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Message-ID: <de1a6799-1545-b9d9-915a-2d5184db001e@digikod.net>
Date: Thu, 29 Oct 2020 12:38:26 +0100
From: Mickaël Salaün <mic@...ikod.net>
To: Jann Horn <jannh@...gle.com>
Cc: James Morris <jmorris@...ei.org>, "Serge E . Hallyn" <serge@...lyn.com>,
Al Viro <viro@...iv.linux.org.uk>, Andy Lutomirski <luto@...capital.net>,
Anton Ivanov <anton.ivanov@...bridgegreys.com>, Arnd Bergmann
<arnd@...db.de>, Casey Schaufler <casey@...aufler-ca.com>,
Jeff Dike <jdike@...toit.com>, Jonathan Corbet <corbet@....net>,
Kees Cook <keescook@...omium.org>, Michael Kerrisk <mtk.manpages@...il.com>,
Richard Weinberger <richard@....at>, Shuah Khan <shuah@...nel.org>,
Vincent Dagonneau <vincent.dagonneau@....gouv.fr>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>,
Linux API <linux-api@...r.kernel.org>,
linux-arch <linux-arch@...r.kernel.org>,
"open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
linux-fsdevel <linux-fsdevel@...r.kernel.org>,
kernel list <linux-kernel@...r.kernel.org>,
"open list:KERNEL SELFTEST FRAMEWORK" <linux-kselftest@...r.kernel.org>,
linux-security-module <linux-security-module@...r.kernel.org>,
the arch/x86 maintainers <x86@...nel.org>,
Mickaël Salaün <mic@...ux.microsoft.com>
Subject: Re: [PATCH v22 12/12] landlock: Add user and kernel documentation
On 29/10/2020 02:07, Jann Horn wrote:
> On Tue, Oct 27, 2020 at 9:04 PM Mickaël Salaün <mic@...ikod.net> wrote:
>> This documentation can be built with the Sphinx framework.
>>
>> Cc: James Morris <jmorris@...ei.org>
>> Cc: Jann Horn <jannh@...gle.com>
>> Cc: Kees Cook <keescook@...omium.org>
>> Cc: Serge E. Hallyn <serge@...lyn.com>
>> Signed-off-by: Mickaël Salaün <mic@...ux.microsoft.com>
>> Reviewed-by: Vincent Dagonneau <vincent.dagonneau@....gouv.fr>
> [...]
>> diff --git a/Documentation/userspace-api/landlock.rst b/Documentation/userspace-api/landlock.rst
> [...]
>> +Landlock rules
>> +==============
>> +
>> +A Landlock rule enables to describe an action on an object. An object is
>
> s/enables to describe/describes/
OK.
>
>> +currently a file hierarchy, and the related filesystem actions are defined in
>> +`Access rights`_. A set of rules is aggregated in a ruleset, which can then
>> +restrict the thread enforcing it, and its future children.
>> +
>> +Defining and enforcing a security policy
>> +----------------------------------------
>> +
>> +We first need to create the ruleset that will contain our rules. For this
>> +example, the ruleset will contain rules which only allow read actions, but
>> +write actions will be denied. The ruleset then needs to handle both of these
>> +kind of actions. To have a backward compatibility, these actions should be
>> +ANDed with the supported ones.
>
> This sounds as if there is a way for userspace to discover which
> actions are supported by the running kernel; but we don't have
> anything like that, right?
Right, it dates from the landlock_get_features(2), which is now gone but
may be replaced by something else in the future. I'll remove that.
>
> If we want to make that possible, we could maybe change
> sys_landlock_create_ruleset() so that if
> ruleset_attr.handled_access_fs contains bits we don't know, we clear
> those bits and then copy the struct back to userspace? And then
> userspace can retry the syscall with the cleared bits? Or something
> along those lines?
Yes, but I would prefer clear syscall which don't read and write from/to
the same argument. I'm working on a more generic solution. It should not
be an issue for now.
>
> [...]
>> +We can now add a new rule to this ruleset thanks to the returned file
>> +descriptor referring to this ruleset. The rule will only enable to read the
>
> s/enable to read/allow reading/
OK.
>
>> +file hierarchy ``/usr``. Without another rule, write actions would then be
>> +denied by the ruleset. To add ``/usr`` to the ruleset, we open it with the
>> +``O_PATH`` flag and fill the &struct landlock_path_beneath_attr with this file
>> +descriptor.
> [...]
>> +Inheritance
>> +-----------
>> +
>> +Every new thread resulting from a :manpage:`clone(2)` inherits Landlock domain
>> +restrictions from its parent. This is similar to the seccomp inheritance (cf.
>> +:doc:`/userspace-api/seccomp_filter`) or any other LSM dealing with task's
>> +:manpage:`credentials(7)`. For instance, one process's thread may apply
>> +Landlock rules to itself, but they will not be automatically applied to other
>> +sibling threads (unlike POSIX thread credential changes, cf.
>> +:manpage:`nptl(7)`).
>> +
>> +When a thread sandbox itself, we have the grantee that the related security
>
> s/sandbox/sandboxes/
> s/grantee/guarantee/
OK.
>
>> +policy will stay enforced on all this thread's descendants. This enables to
>> +create standalone and modular security policies per application, which will
>
> s/enables to create/allows creating/
OK.
>
>
>> +automatically be composed between themselves according to their runtime parent
>> +policies.
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