Virtualization - Sep 2021 - [PATCH v2 2/6] driver core: Add common support to skip probe for un-authorized devices

> 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.


-Andi

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?

Alan Stern