Re: [RFC] Restrict the untrusted devices, to bind to only a set of "whitelisted" drivers

[Rajat Jain <rajatja@google.com>]

On Mon, May 4, 2020 at 4:59 AM Jean-Philippe Brucker
<jean-philippe@linaro.org> wrote:
>
> On Mon, May 04, 2020 at 01:47:27PM +0200, Jean-Philippe Brucker wrote:
> > Hi,
> >
> > On Fri, May 01, 2020 at 04:07:10PM -0700, Rajat Jain wrote:
> > > Hi,
> > >
> > > Currently, the PCI subsystem marks the PCI devices as "untrusted", if
> > > the firmware asks it to:
> > >
> > > 617654aae50e ("PCI / ACPI: Identify untrusted PCI devices")
> > > 9cb30a71acd4 ("PCI: OF: Support "external-facing" property")
> > >
> > > An "untrusted" device indicates a (likely external facing) device that
> > > may be malicious, and can trigger DMA attacks on the system. It may
> > > also try to exploit any vulnerabilities exposed by the driver, that
> > > may allow it to read/write unintended addresses in the host (e.g. if
> > > DMA buffers for the device, share memory pages with other driver data
> > > structures or code etc).
> > >
> > > High Level proposal
> > > ===============
> > > Currently, the "untrusted" device property is used as a hint to enable
> > > IOMMU restrictions (on Intel), disable ATS (on ARM) etc. We'd like to
> > > go a step further, and allow the administrator to build a list of
> > > whitelisted drivers for these "untrusted" devices.
> >
> > How about letting the administrator whitelist devices that are trusted,
> > rather than whitelisting drivers?
>
> Uh, I completely missed the point. Your proposal is about preventing from
> binding any untrusted devices to non-whitelisted drivers. Please disregard
> my reply :)

Yes, my proposal is about ensuring untrusted devices can be bound to
only trusted drivers (if the administrator so desires).

Thanks for the links though - I think they may be additionally useful
for what we're trying to do.

Thanks,

Rajat


>
> Thanks,
> Jean
>
> >
> > The "thunderclap" attack [1] emulates an existing device using an FPGA in
> > order to get probed by the device driver, and then bypasses a weakened
> > IOMMU. By design the driver cannot differentiate a well-behaved device
> > from a malicious one, so changing the trust level of the driver doesn't
> > feel like the right way. What the admin wants to say is "I trust this
> > port, no one is plugging any malicious device in here."
> >
> > Then you could also make the option 3-way: either keep the default trust
> > fixed by FW, or manually set "trusted" or "untrusted".
> >
> > For reference there have been several discussions, recently, about letting
> > admins change IOMMU configuration for a device. A PCI command-line option
> > [2] was suggested, but I think the current proposal is a sysfs knob on
> > IOMMU groups [3], that can be changed while devices are unbound from
> > drivers. It's not completely relevant since the "untrusted" property isn't
> > tied to the IOMMU subsystem, but seemed worth mentioning.
> >
> > [1] https://thunderclap.io/thunderclap-paper-ndss2019.pdf
> > [2] https://lore.kernel.org/linux-iommu/20200101052648.14295-3-baolu.lu@linux.intel.com/
> > [3] https://lore.kernel.org/linux-iommu/5aa5ef20ff81f706aafa9a6af68cef98fe60ad0f.1581619464.git.sai.praneeth.prakhya@intel.com/
> >
> > Thanks,
> > Jean
> >
> > > This whitelist of
> > > drivers are the ones that he trusts enough to have little or no
> > > vulnerabilities. (He may have built this list of whitelisted drivers
> > > by a combination of code analysis of drivers, or by extensive testing
> > > using PCIe fuzzing etc). We propose that the administrator be allowed
> > > to specify this list of whitelisted drivers to the kernel, and the PCI
> > > subsystem to impose this behavior:
> > >
> > > 1) The "untrusted" devices can bind to only "whitelisted drivers".
> > > 2) The other devices (i.e. dev->untrusted=0) can bind to any driver.
> > >
> > > Of course this behavior is to be imposed only if such a whitelist is
> > > provided by the administrator.
> > >
> > > Details
> > > ======
> > >
> > > 1) A kernel argument ("pci.impose_driver_whitelisting") to enable
> > > imposing of whitelisting by PCI subsystem.
> > >
> > > 2) Add a flag ("whitelisted") in struct pci_driver to indicate whether
> > > the driver is whitelisted.
> > >
> > > 3) Use the driver's "whitelisted" flag and the device's "untrusted"
> > > flag, to make a decision about whether to bind or not in
> > > pci_bus_match() or similar.
> > >
> > > 4) A mechanism to allow the administrator to specify the whitelist of
> > > drivers. I think this needs more thought as there are multiple
> > > options.
> > >
> > > a) Expose individual driver's "whitelisted" flag to userspace so a
> > > boot script can whitelist that driver. There are questions that still
> > > need answered though e.g. what to do about the devices that may have
> > > already been enumerated and rejected by then? What to do with the
> > > already bound devices, if the user changes a driver to remove it from
> > > the whitelist. etc.
> > >
> > >       b) Provide a way to specify the whitelist via the kernel command
> > > line. Accept a ("pci.whitelist") kernel parameter which is a comma
> > > separated list of driver names (just like "module_blacklist"), and
> > > then use it to initialize each driver's "whitelisted" flag as the
> > > drivers are registered. Essentially this would mean that the whitelist
> > > of devices cannot be changed after boot.
> > >
> > > To me (b) looks a better option but I think a future requirement would
> > > be the ability to remove the drivers from the whitelist after boot
> > > (adding drivers to whitelist at runtime may not be that critical IMO)
> > >
> > >  WDYT?
> > >
> > > Thanks,
> > >
> > > Rajat