Re: [RFC] Restrict the untrusted devices, to bind to only a set of "whitelisted" drivers
From: Greg Kroah-Hartman
Date: Sun Jun 07 2020 - 07:37:02 EST
On Fri, Jun 05, 2020 at 06:08:28PM -0700, Rajat Jain wrote:
> Hello Greg,
> Thank you for continuing to work with me through this.
> On Fri, Jun 5, 2020 at 1:02 AM Greg Kroah-Hartman
> <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> > On Thu, Jun 04, 2020 at 12:38:18PM -0700, Rajat Jain wrote:
> > > Hello,
> > >
> > > I spent some more thoughts into this...
> > >
> > > On Wed, Jun 3, 2020 at 5:16 AM Greg Kroah-Hartman
> > > <gregkh@xxxxxxxxxxxxxxxxxxx> wrote:
> > > >
> > > > On Wed, Jun 03, 2020 at 04:51:18AM -0700, Rajat Jain wrote:
> > > > > Hello,
> > > > >
> > > > > >
> > > > > > > Thanks for the pointer! I'm still looking at the details yet, but a
> > > > > > > quick look (usb_dev_authorized()) seems to suggest that this API is
> > > > > > > "device based". The multiple levels of "authorized" seem to take shape
> > > > > > > from either how it is wired or from userspace choice. Once authorized,
> > > > > > > USB device or interface is authorized to be used by *anyone* (can be
> > > > > > > attached to any drivers). Do I understand it right that it does not
> > > > > > > differentiate between drivers?
> > > > > >
> > > > > > Yes, and that is what you should do, don't fixate on drivers. Users
> > > > > > know how to control and manage devices. Us kernel developers are
> > > > > > responsible for writing solid drivers and getting them merged into the
> > > > > > kernel tree and maintaining them over time. Drivers in the kernel
> > > > > > should always be trusted, ...
> > > > >
> > > > > 1) Yes, I agree that this would be ideal, and this should be our
> > > > > mission. I should clarify that I may have used the wrong term
> > > > > "Trusted/Certified drivers". I didn't really mean that the drivers may
> > > > > be malicious by intent. What I really meant is that a driver may have
> > > > > an attack surface, which is a vulnerability that may be exploited.
> > > >
> > > > Any code has such a thing, proving otherwise is a tough problem :)
> > > >
> > > > > Realistically speaking, finding vulnerabilities in drivers, creating
> > > > > attacks to exploit them, and fixing them is a never ending cat and
> > > > > mouse game. At Least "identifying the vulnerabilities" part is better
> > > > > performed by security folks rather than driver writers.
> > > >
> > > > Are you sure about that? It's hard to prove a negative :)
> > > >
> > > > > Earlier in the
> > > > > thread I had mentioned certain studies/projects that identified and
> > > > > exploited such vulnerabilities in the drivers. I should have used the
> > > > > term "Vetted Drivers" maybe to convey the intent better - drivers that
> > > > > have been vetted by a security focussed team (admin). What I'm
> > > > > advocating here is an administrator's right to control the drivers
> > > > > that he wants to allow for external ports on his systems.
> > > >
> > > > That's an odd thing, but sure, if you want to write up such a policy for
> > > > your systems, great. But that policy does not belong in the kernel, it
> > > > belongs in userspace.
> > > >
> > > > > 2) In addition to the problem of driver negligences / vulnerabilities
> > > > > to be exploited, we ran into another problem with the "whitelist
> > > > > devices only" approach. We did start with the "device based" approach
> > > > > only initially - but quickly realized that anything we use to
> > > > > whitelist an external device can only be based on the info provided by
> > > > > *that device* itself. So until we have devices that exchange
> > > > > certificates with kernel , it is easy for a malicious device to
> > > > > spoof a whitelisted device (by presenting the same VID:DID or any
> > > > > other data that is used by us to whitelist it).
> > > > >
> > > > >  https://www.intel.com/content/www/us/en/io/pci-express/pcie-device-security-enhancements-spec.html
> > > > >
> > > > > I hope that helps somewhat clarify how / why we reached here?
