Re: [PATCH bpf-next v8 00/11] Landlock LSM: Toward unprivileged sandboxing

["Mickaël Salaün" <mic@digikod.net>]

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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

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.

> 
> But using seccomp for this is indeed annoying.  It would be nice to
> use Landlock's ability to filter based on the filesystem type, for
> example.  So Tycho could write a Landlock rule like:
> 
> bool filter_mount(...)
> {
>   if (path needs emulation)
>     call_user_notifier();
> }
> 
> And it should work.
> 
> This means that, if both seccomp user notifiers and Landlock make it
> upstream, then there should probably be a way to have a user notifier
> bound to a seccomp filter and a set of landlock filters.
> 

Using seccomp filters and Landlock programs may be powerful. However,
for this use case, I think a *post-syscall* vDSO-like (which could get
some data returned by a Landlock program) may be much more flexible
(with less kernel code). What is needed here is a way to know the kernel
semantic (Landlock) and a way to patch userland without patching its
code (vDSO-like).


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