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

[Julien Tinnes]

On Mon, Nov 3, 2014 at 9:37 AM, David Drysdale <drysdale@xxxxxxxxxx> wrote:
> On Mon, Nov 3, 2014 at 3:42 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> 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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