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SubjectRe: [PATCH bpf-next v8 00/11] Landlock LSM: Toward unprivileged sandboxing
On Tue, Mar 6, 2018 at 10:25 PM, Mickaël Salaün <mic@digikod.net> wrote:
>
>
> On 28/02/2018 00:09, Andy Lutomirski wrote:
>> On Tue, Feb 27, 2018 at 10:03 PM, Mickaël Salaün <mic@digikod.net> wrote:
>>>
>>> On 27/02/2018 05:36, Andy Lutomirski wrote:
>>>> On Tue, Feb 27, 2018 at 12:41 AM, Mickaël Salaün <mic@digikod.net> wrote:
>>>>> Hi,
>>>>>
>>
>>>>>
>>>>> ## Why use the seccomp(2) syscall?
>>>>>
>>>>> Landlock use the same semantic as seccomp to apply access rule
>>>>> restrictions. It add a new layer of security for the current process
>>>>> which is inherited by its children. It makes sense to use an unique
>>>>> access-restricting syscall (that should be allowed by seccomp filters)
>>>>> which can only drop privileges. Moreover, a Landlock rule could come
>>>>> from outside a process (e.g. passed through a UNIX socket). It is then
>>>>> useful to differentiate the creation/load of Landlock eBPF programs via
>>>>> bpf(2), from rule enforcement via seccomp(2).
>>>>
>>>> This seems like a weak argument to me. Sure, this is a bit different
>>>> from seccomp(), and maybe shoving it into the seccomp() multiplexer is
>>>> awkward, but surely the bpf() multiplexer is even less applicable.
>>>
>>> I think using the seccomp syscall is fine, and everyone agreed on it.
>>>
>>
>> Ah, sorry, I completely misread what you wrote. My apologies. You
>> can disregard most of my email.
>>
>>>
>>>>
>>>> Also, looking forward, I think you're going to want a bunch of the
>>>> stuff that's under consideration as new seccomp features. Tycho is
>>>> working on a "user notifier" feature for seccomp where, in addition to
>>>> accepting, rejecting, or kicking to ptrace, you can send a message to
>>>> the creator of the filter and wait for a reply. I think that Landlock
>>>> will want exactly the same feature.
>>>
>>> I don't think why this may be useful at all her. Landlock does not
>>> filter at the syscall level but handles kernel object and actions as
>>> does an LSM. That is the whole purpose of Landlock.
>>
>> Suppose I'm writing a container manager. I want to run "mount" in the
>> container, but I don't want to allow moun() in general and I want to
>> emulate certain mount() actions. I can write a filter that catches
>> mount using seccomp and calls out to the container manager for help.
>> This isn't theoretical -- Tycho wants *exactly* this use case to be
>> supported.
>
> Well, I think this use case should be handled with something like
> LD_PRELOAD and a helper library. FYI, I did something like this:
> https://github.com/stemjail/stemshim

I doubt that will work for containers. Containers that use user
namespaces and, for example, setuid programs aren't going to honor
LD_PRELOAD.

>
> Otherwise, we should think about enabling a process to (dynamically)
> extend/patch the vDSO (similar to LD_PRELOAD but at the syscall level
> and works with static binaries) for a subset of processes (the same way
> seccomp filters are inherited). It may be more powerful and flexible
> than extending the kernel/seccomp to patch (buggy?) userland.

Egads!

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