linux-doc.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Andrea Arcangeli <aarcange@redhat.com>
To: "Michael S. Tsirkin" <mst@redhat.com>
Cc: Daniel Colascione <dancol@google.com>,
	Jonathan Corbet <corbet@lwn.net>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	Luis Chamberlain <mcgrof@kernel.org>,
	Kees Cook <keescook@chromium.org>,
	Iurii Zaikin <yzaikin@google.com>,
	Mauro Carvalho Chehab <mchehab+samsung@kernel.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Andy Shevchenko <andy.shevchenko@gmail.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	Mel Gorman <mgorman@techsingularity.net>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Peter Xu <peterx@redhat.com>, Mike Rapoport <rppt@linux.ibm.com>,
	Jerome Glisse <jglisse@redhat.com>, Shaohua Li <shli@fb.com>,
	linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
	linux-fsdevel@vger.kernel.org, timmurray@google.com,
	minchan@google.com, sspatil@google.com, lokeshgidra@google.com
Subject: Re: [PATCH 2/2] Add a new sysctl knob: unprivileged_userfaultfd_user_mode_only
Date: Wed, 20 May 2020 00:59:38 -0400	[thread overview]
Message-ID: <20200520045938.GC26186@redhat.com> (raw)
In-Reply-To: <20200508125314-mutt-send-email-mst@kernel.org>

Hello everyone,

On Fri, May 08, 2020 at 12:54:03PM -0400, Michael S. Tsirkin wrote:
> On Fri, May 08, 2020 at 12:52:34PM -0400, Michael S. Tsirkin wrote:
> > On Wed, Apr 22, 2020 at 05:26:32PM -0700, Daniel Colascione wrote:
> > > This sysctl can be set to either zero or one. When zero (the default)
> > > the system lets all users call userfaultfd with or without
> > > UFFD_USER_MODE_ONLY, modulo other access controls. When
> > > unprivileged_userfaultfd_user_mode_only is set to one, users without
> > > CAP_SYS_PTRACE must pass UFFD_USER_MODE_ONLY to userfaultfd or the API
> > > will fail with EPERM. This facility allows administrators to reduce
> > > the likelihood that an attacker with access to userfaultfd can delay
> > > faulting kernel code to widen timing windows for other exploits.
> > > 
> > > Signed-off-by: Daniel Colascione <dancol@google.com>
> > 
> > The approach taken looks like a hard-coded security policy.
> > For example, it won't be possible to set the sysctl knob
> > in question on any sytem running kvm. So this is
> > no good for any general purpose system.
> > 
> > What's wrong with using a security policy for this instead?
> 
> In fact I see the original thread already mentions selinux,
> so it's just a question of making this controllable by
> selinux.

I agree it'd be preferable if it was not hardcoded, but then this
patchset is also much simpler than the previous controlling it through
selinux..

I was thinking, an alternative policy that could control it without
hard-coding it, is a seccomp-bpf filter, then you can drop 2/2 as
well, not just 1/6-4/6.

If you keep only 1/2, can't seccomp-bpf enforce userfaultfd to be
always called with flags==0x1 without requiring extra modifications in
the kernel?

Can't you get the feature party with the CAP_SYS_PTRACE capability
too, if you don't wrap those tasks with the ptrace capability under
that seccomp filter?

As far as I can tell, it's unprecedented to create a flag for a
syscall API, with the only purpose of implementing a seccomp-bpf
filter verifying such flag is set, but then if you want to control it
with LSM it's even more complex than doing it with seccomp-bpf, and it
requires more kernel code too. We could always add 2/2 later, such
possibility won't disappear, in fact we could also add 1/6-4/6 later
too if that is not enough.

If we could begin by merging only 1/2 from this new series and be done
with the kernel changes, because we offload the rest of the work to
the kernel eBPF JIT, I think it'd be ideal.

Thanks,
Andrea


  reply	other threads:[~2020-05-20  4:59 UTC|newest]

Thread overview: 29+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-04-23  0:26 [PATCH 0/2] Control over userfaultfd kernel-fault handling Daniel Colascione
2020-04-23  0:26 ` [PATCH 1/2] Add UFFD_USER_MODE_ONLY Daniel Colascione
2020-07-24 14:28   ` Michael S. Tsirkin
2020-07-24 14:46     ` Lokesh Gidra
2020-07-26 10:09       ` Michael S. Tsirkin
2020-04-23  0:26 ` [PATCH 2/2] Add a new sysctl knob: unprivileged_userfaultfd_user_mode_only Daniel Colascione
2020-05-06 19:38   ` Peter Xu
2020-05-07 19:15     ` Jonathan Corbet
2020-05-20  4:06       ` Andrea Arcangeli
2020-05-08 16:52   ` Michael S. Tsirkin
2020-05-08 16:54     ` Michael S. Tsirkin
2020-05-20  4:59       ` Andrea Arcangeli [this message]
2020-05-20 18:03         ` Kees Cook
2020-05-20 19:48           ` Andrea Arcangeli
2020-05-20 19:51             ` Andrea Arcangeli
2020-05-20 20:17               ` Lokesh Gidra
2020-05-20 21:16                 ` Andrea Arcangeli
2020-07-17 12:57                   ` Jeffrey Vander Stoep
2020-07-23 17:30                     ` Lokesh Gidra
2020-07-24  0:13                       ` Nick Kralevich
2020-07-24 13:40                         ` Michael S. Tsirkin
2020-08-06  0:43                           ` Nick Kralevich
2020-08-06  5:44                             ` Michael S. Tsirkin
2020-08-17 22:11                               ` Lokesh Gidra
2020-09-04  3:34                                 ` Andrea Arcangeli
2020-09-05  0:36                                   ` Lokesh Gidra
2020-09-19 18:14                                     ` Nick Kralevich
2020-07-24 14:01 ` [PATCH 0/2] Control over userfaultfd kernel-fault handling Michael S. Tsirkin
2020-07-24 14:41   ` Lokesh Gidra

Reply instructions:

You may reply publicly 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=20200520045938.GC26186@redhat.com \
    --to=aarcange@redhat.com \
    --cc=akpm@linux-foundation.org \
    --cc=andy.shevchenko@gmail.com \
    --cc=bigeasy@linutronix.de \
    --cc=corbet@lwn.net \
    --cc=dancol@google.com \
    --cc=jglisse@redhat.com \
    --cc=keescook@chromium.org \
    --cc=linux-doc@vger.kernel.org \
    --cc=linux-fsdevel@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=lokeshgidra@google.com \
    --cc=mcgrof@kernel.org \
    --cc=mchehab+samsung@kernel.org \
    --cc=mgorman@techsingularity.net \
    --cc=minchan@google.com \
    --cc=mst@redhat.com \
    --cc=peterx@redhat.com \
    --cc=rppt@linux.ibm.com \
    --cc=shli@fb.com \
    --cc=sspatil@google.com \
    --cc=timmurray@google.com \
    --cc=vbabka@suse.cz \
    --cc=viro@zeniv.linux.org.uk \
    --cc=yzaikin@google.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
Be sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).