linux-kernel - Re: [PATCH 1/3] fs: add O

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

On Mon, Nov 3, 2014 at 9:37 AM, David Drysdale <drysdale@...gle.com> wrote:
> On Mon, Nov 3, 2014 at 3:42 PM, Andy Lutomirski <luto@...capital.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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