Re: [PATCH v2 1/1] userfaultfd/sysctl: add vm.unprivileged_userfaultfd

From: Peter Xu <peterx@redhat.com>
To: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: Andrew Morton <akpm@linux-foundation.org>,
	linux-kernel@vger.kernel.org, Paolo Bonzini <pbonzini@redhat.com>,
	Hugh Dickins <hughd@google.com>,
	Luis Chamberlain <mcgrof@kernel.org>,
	Maxime Coquelin <maxime.coquelin@redhat.com>,
	Maya Gokhale <gokhale2@llnl.gov>,
	Jerome Glisse <jglisse@redhat.com>,
	Pavel Emelyanov <xemul@virtuozzo.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Martin Cracauer <cracauer@cons.org>,
	Denis Plotnikov <dplotnikov@virtuozzo.com>,
	linux-mm@kvack.org, Marty McFadden <mcfadden8@llnl.gov>,
	Mike Kravetz <mike.kravetz@oracle.com>,
	Andrea Arcangeli <aarcange@redhat.com>,
	Mike Rapoport <rppt@linux.vnet.ibm.com>,
	Kees Cook <keescook@chromium.org>, Mel Gorman <mgorman@suse.de>,
	"Kirill A . Shutemov" <kirill@shutemov.name>,
	linux-api@vger.kernel.org, linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v2 1/1] userfaultfd/sysctl: add vm.unprivileged_userfaultfd
Date: Wed, 20 Mar 2019 08:20:43 +0800	[thread overview]
Message-ID: <20190320002042.GA8956@xz-x1> (raw)
In-Reply-To: <20190319182822.GK2727@work-vm>

On Tue, Mar 19, 2019 at 06:28:23PM +0000, Dr. David Alan Gilbert wrote:
> * Andrew Morton (akpm@linux-foundation.org) wrote:
> > On Tue, 19 Mar 2019 11:07:22 +0800 Peter Xu <peterx@redhat.com> wrote:
> > 
> > > Add a global sysctl knob "vm.unprivileged_userfaultfd" to control
> > > whether userfaultfd is allowed by unprivileged users.  When this is
> > > set to zero, only privileged users (root user, or users with the
> > > CAP_SYS_PTRACE capability) will be able to use the userfaultfd
> > > syscalls.
> > 
> > Please send along a full description of why you believe Linux needs
> > this feature, for me to add to the changelog.  What is the benefit to
> > our users?  How will it be used?
> > 
> > etcetera.  As it was presented I'm seeing no justification for adding
> > the patch!
> 
> How about:
> 
> ---
> Userfaultfd can be misued to make it easier to exploit existing use-after-free
> (and similar) bugs that might otherwise only make a short window
> or race condition available.  By using userfaultfd to stall a kernel
> thread, a malicious program can keep some state, that it wrote, stable
> for an extended period, which it can then access using an existing
> exploit.   While it doesn't cause the exploit itself, and while it's not
> the only thing that can stall a kernel thread when accessing a memory location,
> it's one of the few that never needs priviledge.
> 
> Add a flag, allowing userfaultfd to be restricted, so that in general 
> it won't be useable by arbitrary user programs, but in environments that
> require userfaultfd it can be turned back on.

Thanks for the quick write up, Dave!  I definitely should have some
justification in the cover letter and carry it until the last version.
Sorry to be unclear at the first glance.

-- 
Peter Xu