Starting the kdbus discussions

[Lennart Poettering]

On Tue, 07.01.14 15:24, Tyler Hicks (tyhicks at canonical.com) wrote:

> Hi Lennart! I've added Greg to cc since John Johansen and I spoke with
> him about this in a hallway at Plumbers last year.
> 
> On 2014-01-02 18:08:42, Lennart Poettering wrote:
> > I am sorry, but to make this very clear: this is explicitly not an
> > option. There will not be a payload parser for kdbus in the kernel, as
> > long as the four developers who are working on it have any say. The
> > entire design is based on the concept that the payload is irrelevant to
> > the kernel, and routing is done only according to the metadata we
> > attach. This is a fundamental design decision of kdbus, and the four of
> > us (Kay, Daniel, Greg and I) will refuse this.
> 
> I agree that parsing the payload in the kernel is not something that we
> should do. However, I do think that there are some important metadata
> fields hidden away in the opaque sd_bus_message that should be exposed
> to the kernel by way of moving them to the kdbus_msg.
> 
> I'm proposing that the following fields be moved to the kdbus_msg:
> 
>  - message type
>  - destination
>  - sender

These three you have anyway, in one way or another. (or can mostly
reconstruct from the data available to the kernel, since the kernel can
distuingish method calls from method replies and broadcast signals --
though you cannot distuingish methods replies from method errors).

>  - path
>  - interface
>  - member

Nope. This is really not how this is supposed to work. These concepts
are opaque to the kernel on purpose. And the kernel does not use them
for anything. We are certainly not adding arbitrary fields to the kernel
structures, which nobody needs, which cannot be checked or validated by
the kernel, and complicate things and create a false sense of security
via a flawed security model... 

Really, we will not parse dbus messages in the kernel, and we will not
work around this design choice by then just moving arbitrary fields from
the userspace to the kernelspace header. 

You can do such things with dbus-daemon, or you can create a bus proxy,
like we did for compat reason, but the kernel should not look into the
payload like this or even have an understanding of so many userspace
details.

It's a bit like you'd write a letter with salutation, sender/receiver
addresses and valediction, which you then seal into an envelope, on
which you write sender and reciever address, then break the seal again,
and fax both the envelope and the letter to the destination....

>  1) The kernel provides a centralized, trustworthy location for making
>     security decisions
>     - The security burden isn't placed solely on the clients/services
>     - LSMs could hook into kdbus and perform fine-grained filtering,
>       much like what's possible in today's dbus-daemon
>     - kdbus security policy for an application can be located alongside
>       the security policy for other kernel-managed objects (files,
>       network permissions, IPC objects, etc.)
>     - The D-Bus security policy could be fine-grained, allowing filter
>       on certain interfaces, members, etc.
>     - I suppose that someone could even write an LSM to implement the XML
>       bus policy if they had a dependence on it

This really misses the point. We get rid of the XML policy because it is
misdesigned, and not just because we want to replace XML by somer other
formatting.

You cannot replace decisions PolicyKit is capable of by a decision
engine in the kernel. That's because PolicyKit can ask questions
interactively, can ask the network and whatnot. Trying to avoid fixing
things properly by moving all complexity and all understanding of the
userspace semantics into kernel space is never going to work. In
addition, in many many cases looking at the mere method call
interactions between clients and servers will not work anyway, if you
don't have any context for it.

>     - If the burden of auditing is placed on a client/service, they
>       will not likely have the required privileges (CAP_AUDIT_WRITE) to
>       write to the audit subsystem

auditd can also just subscribe to the monitor stream and get the data
without further intervention of the kernel, if auditing on that level is
really desired.

Sorry, but this is not happening. Find a different solution. Use
dbus-daemon, provide a bus proxy that enforces your ruleset, or hack up
your own kernel IPC, but kdbus is not going to do what you want it to
do.

Lennart

-- 
Lennart Poettering, Red Hat