> > > >
> > > > Kind of, I still think all you need to do is worry about controling the
> > > > devices and if a driver should bind to it or not. Again, much like USB
> > > > has been doing for a very long time now. The idea of "spoofing" ids
> > > > also is not new, and has been around for a very long time as well, and
> > > > again, the controls that the USB core gives you allows you to make any
> > > > type of policy decision you want to, in userspace.
> > >
> > > Er, *currently* it doesn't allow the userspace to make the particular
> > > policy I want to, right? Specifically, today an administrator can not
> > > control which USB *drivers* he wants to allow on an *external* USB
> > > port.
> > Not true, you can do that today with the explicit binding/unbinding of
> > devices to drivers in userspace. Been there for many decades :)
> Not sure if I understood. Can you please elaborate how that helps
> implement the policy I want?
> > But, think this through, since when do you have _multiple_ drivers that
> > have support to control the same type of device? We almost never allow
> > that in the kernel today as that way lies madness (no heiarchy of
> > drivers to bind to what devices and so on.)
> > We always strive to keep a one-to-one mapping of "this device is only
> > allowed to be controlled by this one driver" today, why would you want
> > to change that basic premise now?
> No, I don't want to change that premise. Multiple drivers for a single
> device is not the goal at all.
> > > He can only control which USB devices he wants to authorize, but
> > > once authorized, they are free to bind to any of the USB drivers.
> > Since when do different drivers control the same type of USB device? :)
> Sorry, I should have used better wording:
> "..., a malicious device can choose which (authorized/whitelisted)
> device to spoof, and *thus* choose a driver to attach to"
> Since the only data admin can use to decide to authorize the device,
> is provided by device itself, authorization is really just a farce.
> >From Documentation/usb/authorization.rst:
> "Just checking if the class, type and protocol match something is the worse
> security verification you can make (or the best, for someone willing
> to break it). If you need something secure, use crypto and Certificate
> Authentication or stuff like that."
> Truth be told, there is nothing else really available today. While
> certificate exchanges may be the future, the challenge is to deal with
> devices at hand.
> > > So if I want to allow the administrator to implement a policy that
> > > allows him to control the drivers for external ports, we'll need to
> > > enhance the current code (whether we want to do it specific to a bus,
> > > or more generically in the driver core). Are we on the same page?
> > >
> > > To implement the policy that I want to in the driver core, what is
> > > missing today in driver core is a distinction between "internal" and
> > > "external" devices. Some buses have this knowledge locally today (PCI
> > > has "untrusted" flag which can be used, USB uses hcd->wireless and
> > > hub->port->connect_type) but it is not shared with the core.
> > Note the wireless USB code should now be gone from the tree. If you see
> > any remants of it floating around, let me know and I will remove them, I
> > think there might be a few bits left that I missed.
> > > So just to make sure if I'm thinking in the right direction, this is
> > > what I'm thinking:
> > >
> > > 1) The device core needs a notion of internal vs external devices (a
> > > flag) - a knowledge that needs to be filled in by the bus as it
> > > discovers the device.
> > Nope, don't go down this path. We tried to do this for USB where the
> > BIOS tells us that a device is "internal" vs. "external" but in reality,
> > BIOSes get this wrong and it's not always all that useful.
> > And why would you somehow "trust" a device that is in your system more
> > than one is not? The same driver binds to it no matter what (as I state
> > above), so you should be able to trust it the same.
> There are multiple reasons for trust level to be different for
> "internal" vs "external" devices. Speaking for the laptop world at
> least (and I suspect same is true for most of OEM products):
> 1) The hardware, firmware, and in some cases even the supply chain is
> quite tightly controlled and audited for "internal devices". OTOH, we
> don't even know what an "external device" may look like.
> 2) Most of the internal devices are soldered on board, and can't be
> accessed by a malicious person, atleast without the owner knowing.
> OTOH, external device attack is very easy (Imagine malicious user
> plugging in a USB stick on an unattended laptop / internet cafes /
> airports etc) - the owner wouldn't even know.
> 3) Internal devices do not physically travel to another system. OTOH,
> external devices, even if not malicious, may get infected since they
> travel between multiple systems.
