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