linux-kernel - Re: [PATCH 2/2] Add a new sysctl knob: unprivileged_userfaultfd_user_mode

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Date:   Wed, 6 May 2020 15:38:16 -0400
From:   Peter Xu <peterx@...hat.com>
To:     Daniel Colascione <dancol@...gle.com>
Cc:     Jonathan Corbet <corbet@....net>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Luis Chamberlain <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Iurii Zaikin <yzaikin@...gle.com>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Shevchenko <andy.shevchenko@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Mike Rapoport <rppt@...ux.ibm.com>,
        Jerome Glisse <jglisse@...hat.com>, Shaohua Li <shli@...com>,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, timmurray@...gle.com,
        minchan@...gle.com, sspatil@...gle.com, lokeshgidra@...gle.com
Subject: Re: [PATCH 2/2] Add a new sysctl knob:
 unprivileged_userfaultfd_user_mode_only

On Wed, Apr 22, 2020 at 05:26:32PM -0700, Daniel Colascione wrote:
> +unprivileged_userfaultfd_user_mode_only
> +========================================
> +
> +This flag controls whether unprivileged users can use the userfaultfd
> +system calls to handle page faults in kernel mode.  If set to zero,
> +userfaultfd works with or without UFFD_USER_MODE_ONLY, modulo
> +unprivileged_userfaultfd above.  If set to one, users without
> +SYS_CAP_PTRACE must pass UFFD_USER_MODE_ONLY in order for userfaultfd
> +to succeed.  Prohibiting use of userfaultfd for handling faults from
> +kernel mode may make certain vulnerabilities more difficult
> +to exploit.
> +
> +The default value is 0.

If this is going to be added... I am thinking whether it should be easier to
add another value for unprivileged_userfaultfd, rather than a new sysctl. E.g.:

  "0": unprivileged userfaultfd forbidden
  "1": unprivileged userfaultfd allowed (both user/kernel faults)
  "2": unprivileged userfaultfd allowed (only user faults)

Because after all unprivileged_userfaultfd_user_mode_only will be meaningless
(iiuc) if unprivileged_userfaultfd=0.  The default value will also be the same
as before ("1") then.

Thanks,

-- 
Peter Xu