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

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Date:   Thu, 30 Sep 2021 12:23:36 -0700
From:   Andi Kleen <ak@...ux.intel.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>,
        Alan Stern <stern@...land.harvard.edu>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Kuppuswamy Sathyanarayanan 
        <sathyanarayanan.kuppuswamy@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>, x86@...nel.org,
        Bjorn Helgaas <bhelgaas@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Andreas Noever <andreas.noever@...il.com>,
        Michael Jamet <michael.jamet@...el.com>,
        Yehezkel Bernat <YehezkelShB@...il.com>,
        "Rafael J . Wysocki" <rafael@...nel.org>,
        Mika Westerberg <mika.westerberg@...ux.intel.com>,
        Jonathan Corbet <corbet@....net>,
        Jason Wang <jasowang@...hat.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Kuppuswamy Sathyanarayanan <knsathya@...nel.org>,
        linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-usb@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        "Reshetova, Elena" <elena.reshetova@...el.com>
Subject: Re: [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