linux-kernel - Re: [PATCH 1/1] userfaultfd: require CAP_SYS_PTRACE for UFFD_FEATURE_EVENT

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Date:   Tue, 5 Nov 2019 07:55:17 -0800
From:   Daniel Colascione <dancol@...gle.com>
To:     Mike Rapoport <rppt@...ux.ibm.com>
Cc:     linux-kernel <linux-kernel@...r.kernel.org>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Andy Lutomirski <luto@...nel.org>,
        Jann Horn <jannh@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Lokesh Gidra <lokeshgidra@...gle.com>,
        Nick Kralevich <nnk@...gle.com>,
        Nosh Minwalla <nosh@...gle.com>,
        Pavel Emelyanov <ovzxemul@...il.com>,
        Tim Murray <timmurray@...gle.com>,
        Linux API <linux-api@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>
Subject: Re: [PATCH 1/1] userfaultfd: require CAP_SYS_PTRACE for UFFD_FEATURE_EVENT_FORK

On Tue, Nov 5, 2019 at 7:29 AM Mike Rapoport <rppt@...ux.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?