Re: [PATCH v2 2/6] driver core: Add common support to skip probe for un-authorized devices

[Dan Williams]

On Thu, Sep 30, 2021 at 1:44 PM Alan Stern <stern@xxxxxxxxxxxxxxxxxxx> wrote:
>
> On Thu, Sep 30, 2021 at 12:23:36PM -0700, Andi Kleen wrote:
> >
> > > I don't think the current mitigations under discussion here are about
> > > keeping the system working. In fact most encrypted VM configs tend to
> > > stop booting as a preferred way to handle security issues.
> >
> > Maybe we should avoid the "trusted" term here. We're only really using it
> > because USB is using it and we're now using a common framework like Greg
> > requested. But I don't think it's the right way to think about it.
> >
> > We usually call the drivers "hardened". The requirement for a hardened
> > driver is that all interactions through MMIO/port/config space IO/MSRs are
> > sanitized and do not cause memory safety issues or other information leaks.
> > Other than that there is no requirement on the functionality. In particular
> > DOS is ok since a malicious hypervisor can decide to not run the guest at
> > any time anyways.
> >
> > Someone loading an malicious driver inside the guest would be out of scope.
> > If an attacker can do that inside the guest you already violated the
> > security mechanisms and there are likely easier ways to take over the guest
> > or leak data.
> >
> > The goal of the device filter mechanism is to prevent loading unhardened
> > drivers that could be exploited without them being themselves malicious.
>
> If all you want to do is prevent someone from loading a bunch of
> drivers that you have identified as unhardened, why not just use a
> modprobe blacklist?  Am I missing something?

modules != drivers (i.e. multi-driver modules are a thing) and builtin
modules do not adhere to modprobe policy.

There is also a desire to be able to support a single kernel image
across hosts and guests. So, if you were going to say, "just compile
all unnecessary drivers as modules" that defeats the common kernel
image goal. For confidential computing the expectation is that the
necessary device set is small. As you can see in the patches in this
case it's just a few lines of PCI ids and a hack to the virtio bus to
achieve the goal of disabling all extraneous devices by default.