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Date: Thu, 8 Mar 2018 23:53:52 +0000
From: Andy Lutomirski <>
To: Mickaël Salaün <>
Cc: Tycho Andersen <>, LKML <>, 
	Alexei Starovoitov <>, Arnaldo Carvalho de Melo <>, 
	Casey Schaufler <>, Daniel Borkmann <>, 
	David Drysdale <>, "David S . Miller" <>, 
	"Eric W . Biederman" <>, James Morris <>, 
	Jann Horn <>, Jonathan Corbet <>, 
	Michael Kerrisk <>, Kees Cook <>, 
	Paul Moore <>, Sargun Dhillon <>, 
	"Serge E . Hallyn" <>, Shuah Khan <>, Tejun Heo <>, 
	Thomas Graf <>, Will Drewry <>, 
	Kernel Hardening <>, Linux API <>, 
	LSM List <>, 
	Network Development <>
Subject: Re: [PATCH bpf-next v8 00/11] Landlock LSM: Toward unprivileged sandboxing

On Thu, Mar 8, 2018 at 11:51 PM, Mickaël Salaün <> wrote:
> On 07/03/2018 02:21, Andy Lutomirski wrote:
>> On Tue, Mar 6, 2018 at 11:06 PM, Mickaël Salaün <> 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:
>>>>> I doubt that will work for containers.  Containers that use user
>>>>> namespaces and, for example, setuid programs aren't going to honor
>>>> 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.

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