Re: kdbus: add code to gather metadata

[Daniel Mack]

Hi Andy,

On 10/30/2014 10:01 PM, Andy Lutomirski wrote:
> On Thu, Oct 30, 2014 at 8:54 AM, Daniel Mack <daniel@xxxxxxxxxx> wrote:

>> We merely allow user that are connected to a bus to query the
>> credentials of the creator of the bus they're connected to.
> 
> Why do you allow this?  What purpose does it serve?  Is that idea that
> each application will own one bus?  If so, what goes wrong if you only
> capture the specific credentials that the creator of a given bus asks
> to have captured?

There are different kinds of buses. There is the system bus and a number
of user buses. It's really useful to be able to identify the user who
owns one of these user buses, for the sake of access control. More
specifically, we have a compatibility service called "bus-proxy" that
speaks the old D-Bus socket protocol on one side and translates all
messages to kdbus messages onto the other. For that, it needs to enforce
the old D-Bus access control semantics, which is described in XML, and
has quite elaborate checks. In order to enforce it, it's relevant to be
able to compare peer credentials with bus owner credentials, because
there's usually a rule that the bus owner UID is allowed more than other
peers.

Sure, there are other ways to figure out the identity of the bus, but
it's really nice to have similar semantics for identifying the bus
owner. kdbus internally has that piece of information anyway, so we
decided to export it, optionally. However, that's really a minor detail
after all.

>> For example, if a privileged service can reboot the system, it checks
>> whether the asking peer has CAP_SYS_BOOT set. If it checks for uid==0
>> instead, and it works in your tests because you happen to test as root,
>> that just a bug in the service, right? But I might have missed your point.
> 
> The issue is the following: if have the privilege needed to talk to
> journald, I may want to enhance security by opening a connection to
> journald (and capture that privilege) and then drop privilege.  I
> should still be able to talk to journald.

Hmm, this is not how D-Bus works, and kdbus stays close the design
principles of D-Bus. There's no concept of 'opening connections to
services'. You just connect to a bus, and then on that bus, you send
individual messages and method calls to other services.

The design of Binder and D-Bus are fundamentally different in that
regard. On D-Bus, the focus is really about method call transaction (a
method call message and a corresponding reply message), and there's no
way to continuously reference a peer via the concept of a 'connection'.

This is why we have this functionality of passing over the caller creds
every time a method call is made. The focus is really on the individual
method call transaction, each one is individually routed, dispatched and
checked for permission. Hence, it should carry individual credential
information from the time the call is issued.

So, back to your example: you cannot 'open a connection to journald'.
You can only connect to a bus and send messages to journald.

> Alternatively, if the privilege needed to reboot is CAP_SYS_BOOT, then
> clients should send that capability bit.  Capturing extra information
> to try to give daemons the flexibility to change their authorization
> conditions later on just moves the problem if you need to change
> policy down the line.

This would be similar to changing the Linux kernel so that each system
call gets a set of capabilities passed in explicitly. RPC calls are very
much comparable to syscalls, though they don't transition into kernel
space, but simply into another process.

We've been augmenting what syscalls check on for access control ever
since. Initially, it check was UID based, then became capability based,
and nowadays we have a concept of MACs, that are actually different on
every system, because some systems use SElinux, some SMACK or some other
MAC. This is why this metadata should be implicit and controlled by the
receiver, not by the sender, because the implementation of the policy
might change eventually, be extended with more sophisticated access
control etc.

But it's not only about access control, there's also auditing. In order
to generate useful audit logs, a system service that is offering
privileged operation to certain clients needs to know the audit
credentials (sessionid and loginuid). Hence, this information needs to
be implicitly appended, controlled by that service. Because if it is not
implicitly appended, than it will more often be missing than expected
(simply because in real-life very few people actually use auditing), and
the system service would not be able to log about it.

So, this is not a metadata leak by accident but metadata that system
services need to know about in order to work properly. Individual
services will require slightly different components of these
credentials, but if you combine things, they need to know pretty much
all of the details we currently offer as implied metadata.

The system bus is about unprivileged apps asking system services for
system operations, in which case the system services must have a way to
know who wants them to do what. The purpose of a system bus is _not_ to
allow unprivileged peers to talk to each other, that's actually even
forbidden in the default policy. That's what user buses are for, and on
those, pretty much every client will have the same privileges anyway,
hence there's no information leak there either.

> What's the actual problem for logging?  I very much understand why a
> logging service never wants to log an incorrect credential (and legacy
> syslog has serious problems here because it doesn't even try to
> capture credential), but what's wrong with having a log that shows the
> uid for legit log messages and that reliably says "declined to state"
> for messages that decline to state.

A system's administrator should be able to gather all sorts of
information about things that happened on a system. Trying to hide
associated metadata is not how things are done anyway - we show it in
numerous places, in /proc, in SCM_CREDENTIALS, by listing /tmp or /home etc.

The separation to limit what is passed around is, in our concept, rather
on the level of connecting to separate buses, PID namespaces, kdbus
domains etc, than to suppress information.

> (Also, I presume that cmdline is for logging.  Keep in mind that the
> cmdline is yanked from user memory and can be freely spoofed.)

Sure. But If a task did that, we want to know about it, and log it
accordingly. Journald provides such features already today, and it's a
great deal in detecting runtime inconsistencies between a task's real
nature and what is displayed in 'ps'.



Thanks,
Daniel
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