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Date: Mon, 2 Apr 2018 00:04:36 +0200
From: Mickaël Salaün <mic@...ikod.net>
To: Andy Lutomirski <luto@...nel.org>
Cc: Tycho Andersen <tycho@...ho.ws>, LKML <linux-kernel@...r.kernel.org>,
Alexei Starovoitov <ast@...nel.org>,
Arnaldo Carvalho de Melo <acme@...nel.org>,
Casey Schaufler <casey@...aufler-ca.com>,
Daniel Borkmann <daniel@...earbox.net>,
David Drysdale
<drysdale@...gle.com>,
"David S . Miller" <davem@...emloft.net>,
"Eric W . Biederman" <ebiederm@...ssion.com>,
James Morris <james.l.morris@...cle.com>, Jann Horn <jann@...jh.net>,
Jonathan Corbet <corbet@....net>,
Michael Kerrisk <mtk.manpages@...il.com>,
Kees Cook <keescook@...omium.org>, Paul Moore <paul@...l-moore.com>,
Sargun Dhillon <sargun@...gun.me>,
"Serge E . Hallyn" <serge@...lyn.com>, Shuah Khan <shuah@...nel.org>,
Tejun Heo <tj@...nel.org>, Thomas Graf <tgraf@...g.ch>,
Will Drewry <wad@...omium.org>,
Kernel Hardening <kernel-hardening@...ts.openwall.com>,
Linux API <linux-api@...r.kernel.org>,
LSM List <linux-security-module@...r.kernel.org>,
Network Development <netdev@...r.kernel.org>
Subject: [PATCH bpf-next v8 00/11] Landlock LSM: Toward unprivileged
sandboxing
On 03/09/2018 12:53 AM, Andy Lutomirski wrote:
> On Thu, Mar 8, 2018 at 11:51 PM, Mickaël Salaün <mic@...ikod.net> wrote:
>>
>> On 07/03/2018 02:21, Andy Lutomirski wrote:
>>> On Tue, Mar 6, 2018 at 11:06 PM, Mickaël Salaün <mic@...ikod.net> 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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