Re: [PATCH 0/3] userfaultfd: allow to forbid unprivileged users

From: Peter Xu <peterx@redhat.com>
To: Mike Rapoport <rppt@linux.ibm.com>
Cc: 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>,
	kvm@vger.kernel.org, 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>,
	Maya Gokhale <gokhale2@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-fsdevel@vger.kernel.org,
	"Dr . David Alan Gilbert" <dgilbert@redhat.com>,
	Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH 0/3] userfaultfd: allow to forbid unprivileged users
Date: Tue, 12 Mar 2019 20:29:27 +0800	[thread overview]
Message-ID: <20190312122927.GF14108@xz-x1> (raw)
In-Reply-To: <20190312070147.GC9497@rapoport-lnx>

On Tue, Mar 12, 2019 at 09:01:47AM +0200, Mike Rapoport wrote:
> Hi Peter,
> 
> On Mon, Mar 11, 2019 at 05:36:58PM +0800, Peter Xu wrote:
> > Hi,
> > 
> > (The idea comes from Andrea, and following discussions with Mike and
> >  other people)
> > 
> > This patchset introduces a new sysctl flag to allow the admin to
> > forbid users from using userfaultfd:
> > 
> >   $ cat /proc/sys/vm/unprivileged_userfaultfd
> >   [disabled] enabled kvm
> > 
> >   - When set to "disabled", all unprivileged users are forbidden to
> >     use userfaultfd syscalls.
> > 
> >   - When set to "enabled", all users are allowed to use userfaultfd
> >     syscalls.
> > 
> >   - When set to "kvm", all unprivileged users are forbidden to use the
> >     userfaultfd syscalls, except the user who has permission to open
> >     /dev/kvm.
> > 
> > This new flag can add one more layer of security to reduce the attack
> > surface of the kernel by abusing userfaultfd.  Here we grant the
> > thread userfaultfd permission by checking against CAP_SYS_PTRACE
> > capability.  By default, the value is "disabled" which is the most
> > strict policy.  Distributions can have their own perferred value.
> > 
> > The "kvm" entry is a bit special here only to make sure that existing
> > users like QEMU/KVM won't break by this newly introduced flag.  What
> > we need to do is simply set the "unprivileged_userfaultfd" flag to
> > "kvm" here to automatically grant userfaultfd permission for processes
> > like QEMU/KVM without extra code to tweak these flags in the admin
> > code.
> > 
> > Patch 1:  The interface patch to introduce the flag
> > 
> > Patch 2:  The KVM related changes to detect opening of /dev/kvm
> > 
> > Patch 3:  Apply the flag to userfaultfd syscalls
>  
> I'd appreciate to see "Patch 4: documentation update" ;-)
> It'd be also great to update the man pages after this is merged.

Oops, sorry!  I should have remembered that.

> 
> Except for the comment to patch 1, feel free to add
> 
> Reviewed-by: Mike Rapoport <rppt@linux.ibm.com>

Thanks Mike!  I'll take it for 2/3 until I got confirmation from you
on patch 1.

Regards,

-- 
Peter Xu