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Date:   Tue, 5 Nov 2019 11:33:16 -0500
From:   Andrea Arcangeli <aarcange@redhat.com>
To:     Daniel Colascione <dancol@google.com>
Cc:     Andy Lutomirski <luto@kernel.org>,
        Mike Rapoport <rppt@linux.ibm.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Jann Horn <jannh@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Lokesh Gidra <lokeshgidra@google.com>,
        Nick Kralevich <nnk@google.com>,
        Nosh Minwalla <nosh@google.com>,
        Pavel Emelyanov <ovzxemul@gmail.com>,
        Tim Murray <timmurray@google.com>,
        Linux API <linux-api@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>
Subject: Re: [PATCH 1/1] userfaultfd: require CAP_SYS_PTRACE for
 UFFD_FEATURE_EVENT_FORK
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On Tue, Nov 05, 2019 at 08:06:49AM -0800, Daniel Colascione wrote:
> Sure, but the same argument applies to all the other permission checks
> that we do at open time, not at ioctl time. For better or for worse,
> the DAC-ish model used in most places is that access checks happen at
> file object creation time and anyone who has the FD can perform those
> operations later. Confusing the model by doing *some* permission
> checks at open time and *some* permission checks at usage time makes
> the system harder to understand.

The only case that requires change is if userland requested the
UFFD_FEATURE_EVENT_FORK feature (which AFIK only CRIU does) and that
request is done in the UFFDIO_API call not during the syscall.

Doing the check in the syscall would then break all non privileged
users like if we'd set /proc/sys/vm/unprivileged_userfaultfd to
zero. Qemu for example rightfully never runs with privilege (with a
few exceptions like Kata which should be fixed in fact) and it never
asks for the UFFD_FEATURE_EVENT_FORK feature either.