LKML Archive on lore.kernel.org
 help / color / Atom feed
From: Andy Lutomirski <luto@kernel.org>
To: Andrea Arcangeli <aarcange@redhat.com>
Cc: Andy Lutomirski <luto@kernel.org>, Jann Horn <jannh@google.com>,
	Daniel Colascione <dancol@google.com>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Pavel Emelyanov <xemul@virtuozzo.com>,
	Lokesh Gidra <lokeshgidra@google.com>,
	Nick Kralevich <nnk@google.com>, Nosh Minwalla <nosh@google.com>,
	Tim Murray <timmurray@google.com>,
	Mike Rapoport <rppt@linux.vnet.ibm.com>,
	Linux API <linux-api@vger.kernel.org>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 3/7] Add a UFFD_SECURE flag to the userfaultfd API.
Date: Wed, 23 Oct 2019 12:21:18 -0700
Message-ID: <CALCETrWY+5ynDct7eU_nDUqx=okQvjm=Y5wJvA4ahBja=CQXGw@mail.gmail.com> (raw)
In-Reply-To: <20191023190959.GA9902@redhat.com>

On Wed, Oct 23, 2019 at 12:10 PM Andrea Arcangeli <aarcange@redhat.com> wrote:
>
> Hello,
>
> On Sat, Oct 12, 2019 at 06:14:23PM -0700, Andy Lutomirski wrote:
> > [adding more people because this is going to be an ABI break, sigh]
>
> That wouldn't break the ABI, no more than when if you boot a kernel
> built with CONFIG_USERFAULTFD=n.
>
> All non-cooperative features can be removed any time in a backwards
> compatible way, the only precaution is to mark their feature bits as
> reserved so they can't be reused for something else later.
>
> > 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.
>
> With UFFD_EVENT_FORK, the newly created uffd that controls the child,
> is not passed to the parent nor to the child. Instead it's passed to
> the CRIU monitor only, which has to be already running as root and is
> fully trusted and acts a hypervisor (despite there is no hypervisor).
>
> By the time execve runs and any suid bit in the execve'd inode becomes
> relevant, well before the new userland executable code can run, the
> kernel throws away the "old_mm" controlled by any uffd and all
> attached uffds are released as well.
>
> All I found is your "A .read implementation MUST NOT ACT ON THE
> CALLING TASK" as an explanation that something is broken but I need
> further clarification.

There are two things going on here.

1. Daniel wants to add LSM labels to userfaultfd objects.  This seems
reasonable to me.  The question, as I understand it, is: who is the
subject that creates a uffd referring to a forked child?  I'm sure
this is solvable in any number of straightforward ways, but I think
it's less important than:

2. The existing ABI is busted independently of #1.  Suppose you call
userfaultfd to get a userfaultfd and enable UFFD_FEATURE_EVENT_FORK.
Then you do:

$ sudo <&[userfaultfd number]

Sudo will read it and get a new fd unexpectedly added to its fd table.
It's worse if SCM_RIGHTS is involved.

So I think we either need to declare that UFFD_FEATURE_EVENT_FORK is
only usable by global root or we need to remove it and maybe re-add it
in some other form.


--Andy

  reply index

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
2019-10-23 19:09           ` Andrea Arcangeli
2019-10-23 19:21             ` Andy Lutomirski [this message]
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

Reply instructions:

You may reply publically to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to='CALCETrWY+5ynDct7eU_nDUqx=okQvjm=Y5wJvA4ahBja=CQXGw@mail.gmail.com' \
    --to=luto@kernel.org \
    --cc=aarcange@redhat.com \
    --cc=dancol@google.com \
    --cc=jannh@google.com \
    --cc=linux-api@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=lokeshgidra@google.com \
    --cc=nnk@google.com \
    --cc=nosh@google.com \
    --cc=rppt@linux.vnet.ibm.com \
    --cc=timmurray@google.com \
    --cc=torvalds@linux-foundation.org \
    --cc=xemul@virtuozzo.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

LKML Archive on lore.kernel.org

Archives are clonable:
	git clone --mirror https://lore.kernel.org/lkml/0 lkml/git/0.git
	git clone --mirror https://lore.kernel.org/lkml/1 lkml/git/1.git
	git clone --mirror https://lore.kernel.org/lkml/2 lkml/git/2.git
	git clone --mirror https://lore.kernel.org/lkml/3 lkml/git/3.git
	git clone --mirror https://lore.kernel.org/lkml/4 lkml/git/4.git
	git clone --mirror https://lore.kernel.org/lkml/5 lkml/git/5.git
	git clone --mirror https://lore.kernel.org/lkml/6 lkml/git/6.git
	git clone --mirror https://lore.kernel.org/lkml/7 lkml/git/7.git

	# If you have public-inbox 1.1+ installed, you may
	# initialize and index your mirror using the following commands:
	public-inbox-init -V2 lkml lkml/ https://lore.kernel.org/lkml \
		linux-kernel@vger.kernel.org
	public-inbox-index lkml

Example config snippet for mirrors

Newsgroup available over NNTP:
	nntp://nntp.lore.kernel.org/org.kernel.vger.linux-kernel


AGPL code for this site: git clone https://public-inbox.org/public-inbox.git