linux-kernel - Re: [PATCH v6 0/3] SELinux support for anonymous inodes and UFFD

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Date:   Thu, 20 Aug 2020 20:48:52 -0700
From:   Lokesh Gidra <lokeshgidra@...gle.com>
To:     James Morris <jmorris@...ei.org>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Stephen Smalley <stephen.smalley.work@...il.com>,
        Casey Schaufler <casey@...aufler-ca.com>,
        Eric Biggers <ebiggers@...nel.org>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Paul Moore <paul@...l-moore.com>,
        Eric Paris <eparis@...isplace.org>,
        Daniel Colascione <dancol@...col.org>,
        Kees Cook <keescook@...omium.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        KP Singh <kpsingh@...gle.com>,
        David Howells <dhowells@...hat.com>,
        Thomas Cedeno <thomascedeno@...gle.com>,
        Anders Roxell <anders.roxell@...aro.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Matthew Garrett <matthewgarrett@...gle.com>,
        Aaron Goidel <acgoide@...ho.nsa.gov>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        YueHaibing <yuehaibing@...wei.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Alexey Budankov <alexey.budankov@...ux.intel.com>,
        Adrian Reber <areber@...hat.com>,
        Aleksa Sarai <cyphar@...har.com>,
        Linux FS Devel <linux-fsdevel@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        LSM List <linux-security-module@...r.kernel.org>,
        SElinux list <selinux@...r.kernel.org>,
        Kalesh Singh <kaleshsingh@...gle.com>,
        Calin Juravle <calin@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Nick Kralevich <nnk@...gle.com>,
        Jeffrey Vander Stoep <jeffv@...gle.com>,
        kernel-team@...roid.com
Subject: Re: [PATCH v6 0/3] SELinux support for anonymous inodes and UFFD

On Thu, Aug 20, 2020 at 11:36 AM James Morris <jmorris@...ei.org> wrote:
>
> On Fri, 7 Aug 2020, Lokesh Gidra wrote:
>
> > Userfaultfd in unprivileged contexts could be potentially very
> > useful. We'd like to harden userfaultfd to make such unprivileged use
> > less risky. This patch series allows SELinux to manage userfaultfd
> > file descriptors and in the future, other kinds of
> > anonymous-inode-based file descriptor.  SELinux policy authors can
> > apply policy types to anonymous inodes by providing name-based
> > transition rules keyed off the anonymous inode internal name (
> > "[userfaultfd]" in the case of userfaultfd(2) file descriptors) and
> > applying policy to the new SIDs thus produced.
>
> Can you expand more on why this would be useful, e.g. use-cases?
>
With SELinux managed userfaultfd file descriptors, an administrator
can control creation and movement of them. In particular, handling of
a userfaultfd descriptor by a different process is essentially a
ptrace access into the process, without any of the
corresponding security_ptrace_access_check() checks. For privacy, the
admin may want to deny such accesses,
which is possible with SELinux support.

I'll add this use case in the cover letter too in the next version.