> 4) New devices' support (new drivers) keeps on getting added in the
> kernel (which is a good thing!). But that means while the "internal
> device space" is fixed at product release, the "external device space"
> is unbounded and keeps on increasing. It is good from a functionality
> point of view, but not from a security point of view.
> > > 2) The driver core needs to allow an admin to provide a whitelist of
> > > drivers for external devices. (Via Command line or a driver flag.
> > > Default = everything is whitelisted).
> > Again, nope, no difference, see above.
> > > 3) While matching a driver to a device, the driver core needs to
> > > impose the whitelist if the device is external, and if the
> > > administrator has provided a whitelist.
> > Ick, no, again, work on a per-device authorized setting. That way it
> > works the same all across the system. Don't get stuck in a "external
> > vs. internal" discussion as this will get messy really quickly (think
> > about "internal" devices with "external" links to them like PCI
> > "drawers" of devices that we currently support on large systems. Or
> > things like thunderbolt hubs with "internal" devices like I have on my
> > desktop right now.
> Good point. I'm not good at terminologies - by "external", I meant
> anything that is not in the physical boundary of the host system as
> shipped. It should be defined by the individual buses who learn it
> from the platform (BIOS / Device tree / Discovery process etc).
> > In summary, if a driver is "trusted enough" to control an internal
> > device, it should be "trusted enough" to control an external device. If
> > not, then fix that driver so that you do "trust" it.
> That is indeed our goal, and we do intend to inspect and send patch
> fixes upstream. However the problem is inspecting drivers for security
> and finding and fixing issues is a long drawn process - and has a lot
> of dependencies on different maintainers. It is not possible to front
> load all the effort and release the product only when *all* drivers
> are fixed (We want to begin with a *NULL* whitelist of drivers and
> then build it *slowly*). OTOH, it would be unfair to block a product
> because not all drivers could be inspected or fixed in time for
> security issues.
> I feel that I have failed to explain the problems clearly. I'm copying
> a blurb earlier from this thread, which explains the context and the
> problem we're trying to solve.
> ================ BEGIN ============================
> So here is our dilemma. In the laptop world:
> 1) Today (Pre-Thunderbolt 3 / Pre-USB4), there is a mix of trusted /
> untrusted drivers that we (or any OEMs) are shipping on their laptops.
> Yes, there is some (calculated) risk that everyone is taking - because
> currently PCI bus does not extend outside the laptop *easily*. Yes I
> understand systems may have external PCI slots, but that is rather
> rare in the laptop world I think. The risks of the existing drivers
> are limited to the devices that were built into the system, and since
> the drivers, firmware updates, (and supply chain in some cases) are
> controlled by us, such internal devices are conceivably more secure
> than something random that the user may plug in. If the user opens the
> chassis to replace a piece of hardware with something else, all bets
> are off. Yes, we're still susceptible to the NIC driver attacks that
> you talk about it along with other potential vulnerabilities, but this
> is just convey our current baseline level of risk/security.
> 2) Now, we want to enable technology of tomorrow :-) (Thunderbolt 3 /
> USB4) on laptops, which allows to very *easily* extend the internal
> PCI bus to the outside devices. Note it doesn't require to open a
> laptop, and anyone can plug a device onto a port. This throws the
> system open to a lot of DMA attacks now, which it did not have to deal
> with earlier. Essentially with the advent of technology to expand PCIe
> outside of system chassis, the attacks have become much more easier,
> we can no longer control or monitor device hardware or firmware, and
> thus the level of risk has clearly increased. So what we are trying to
> find here, is a good path to enable these new technologies, that keeps
> keeps our baseline level of risk/security unchanged, and to also not
> regress in functionality in supporting devices as much as possible.
> 3) Now you are certainly right that one path could be a binary
> decision to ship or not ship a driver, or fix any issues with the
> driver, or change the driver to differentiate between external and
> internal ports. However, there are multiple factors that pose
> practical problems (why regress internal devices that we tightly
> control? Why regress systems that don't have such external ports? Need
> to front load all effort in vetting the drivers before hand before the
> first release. Work with each and individual driver etc).
