linux-kernel - Re: [PATCH 4/7] Teach SELinux about a new userfaultfd class

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Date:   Sat, 12 Oct 2019 17:46:16 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Daniel Colascione <dancol@...gle.com>
Cc:     Andy Lutomirski <luto@...nel.org>,
        Linux API <linux-api@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Lokesh Gidra <lokeshgidra@...gle.com>,
        Nick Kralevich <nnk@...gle.com>,
        Nosh Minwalla <nosh@...gle.com>,
        Tim Murray <timmurray@...gle.com>
Subject: Re: [PATCH 4/7] Teach SELinux about a new userfaultfd class

On Sat, Oct 12, 2019 at 5:12 PM Daniel Colascione <dancol@...gle.com> wrote:
>
> On Sat, Oct 12, 2019 at 4:09 PM Andy Lutomirski <luto@...nel.org> wrote:
> >
> > On Sat, Oct 12, 2019 at 12:16 PM Daniel Colascione <dancol@...gle.com> wrote:
> > >
> > > Use the secure anonymous inode LSM hook we just added to let SELinux
> > > policy place restrictions on userfaultfd use. The create operation
> > > applies to processes creating new instances of these file objects;
> > > transfer between processes is covered by restrictions on read, write,
> > > and ioctl access already checked inside selinux_file_receive.
> >
> > This is great, and I suspect we'll want it for things like SGX, too.
> > But the current design seems like it will make it essentially
> > impossible for SELinux to reference an anon_inode class whose
> > file_operations are in a module, and moving file_operations out of a
> > module would be nasty.
> >
> > Could this instead be keyed off a new struct anon_inode_class, an
> > enum, or even just a string?
>
> The new LSM hook already receives the string that callers pass to the
> anon_inode APIs; modules can look at that instead of the fops if they
> want. The reason to pass both the name and the fops through the hook
> is to allow LSMs to match using fops comparison (which seems less
> prone to breakage) when possible and rely on string matching when it
> isn't.

I suppose that whoever makes the first module that wants to use this
mechanism can have the fun task of reworking it.  There's nothing
user-visible here that would make it hard to change in the future.