From: Jann Horn <jannh@google.com>
To: Andy Lutomirski <luto@kernel.org>
Cc: Daniel Colascione <dancol@google.com>,
Linus Torvalds <torvalds@linux-foundation.org>,
Andrea Arcangeli <aarcange@redhat.com>,
Pavel Emelyanov <xemul@parallels.com>,
Linux API <linux-api@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
Lokesh Gidra <lokeshgidra@google.com>,
Nick Kralevich <nnk@google.com>, Nosh Minwalla <nosh@google.com>,
Tim Murray <timmurray@google.com>
Subject: Re: [PATCH 3/7] Add a UFFD_SECURE flag to the userfaultfd API.
Date: Mon, 14 Oct 2019 18:04:22 +0200 [thread overview]
Message-ID: <CAG48ez3P27-xqdjKLqfP_0Q_v9K92CgEjU4C=kob2Ax7=NoZbA@mail.gmail.com> (raw)
In-Reply-To: <CALCETrUyq=J37gU-MYXqLdoi7uH7iNNVRjvcGUT11JA1QuTFyg@mail.gmail.com>
On Sun, Oct 13, 2019 at 3:14 AM 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.
[...]
> 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.
next prev parent reply other threads:[~2019-10-14 16:04 UTC|newest]
Thread overview: 44+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-10-12 19:15 [PATCH 0/7] Harden userfaultfd Daniel Colascione
2019-10-12 19:15 ` [PATCH 1/7] Add a new flags-accepting interface for anonymous inodes Daniel Colascione
2019-10-14 4:26 ` kbuild test robot
2019-10-14 15:38 ` Jann Horn
2019-10-14 18:15 ` Daniel Colascione
2019-10-14 18:30 ` Jann Horn
2019-10-15 8:08 ` Christoph Hellwig
2019-10-12 19:15 ` [PATCH 2/7] Add a concept of a "secure" anonymous file Daniel Colascione
2019-10-14 3:01 ` kbuild test robot
2019-10-15 8:08 ` Christoph Hellwig
2019-10-12 19:15 ` [PATCH 3/7] Add a UFFD_SECURE flag to the userfaultfd API Daniel Colascione
2019-10-12 23:10 ` Andy Lutomirski
2019-10-13 0:51 ` Daniel Colascione
2019-10-13 1:14 ` Andy Lutomirski
2019-10-13 1:38 ` Daniel Colascione
2019-10-14 16:04 ` Jann Horn [this message]
2019-10-23 19:09 ` Andrea Arcangeli
2019-10-23 19:21 ` Andy Lutomirski
2019-10-23 21:16 ` Andrea Arcangeli
2019-10-23 21:25 ` Andy Lutomirski
2019-10-23 22:41 ` Andrea Arcangeli
2019-10-23 23:01 ` Andy Lutomirski
2019-10-23 23:27 ` Andrea Arcangeli
2019-10-23 20:05 ` Daniel Colascione
2019-10-24 0:23 ` Andrea Arcangeli
2019-10-23 20:15 ` Linus Torvalds
2019-10-24 9:02 ` Mike Rapoport
2019-10-24 15:10 ` Andrea Arcangeli
2019-10-25 20:12 ` Mike Rapoport
2019-10-22 21:27 ` Daniel Colascione
2019-10-23 4:11 ` Andy Lutomirski
2019-10-23 7:29 ` Cyrill Gorcunov
2019-10-23 12:43 ` Mike Rapoport
2019-10-23 17:13 ` Andy Lutomirski
2019-10-12 19:15 ` [PATCH 4/7] Teach SELinux about a new userfaultfd class Daniel Colascione
2019-10-12 23:08 ` Andy Lutomirski
2019-10-13 0:11 ` Daniel Colascione
2019-10-13 0:46 ` Andy Lutomirski
2019-10-12 19:16 ` [PATCH 5/7] Let userfaultfd opt out of handling kernel-mode faults Daniel Colascione
2019-10-12 19:16 ` [PATCH 6/7] Allow users to require UFFD_SECURE Daniel Colascione
2019-10-12 23:12 ` Andy Lutomirski
2019-10-12 19:16 ` [PATCH 7/7] Add a new sysctl for limiting userfaultfd to user mode faults Daniel Colascione
2019-10-16 0:02 ` [PATCH 0/7] Harden userfaultfd James Morris
2019-11-15 15:09 ` Stephen Smalley
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