Re: [PATCH 1/2] rtnetlink: gate MAC address with an LSM hook

[Paul Moore <paul@paul-moore.com>]

On Fri, Aug 23, 2019 at 7:41 AM Jeffrey Vander Stoep <jeffv@google.com> wrote:
> On Fri, Aug 23, 2019 at 1:19 AM David Miller <davem@davemloft.net> wrote:
> > From: Jeff Vander Stoep <jeffv@google.com>
> > Date: Wed, 21 Aug 2019 15:45:47 +0200
> >
> > > MAC addresses are often considered sensitive because they are
> > > usually unique and can be used to identify/track a device or
> > > user [1].
> > >
> > > The MAC address is accessible via the RTM_NEWLINK message type of a
> > > netlink route socket[2]. Ideally we could grant/deny access to the
> > > MAC address on a case-by-case basis without blocking the entire
> > > RTM_NEWLINK message type which contains a lot of other useful
> > > information. This can be achieved using a new LSM hook on the netlink
> > > message receive path. Using this new hook, individual LSMs can select
> > > which processes are allowed access to the real MAC, otherwise a
> > > default value of zeros is returned. Offloading access control
> > > decisions like this to an LSM is convenient because it preserves the
> > > status quo for most Linux users while giving the various LSMs
> > > flexibility to make finer grained decisions on access to sensitive
> > > data based on policy.
> > >
> > > [1] https://adamdrake.com/mac-addresses-udids-and-privacy.html
> > > [2] Other access vectors like ioctl(SIOCGIFHWADDR) are already covered
> > > by existing LSM hooks.
> > >
> > > Signed-off-by: Jeff Vander Stoep <jeffv@google.com>
> >
> > I'm sure the MAC address will escape into userspace via other means,
> > dumping pieces of networking config in other contexts, etc.  I mean,
> > if I can get a link dump, I can dump the neighbor table as well.
>
> These are already gated by existing LSM hooks and capability checks.
> They are not allowed on mandatory access control systems unless explicitly
> granted.
>
> > I kinda think this is all very silly whack-a-mole kind of stuff, to
> > be quite honest.
>
> We evaluated mechanisms for the MAC to reach unprivileged apps.
> A number of researchers have published on this as well such as:
> https://www.usenix.org/conference/usenixsecurity19/presentation/reardon
>
> Three "leaks" were identified, two have already been fixed.
> -ioctl(SIOCGIFHWADDR). Fixed using finer grained LSM checks
> on socket ioctls (similar to this change).
> -IPv6 IP addresses. Fixed by no longer including the MAC as part
> of the IP address.
> -RTM_NEWLINK netlink route messages. The last mole to be whacked.
>
> > And like others have said, tomorrow you'll be like "oh crap, we should
> > block X too" and we'll get another hook, another config knob, another
> > rulset update, etc.
>
> This seems like an issue inherent with permissions/capabilities. I don’t
> think we should abandon the concept of permissions because someone
> can forget to add a check.  Likewise, if someone adds new code to the
> kernel and omits a capable(CAP_NET_*) check, I would expect it to be
> fixed like any other bug without the idea of capability checks being tossed
> out.
>
> We need to do something because this information is being abused. Any
> recommendations? This seemed like the simplest approach, but I can
> definitely appreciate that it has downsides.
>
> I could make this really generic by adding a single hook to the end of
> sock_msgrecv() which would allow an LSM to modify the message to omit
> the MAC address and any other information that we deem as sensitive in the
> future. Basically what Casey was suggesting. Thoughts on that approach?

I apologize for the delay in responding; I'm blaming LSS-NA travel.

I'm also not a big fan of inserting the hook in rtnl_fill_ifinfo(); as
presented it is way too specific for a LSM hook for me to be happy.
However, I do agree that giving the LSMs some control over netlink
messages makes sense.  As others have pointed out, it's all a matter
of where to place the hook.

If we only care about netlink messages which leverage nlattrs I
suppose one option that I haven't seen mentioned would be to place a
hook in nla_put().  While it is a bit of an odd place for a hook, it
would allow the LSM easy access to the skb and attribute type to make
decisions, and all of the callers should already be checking the
return code (although we would need to verify this).  One notable
drawback (not the only one) is that the hook is going to get hit
multiple times for each message.

--
paul moore
www.paul-moore.com