> 4) The other path that this proposal aims to take is that by applying
> a whitelist of drivers to external ports only, we're going to be able
> to *slowly* build this whitelist. We can start with a NULL whitelist.
> Which means that existing internal devices continue to work, and
> external devices on PCI don't pose a risk. With ACS and IOMMU
> restrictions in place, the security/risk baseline remains unchanged.
> The existing devices are not regressed. As we vet and whitelist the
> drivers, we start supporting more and more USB4 and Thunderbolt3
> devices. Until then, those devices when plugged, can continue to work
> in the "USB / legacy mode" (I forgot what it is called).
> 5) To give an example, assume we don't trust the PCI nvme driver and
> don't want to whitelist it for external devices given there are so
> many off the shelf devices with questionable firmware. But we
> certainly need to enable it for internal NVME devices (that we may
> have audited the firmware for, and control our supply chain) in order
> to boot. With my proposal, until we whitelist it, the internal devices
> continue to work, the external NVMEs switch to "USB storage device"
> mode and thus go via a USB bridge so they cannot directly DMA into
> host memory directly. Keep in mind that whitelisting a driver may be
> handled by a separate security team, and may take long time depending
> on the driver. The proposal allows us to release laptops with
> Thunderbolt3/USB4 support and add peripheral support as we go.
> 6) Also small nit: consider the other scenario (I think this may not
> be as important but still worth a thought). Assume the security team
> finds a new vulnerability in a whitelisted driver, and want to take it
> out of whitelist. Now, this really isn't possible if there was no
> distinction between internal / external devices, and an internal
> device uses that driver to boot.
> ================ END ============================
> I feel I've described a problem, looked around to see what is
> available, and tried to explain why it doesn't work for us given the
> constraints. When it comes to security, the world is far from ideal.
> Though in principle a "driver vulnerability" applies equally to any
> device, in reality the threat vector is different for "internal" vs
> "external" devices.
Your "problem" I think can be summed up a bit more concise:
- you don't trust kernel drivers to be "secure" for untrusted
- you only want to bind kernel drivers to "internal" devices
automatically as you "trust" drivers in that situation.
- you want to only bind specific kernel drivers that you somehow
feel are "secure" to untrusted devices "outside" of a system
when those devices are added to the system.
Is that correct?
If so, fine, you can do that today with the bind/unbind ability of
drivers, right? After boot with your "trusted" drivers bound to
"internal" devices, turn off autobind of drivers to devices and then
manually bind them when you see new devices show up, as those "must" be
from external devices (see the bind/unbind files that all drivers export
for how to do this, and old lwn.net articles, this feature has been
around for a very long time.)
I know for USB you can do this, odds are PCI you can turn off
autobinding as well, as I think this is a per-bus flag somewhere. If
that's not exported to userspace, should be trivial to do so, should be
somewere in the driver model already...
Ah, yes, look at the "drivers_autoprobe" and "drivers_probe" files in
sysfs for all busses. Do those not work for you?
My other points are the fact that you don't want to put policy in the
kernel, and I think that you can do everything you want in userspace
today, except maybe the fact that trying to determine what is "inside"
and "outside" is not always easy given that most hardware does not
export this information properly, if at all. Go work with the firmware
people on that issue please, that would be most helpful for everyone
involved to get that finally straightened out.
> I feel a lot of resistance to the proposal, however, I'm not hearing
> any realistic solutions that may help us to move forward. We want to
> go with a solution that is acceptable upstream as that is our mission,
> and also helps the community, however the behemoth task of "inspect
> all drivers and fix them" before launching a product is really an
> unfair ask I feel :-(. Can you help us by suggesting a proposal that
> does not require us to trust a driver equally for internal / external
I have no idea why you feel you have to "inspect all drivers" other than
the fact that for some reason _you_ do not feel they are secure today.
What type of "assurance" are you, or anyone else going to be able to
provide for any kernel driver that would meet such a "I feel good now"
level? Have you done that work for any specific driver already so that
you can show us what you mean by this effort? Perhaps it's as simple as
"oh look, this tool over here runs 'clean' on the source code, all is
good!", or not, I really have no idea.