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

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Date:   Wed, 20 May 2020 17:16:34 -0400
From:   Andrea Arcangeli <aarcange@...hat.com>
To:     Lokesh Gidra <lokeshgidra@...gle.com>
Cc:     Suren Baghdasaryan <surenb@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Daniel Colascione <dancol@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Luis Chamberlain <mcgrof@...nel.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>,
        Peter Xu <peterx@...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, Tim Murray <timmurray@...gle.com>,
        Minchan Kim <minchan@...gle.com>,
        Sandeep Patil <sspatil@...gle.com>, kernel@...roid.com
Subject: Re: [PATCH 2/2] Add a new sysctl knob:
 unprivileged_userfaultfd_user_mode_only

On Wed, May 20, 2020 at 01:17:20PM -0700, Lokesh Gidra wrote:
> Adding the Android kernel team in the discussion.

Unless I'm mistaken that you can already enforce bit 1 of the second
parameter of the userfaultfd syscall to be set with seccomp-bpf, this
would be more a question to the Android userland team.

The question would be: does it ever happen that a seccomp filter isn't
already applied to unprivileged software running without
SYS_CAP_PTRACE capability?

If answer is "no" the behavior of the new sysctl in patch 2/2 (in
subject) should be enforceable with minor changes to the BPF
assembly. Otherwise it'd require more changes.

Thanks!
Andrea