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Date:   Wed, 6 May 2020 15:38:16 -0400
From:   Peter Xu <peterx@redhat.com>
To:     Daniel Colascione <dancol@google.com>
Cc:     Jonathan Corbet <corbet@lwn.net>,
        Alexander Viro <viro@zeniv.linux.org.uk>,
        Luis Chamberlain <mcgrof@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Iurii Zaikin <yzaikin@google.com>,
        Mauro Carvalho Chehab <mchehab+samsung@kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Shevchenko <andy.shevchenko@gmail.com>,
        Vlastimil Babka <vbabka@suse.cz>,
        Mel Gorman <mgorman@techsingularity.net>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Mike Rapoport <rppt@linux.ibm.com>,
        Jerome Glisse <jglisse@redhat.com>, Shaohua Li <shli@fb.com>,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-fsdevel@vger.kernel.org, timmurray@google.com,
        minchan@google.com, sspatil@google.com, lokeshgidra@google.com
Subject: Re: [PATCH 2/2] Add a new sysctl knob:
 unprivileged_userfaultfd_user_mode_only
Message-ID: <20200506193816.GB228260@xz-x1>
References: <20200423002632.224776-1-dancol@google.com>
 <20200423002632.224776-3-dancol@google.com>
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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