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

[Andy Lutomirski]

On Tue, Mar 6, 2018 at 11:06 PM, MickaÃl SalaÃn <mic@xxxxxxxxxxx> wrote:
>
> On 06/03/2018 23:46, Tycho Andersen wrote:
>> On Tue, Mar 06, 2018 at 10:33:17PM +0000, Andy Lutomirski wrote:
>>>>> 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.
>>
>> Or anything that calls syscalls directly, like go programs.
>
> That's why the vDSO-like approach. Enforcing an access control is not
> the issue here, patching a buggy userland (without patching its code) is
> the issue isn't it?
>
> As far as I remember, the main problem is to handle file descriptors
> while "emulating" the kernel behavior. This can be done with a "shim"
> code mapped in every processes. Chrome used something like this (in a
> previous sandbox mechanism) as a kind of emulation (with the current
> seccomp-bpf ). I think it should be doable to replace the (userland)
> emulation code with an IPC wrapper receiving file descriptors through
> UNIX socket.
>

Can you explain exactly what you mean by "vDSO-like"?

When a 64-bit program does a syscall, it just executes the SYSCALL
instruction.  The vDSO isn't involved at all.  32-bit programs usually
go through the vDSO, but not always.

It could be possible to force-load a DSO into an entire container and
rig up seccomp to intercept all SYSCALLs not originating from the DSO
such that they merely redirect control to the DSO, but that seems
quite messy.