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Date: Mon, 3 Nov 2014 10:26:43 -0800
Message-ID: <CAKyRK=j=yrWxwgAaqTiM+sEaf+-kNmYm3efK9N83oXafEwFa_Q@mail.gmail.com>
Subject: Re: [PATCH 1/3] fs: add O_BENEATH flag to openat(2)
From: Julien Tinnes <jln@chromium.org>
To: David Drysdale <drysdale@google.com>
Cc: Andy Lutomirski <luto@amacapital.net>, Al Viro <viro@zeniv.linux.org.uk>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Meredydd Luff <meredydd@senatehouse.org>,
        Will Drewry <wad@chromium.org>,
        Jorge Lucangeli Obes <jorgelo@google.com>,
        Ricky Zhou <rickyz@google.com>, Lee Campbell <leecam@google.com>,
        Mike Depinet <mdepinet@google.com>,
        James Morris <james.l.morris@oracle.com>,
        Paolo Bonzini <pbonzini@redhat.com>, Paul Moore <paul@paul-moore.com>,
        Christoph Hellwig <hch@infradead.org>,
        "Eric W. Biederman" <ebiederm@xmission.com>,
        Linux API <linux-api@vger.kernel.org>,
        LSM List <linux-security-module@vger.kernel.org>
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Sender: linux-kernel-owner@vger.kernel.org

On Mon, Nov 3, 2014 at 9:37 AM, David Drysdale <drysdale@google.com> wrote:
> On Mon, Nov 3, 2014 at 3:42 PM, Andy Lutomirski <luto@amacapital.net> wrote:
>> This is extremely useful in conjunction with seccomp.
>
> Yes, that was my understanding of how the Chrome[OS] folk wanted
> to use it.

Yes, exactly. Without this, if we want to give a sandboxed process A
access to a directory, we need to:
1. Create a new 'broker" process B
2. Make sure to have an IPC channel between A and B.
3. SIGSYS open() and openat() in A via seccomp-bpf
4. Have an async-signal-safe handler that can IPC open / openat.

There is a lot of hidden complexity in such a set-up. For instance, if
you need to prevent contention, the number of threads in the broker B
should scale automatically.

This is 'fine' (but undesirable) for a big beast such as Chromium
which needs such a complex set-ups anyways, but David's patch would
make it a lot easier to build a sandbox and whitelist directories for
everyone, simply by enforcing O_BENEATH in seccomp and whitelisting
open directory file descriptors in the sandboxed process.

Julien
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