linux-kernel - Re: [PATCH v1] security: Add a new hook: inode_touch

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Message-ID: <20161222090652.GA8715@infradead.org>
Date:   Thu, 22 Dec 2016 01:06:52 -0800
From:   Christoph Hellwig <hch@...radead.org>
To:     Mickaël Salaün <mic@...ikod.net>
Cc:     Al Viro <viro@...IV.linux.org.uk>,
        Casey Schaufler <casey@...aufler-ca.com>,
        linux-kernel@...r.kernel.org,
        Andreas Gruenbacher <agruenba@...hat.com>,
        Andy Lutomirski <luto@...capital.net>,
        Dmitry Kasatkin <dmitry.kasatkin@...il.com>,
        Eric Paris <eparis@...isplace.org>,
        James Morris <james.l.morris@...cle.com>,
        John Johansen <john.johansen@...onical.com>,
        Kees Cook <keescook@...omium.org>,
        Kentaro Takeda <takedakn@...data.co.jp>,
        Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        Paul Moore <paul@...l-moore.com>,
        "Serge E . Hallyn" <serge@...lyn.com>,
        Stephen Smalley <sds@...ho.nsa.gov>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        Vivek Goyal <vgoyal@...hat.com>, linux-fsdevel@...r.kernel.org,
        linux-security-module@...r.kernel.org,
        Christoph Hellwig <hch@...radead.org>
Subject: Re: [PATCH v1] security: Add a new hook: inode_touch_atime

On Thu, Dec 22, 2016 at 09:58:42AM +0100, Mickaël Salaün wrote:
> Of course a read-only mount point can do the trick (except for anonymous
> inodes). However, a security policy (e.g. for SELinux) should not (and
> can't always) rely on mount options. For example, a security policy can
> come from a distro but they may not want to tie mount options with this
> policy. We may also not want a sandbox to being able to change mount
> options (even with user namespaces).
> 
> Being able to write (meta-)data, whereas a security policy said that
> it's not allowed, seems like a flaw in this policy. Moreover, modifying
> access time is an easy way to create cover-channels without any LSM
> being able to notice it.

A security policy must not mess with the readonly state of a file system
or mount, period.  You're overstepping your boundaries.