...nel.org>, Casey Schaufler <casey@...aufler-ca.com>,
Daniel Borkmann <daniel@...earbox.net>, David Drysdale <drysdale@...gle.com>,
"David S . Miller" <davem@...emloft.net>, "Eric W . Biederman" <ebiederm@...ssion.com>,
James Morris <james.l.morris@...cle.com>, Jann Horn <jann@...jh.net>,
Jonathan Corbet <corbet@....net>, Michael Kerrisk <mtk.manpages@...il.com>,
Kees Cook <keescook@...omium.org>, Paul Moore <paul@...l-moore.com>,
Sargun Dhillon <sargun@...gun.me>, "Serge E . Hallyn" <serge@...lyn.com>, Shuah Khan <shuah@...nel.org>,
Tejun Heo <tj@...nel.org>, Thomas Graf <tgraf@...g.ch>, Tycho Andersen <tycho@...ho.ws>,
Will Drewry <wad@...omium.org>, Kernel Hardening <kernel-hardening@...ts.openwall.com>,
Linux API <linux-api@...r.kernel.org>,
LSM List <linux-security-module@...r.kernel.org>,
Network Development <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next v8 08/11] landlock: Add ptrace restrictions
On Tue, Feb 27, 2018 at 10:14 PM, Mickaël Salaün <mic@...ikod.net> wrote:
>
> On 27/02/2018 06:01, Andy Lutomirski wrote:
>>
>>
>>> On Feb 26, 2018, at 8:17 PM, Andy Lutomirski <luto@...capital.net> wrote:
>>>
>>>> On Tue, Feb 27, 2018 at 12:41 AM, Mickaël Salaün <mic@...ikod.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@...ikod.net>
>>>> Cc: Alexei Starovoitov <ast@...nel.org>
>>>> Cc: Andy Lutomirski <luto@...capital.net>
>>>> Cc: Daniel Borkmann <daniel@...earbox.net>
>>>> Cc: David S. Miller <davem@...emloft.net>
>>>> Cc: James Morris <james.l.morris@...cle.com>
>>>> Cc: Kees Cook <keescook@...omium.org>
>>>> Cc: Serge E. Hallyn <serge@...lyn.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@...ikod.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.
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