Re: [PATCH v4 0/2] Control over userfaultfd kernel-fault handling

From: Lokesh Gidra <lokeshgidra@google.com>
To: Kees Cook <keescook@chromium.org>,
	Jonathan Corbet <corbet@lwn.net>, Peter Xu <peterx@redhat.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
	Andrew Morton <akpm@linux-foundation.org>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>,
	Stephen Smalley <stephen.smalley.work@gmail.com>,
	Eric Biggers <ebiggers@kernel.org>,
	Daniel Colascione <dancol@dancol.org>,
	"Joel Fernandes (Google)" <joel@joelfernandes.org>,
	Linux FS Devel <linux-fsdevel@vger.kernel.org>,
	linux-kernel <linux-kernel@vger.kernel.org>,
	linux-doc@vger.kernel.org, Kalesh Singh <kaleshsingh@google.com>,
	Calin Juravle <calin@google.com>,
	Suren Baghdasaryan <surenb@google.com>,
	Nick Kralevich <nnk@google.com>,
	Jeffrey Vander Stoep <jeffv@google.com>,
	"Cc: Android Kernel" <kernel-team@android.com>,
	Mike Rapoport <rppt@linux.vnet.ibm.com>, Shaohua Li <shli@fb.com>,
	Jerome Glisse <jglisse@redhat.com>,
	Mauro Carvalho Chehab <mchehab+huawei@kernel.org>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Mel Gorman <mgorman@techsingularity.net>,
	Nitin Gupta <nigupta@nvidia.com>,
	Vlastimil Babka <vbabka@suse.cz>,
	Iurii Zaikin <yzaikin@google.com>,
	Luis Chamberlain <mcgrof@kernel.org>
Subject: Re: [PATCH v4 0/2] Control over userfaultfd kernel-fault handling
Date: Wed, 7 Oct 2020 13:26:55 -0700	[thread overview]
Message-ID: <CA+EESO7kCqtJf+ApoOcceFT+NX8pBwGmOr0q0PVnJf9Dnkrp6A@mail.gmail.com> (raw)
In-Reply-To: <20200924065606.3351177-1-lokeshgidra@google.com>

On Wed, Sep 23, 2020 at 11:56 PM Lokesh Gidra <lokeshgidra@google.com> wrote:
>
> This patch series is split from [1]. The other series enables SELinux
> support for userfaultfd file descriptors so that its creation and
> movement can be controlled.
>
> It has been demonstrated on various occasions that suspending kernel
> code execution for an arbitrary amount of time at any access to
> userspace memory (copy_from_user()/copy_to_user()/...) can be exploited
> to change the intended behavior of the kernel. For instance, handling
> page faults in kernel-mode using userfaultfd has been exploited in [2, 3].
> Likewise, FUSE, which is similar to userfaultfd in this respect, has been
> exploited in [4, 5] for similar outcome.
>
> This small patch series adds a new flag to userfaultfd(2) that allows
> callers to give up the ability to handle kernel-mode faults with the
> resulting UFFD file object. It then adds a 'user-mode only' option to
> the unprivileged_userfaultfd sysctl knob to require unprivileged
> callers to use this new flag.
>
> The purpose of this new interface is to decrease the chance of an
> unprivileged userfaultfd user taking advantage of userfaultfd to
> enhance security vulnerabilities by lengthening the race window in
> kernel code.
>
> [1] https://lore.kernel.org/lkml/20200211225547.235083-1-dancol@google.com/
> [2] https://duasynt.com/blog/linux-kernel-heap-spray
> [3] https://duasynt.com/blog/cve-2016-6187-heap-off-by-one-exploit
> [4] https://googleprojectzero.blogspot.com/2016/06/exploiting-recursion-in-linux-kernel_20.html
> [5] https://bugs.chromium.org/p/project-zero/issues/detail?id=808
>
> Changes since v3:
>
>   - Modified the meaning of value '0' of unprivileged_userfaultfd
>     sysctl knob. Setting this knob to '0' now allows unprivileged users
>     to use userfaultfd, but can handle page faults in user-mode only.
>   - The default value of unprivileged_userfaultfd sysctl knob is changed
>     to '0'.
>
Request reviewers and maintainers to please take a look.

> Changes since v2:
>
>   - Removed 'uffd_flags' and directly used 'UFFD_USER_MODE_ONLY' in
>     userfaultfd().
>
> Changes since v1:
>
>   - Added external references to the threats from allowing unprivileged
>     users to handle page faults from kernel-mode.
>   - Removed the new sysctl knob restricting handling of page
>     faults from kernel-mode, and added an option for the same
>     in the existing 'unprivileged_userfaultfd' knob.
>
> Lokesh Gidra (2):
>   Add UFFD_USER_MODE_ONLY
>   Add user-mode only option to unprivileged_userfaultfd sysctl knob
>
>  Documentation/admin-guide/sysctl/vm.rst | 15 ++++++++++-----
>  fs/userfaultfd.c                        | 12 +++++++++---
>  include/uapi/linux/userfaultfd.h        |  9 +++++++++
>  3 files changed, 28 insertions(+), 8 deletions(-)
>
> --
> 2.28.0.681.g6f77f65b4e-goog
>