Re: [PATCH 1/3] fs: add O_BENEATH flag to openat(2)

[Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org>]

On Mon, Nov 3, 2014 at 10:25 AM, Julien Tinnes <jln-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org> wrote:
> On Mon, Nov 3, 2014 at 9:37 AM, David Drysdale <drysdale-hpIqsD4AKlfQT0dZR+AlfA@public.gmane.org> wrote:
>>
>> On Mon, Nov 3, 2014 at 3:42 PM, Andy Lutomirski <luto-kltTT9wpgjJwATOyAt5JVQ@public.gmane.org>
>> wrote:
>> > On Mon, Nov 3, 2014 at 7:20 AM, Al Viro <viro-RmSDqhL/yNMiFSDQTTA3OLVCufUGDwFn@public.gmane.org> wrote:
>> >> On Mon, Nov 03, 2014 at 11:48:23AM +0000, David Drysdale wrote:
>> >>> Add a new O_BENEATH flag for openat(2) which restricts the
>> >>> provided path, rejecting (with -EACCES) paths that are not beneath
>> >>> the provided dfd.  In particular, reject:
>> >>>  - paths that contain .. components
>> >>>  - paths that begin with /
>> >>>  - symlinks that have paths as above.
>> >>
>> >> Yecch...  The degree of usefulness aside (and I'm not convinced that it
>> >> is non-zero),
>> >
>> > 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.

You can do this with user namespaces, too.  But this is way more
complicated than it should be, and it has a lot more overhead.

--Andy

>
> 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



-- 
Andy Lutomirski
AMA Capital Management, LLC