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

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Date:   Mon, 14 Oct 2019 18:04:22 +0200
From:   Jann Horn <jannh@...gle.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Daniel Colascione <dancol@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Pavel Emelyanov <xemul@...allels.com>,
        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 3/7] Add a UFFD_SECURE flag to the userfaultfd API.

On Sun, Oct 13, 2019 at 3:14 AM Andy Lutomirski <luto@...nel.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@...gle.com> wrote:
> > On Sat, Oct 12, 2019 at 4:10 PM Andy Lutomirski <luto@...nel.org> wrote:
> > > On Sat, Oct 12, 2019 at 12:16 PM Daniel Colascione <dancol@...gle.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.
[...]
> Now that you've pointed this mechanism out, it is utterly and
> completely broken and should be removed from the kernel outright or at
> least severely restricted.  A .read implementation MUST NOT ACT ON THE
> CALLING TASK.  Ever.  Just imagine the effect of passing a userfaultfd
> as stdin to a setuid program.
>
> So I think the right solution might be to attempt to *remove*
> UFFD_EVENT_FORK.  Maybe the solution is to say that, unless the
> creator of a userfaultfd() has global CAP_SYS_ADMIN, then it cannot
> use UFFD_FEATURE_EVENT_FORK) and print a warning (once) when
> UFFD_FEATURE_EVENT_FORK is allowed.  And, after some suitable
> deprecation period, just remove it.  If it's genuinely useful, it
> needs an entirely new API based on ioctl() or a syscall.  Or even
> recvmsg() :)

FWIW, <https://codesearch.debian.net/search?q=UFFD_FEATURE_EVENT_FORK&literal=1>
just shows the kernel, kernel selftests, and strace code for decoding
syscall arguments. CRIU uses it though (probably for postcopy live
migration / lazy migration?), I guess that code isn't in debian for
some reason.