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

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Message-ID: <20200507131503.02aba5a6@lwn.net>
Date:   Thu, 7 May 2020 13:15:03 -0600
From:   Jonathan Corbet <corbet@....net>
To:     Peter Xu <peterx@...hat.com>
Cc:     Daniel Colascione <dancol@...gle.com>,
        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, 6 May 2020 15:38:16 -0400
Peter Xu <peterx@...hat.com> wrote:

> 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.

It occurs to me to wonder whether this interface should also let an admin
block *privileged* user from handling kernel-space faults?  In a
secure-boot/lockdown setting, this could be a hardening measure that keeps
a (somewhat) restricted root user from expanding their privilege...?

jon