Re: [PATCH 3/7] Add a UFFD_SECURE flag to the userfaultfd API.

[Andy Lutomirski <luto@kernel.org>]

On Wed, Oct 23, 2019 at 5:44 AM Mike Rapoport <rppt@linux.ibm.com> wrote:
>
> On Wed, Oct 23, 2019 at 10:29:20AM +0300, Cyrill Gorcunov wrote:
> > On Tue, Oct 22, 2019 at 09:11:04PM -0700, Andy Lutomirski wrote:
> > > Trying again.  It looks like I used the wrong address for Pavel.
> >
> > Thanks for CC Andy! I must confess I didn't dive into userfaultfd engine
> > personally but let me CC more people involved from criu side. (overquoting
> > left untouched for their sake).
>
> Thanks for CC Cyrill!
>
>
> > > On Sat, Oct 12, 2019 at 6:14 PM Andy Lutomirski <luto@kernel.org> wrote:
> > > >
> > > > [adding more people because this is going to be an ABI break, sigh]
> > > >
> > > > On Sat, Oct 12, 2019 at 5:52 PM Daniel Colascione <dancol@google.com> wrote:
> > > > >
> > > > > On Sat, Oct 12, 2019 at 4:10 PM Andy Lutomirski <luto@kernel.org> wrote:
> > > > > >
> > > > > > On Sat, Oct 12, 2019 at 12:16 PM Daniel Colascione <dancol@google.com> wrote:
> > > > > > >
> > > > > > > The new secure flag makes userfaultfd use a new "secure" anonymous
> > > > > > > file object instead of the default one, letting security modules
> > > > > > > supervise userfaultfd use.
> > > > > > >
> > > > > > > Requiring that users pass a new flag lets us avoid changing the
> > > > > > > semantics for existing callers.
> > > > > >
> > > > > > Is there any good reason not to make this be the default?
> > > > > >
> > > > > >
> > > > > > The only downside I can see is that it would increase the memory usage
> > > > > > of userfaultfd(), but that doesn't seem like such a big deal.  A
> > > > > > lighter-weight alternative would be to have a single inode shared by
> > > > > > all userfaultfd instances, which would require a somewhat different
> > > > > > internal anon_inode API.
> > > > >
> > > > > I'd also prefer to just make SELinux use mandatory, but there's a
> > > > > nasty interaction with UFFD_EVENT_FORK. Adding a new UFFD_SECURE mode
> > > > > which blocks UFFD_EVENT_FORK sidesteps this problem. Maybe you know a
> > > > > better way to deal with it.
> > > > >
> > > > > Right now, when a process with a UFFD-managed VMA using
> > > > > UFFD_EVENT_FORK forks, we make a new userfaultfd_ctx out of thin air
> > > > > and enqueue it on the message queue for the parent process. When we
> > > > > dequeue that context, we get to resolve_userfault_fork, which makes up
> > > > > a new UFFD file object out of thin air in the context of the reading
> > > > > process. Following normal SELinux rules, the SID attached to that new
> > > > > file object would be the task SID of the process *reading* the fork
> > > > > event, not the SID of the new fork child. That seems wrong, because
> > > > > the label we give to the UFFD should correspond to the label of the
> > > > > process that UFFD controls.
>
> I must admit I have no idea about how SELinux works, but what's wrong with
> making the new UFFD object to inherit the properties of the "original" one?
>
> The new file object is created in the context of the same task that owns
> the initial userfault file descriptor and it is used by the same task. So
> if you have a process that registers some of its VMAs with userfaultfd
> and enables UFFD_EVENT_FORK, the same process controls UFFD of itself and
> its children.

I'm not actually convinced this is a problem.

What *is* a problem is touching the file descriptor table at all from
read(2).  That's a big no-no.

--Andy