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From:   Andy Lutomirski <luto@kernel.org>
Date:   Tue, 5 Nov 2019 08:00:26 -0800
Message-ID: <CALCETrX=VmSjD6kLT6tuZQ4Efhc_13vZrw1mo4Z2iKqZTT-bzg@mail.gmail.com>
Subject: Re: [PATCH 1/1] userfaultfd: require CAP_SYS_PTRACE for UFFD_FEATURE_EVENT_FORK
To:     Daniel Colascione <dancol@google.com>
Cc:     Mike Rapoport <rppt@linux.ibm.com>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Andrea Arcangeli <aarcange@redhat.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Andy Lutomirski <luto@kernel.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>
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On Tue, Nov 5, 2019 at 7:55 AM Daniel Colascione <dancol@google.com> wrote:
>
> On Tue, Nov 5, 2019 at 7:29 AM Mike Rapoport <rppt@linux.ibm.com> wrote:
> >
> > Current implementation of UFFD_FEATURE_EVENT_FORK modifies the file
> > descriptor table from the read() implementation of uffd, which may have
> > security implications for unprivileged use of the userfaultfd.
> >
> > Limit availability of UFFD_FEATURE_EVENT_FORK only for callers that have
> > CAP_SYS_PTRACE.
>
> Thanks. But shouldn't we be doing the capability check at
> userfaultfd(2) time (when we do the other permission checks), not
> later, in the API ioctl?

The ioctl seems reasonable to me.  In particular, if there is anyone
who creates a userfaultfd as root and then drop permissions, a later
ioctl could unexpectedly enable FORK.

This assumes that the code in question is only reachable through
ioctl() and not write().