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

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Date:   Tue, 12 Mar 2019 09:01:47 +0200
From:   Mike Rapoport <rppt@...ux.ibm.com>
To:     Peter Xu <peterx@...hat.com>
Cc:     linux-kernel@...r.kernel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Maxime Coquelin <maxime.coquelin@...hat.com>,
        kvm@...r.kernel.org, Jerome Glisse <jglisse@...hat.com>,
        Pavel Emelyanov <xemul@...tuozzo.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Martin Cracauer <cracauer@...s.org>,
        Denis Plotnikov <dplotnikov@...tuozzo.com>, linux-mm@...ck.org,
        Marty McFadden <mcfadden8@...l.gov>,
        Maya Gokhale <gokhale2@...l.gov>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Mike Rapoport <rppt@...ux.vnet.ibm.com>,
        Kees Cook <keescook@...omium.org>,
        Mel Gorman <mgorman@...e.de>,
        "Kirill A . Shutemov" <kirill@...temov.name>,
        linux-fsdevel@...r.kernel.org,
        "Dr . David Alan Gilbert" <dgilbert@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 0/3] userfaultfd: allow to forbid unprivileged users

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.

Except for the comment to patch 1, feel free to add

Reviewed-by: Mike Rapoport <rppt@...ux.ibm.com>

> All comments would be greatly welcomed.  Thanks,
> 
> Peter Xu (3):
>   userfaultfd/sysctl: introduce unprivileged_userfaultfd
>   kvm/mm: introduce MMF_USERFAULTFD_ALLOW flag
>   userfaultfd: apply unprivileged_userfaultfd check
> 
>  fs/userfaultfd.c               | 121 +++++++++++++++++++++++++++++++++
>  include/linux/sched/coredump.h |   1 +
>  include/linux/userfaultfd_k.h  |   5 ++
>  init/Kconfig                   |  11 +++
>  kernel/sysctl.c                |  11 +++
>  virt/kvm/kvm_main.c            |   7 ++
>  6 files changed, 156 insertions(+)
> 
> -- 
> 2.17.1
> 

-- 
Sincerely yours,
Mike.