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

From: MickaÃl SalaÃn
Date: Sun Apr 01 2018 - 18:06:33 EST

On 03/09/2018 12:53 AM, Andy Lutomirski wrote:
> On Thu, Mar 8, 2018 at 11:51 PM, MickaÃl SalaÃn <mic@xxxxxxxxxxx> wrote:
>> On 07/03/2018 02:21, Andy Lutomirski wrote:
>>> 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.
>> vDSO is a code mapped for all processes. As you said, these processes
>> may use it or not. What I was thinking about is to use the same concept,
>> i.e. map a "shim" code into each processes pertaining to a particular
>> hierarchy (the same way seccomp filters are inherited across processes).
>> With a seccomp filter matching some syscall (e.g. mount, open), it is
>> possible to jump back to the shim code thanks to SECCOMP_RET_TRAP. This
>> shim code should then be able to emulate/patch what is needed, even
>> faking a file opening by receiving a file descriptor through a UNIX
>> socket. As did the Chrome sandbox, the seccomp filter may look at the
>> calling address to allow the shim code to call syscalls without being
>> catched, if needed. However, relying on SIGSYS may not fit with
>> arbitrary code. Using a new SECCOMP_RET_EMULATE (?) may be used to jump
>> to a specific process address, to emulate the syscall in an easier way
>> than only relying on a {c,e}BPF program.
> This could indeed be done, but I think that Tycho's approach is much
> cleaner and probably faster.

I like it too but how does this handle file descriptors?

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