linux-kernel - Re: [PATCH RFC v2 1/2] selinux: Fix selinux_sb_mnt_opts

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Message-ID: <CAFqZXNvny0zJmEMzFeMFuy0DzjAAaB5uqRpQoSMbZwVcUxTDAQ@mail.gmail.com>
Date:   Mon, 31 Jan 2022 13:46:32 +0100
From:   Ondrej Mosnacek <omosnace@...hat.com>
To:     Paul Moore <paul@...l-moore.com>
Cc:     Scott Mayhew <smayhew@...hat.com>,
        SElinux list <selinux@...r.kernel.org>,
        linux-nfs <linux-nfs@...r.kernel.org>,
        Linux kernel mailing list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC v2 1/2] selinux: Fix selinux_sb_mnt_opts_compat()

On Fri, Jan 28, 2022 at 3:28 AM Paul Moore <paul@...l-moore.com> wrote:
> On Thu, Jan 27, 2022 at 4:54 AM Ondrej Mosnacek <omosnace@...hat.com> wrote:
> > I wonder if we could make this all much simpler by *always* doing the
> > label parsing in selinux_add_opt() and just returning an error when
> > !selinux_initialized(&selinux_state). Before the new mount API, mount
> > options were always passed directly to the mount(2) syscall, so it
> > wasn't possible to pass any SELinux mount options before the SELinux
> > policy was loaded. I don't see why we need to jump through hoops here
> > just to support this pseudo-feature of stashing an unparsed label into
> > an fs_context before policy is loaded... Userspace should never need
> > to do that.
>
> I could agree with that, although part of my mind is a little nervous
> about the "userspace should *never* ..." because that always seems to
> bite us.  Although I'm struggling to think of a case where userspace
> would need to set explicit SELinux mount options without having a
> policy loaded.

I get that, but IMO this is enough of an odd "use case" that I
wouldn't worry too much. To be affected by this, someone would need
to:
1. Use the new mount API, which:
    a) doesn't even have man pages yet,
    b) isn't even used by the mount(8) utility yet.
2. Call fsconfig(2) with a SELinux mount option before policy is loaded.
3. Call fsmount(2) with the same fd after policy is loaded.

And racing with the policy load doesn't count - that could fail
randomly with or without the change. I honestly can't imagine any
realistic scenario where someone would do this...

--
Ondrej Mosnacek
Software Engineer, Linux Security - SELinux kernel
Red Hat, Inc.