[Bug 218662] bhyve exposes CPU feature SDBG to guests, causing guest panic on OpenBSD 6.1

[bugzilla-noreply at freebsd.org]

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=218662

Brandon Bergren <freebsd at bdragon.rtk0.net> changed:

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--- Comment #2 from Brandon Bergren <freebsd at bdragon.rtk0.net> ---
Created attachment 187571
  --> https://bugs.freebsd.org/bugzilla/attachment.cgi?id=187571&action=edit
VMM patch (from HardenedBSD)

This affects me too.

HardenedBSD has the most elegant fix at
https://github.com/HardenedBSD/hardenedBSD/commit/e76fcb77ba82649bc6aed808af06d6d2184847d8
-- I have attached a copy of the patch. I can confirm that with just that patch
I can successfully run OpenBSD 6.1 in bhyve on my machine that is otherwise
affected by this problem.

I believe it makes a lot more sense than emulating the MSR (i.e. what
https://github.com/HardenedBSD/hardenedBSD/commit/cc91b57f4d1dabddfbf8b1e7655
does.)

My SuperMicro C7Z170-OCE board actually has bios options for enabling and
disabling the silicon debug status and lock bit, so if I were to misconfigure
my bios, the countermeasure would have actually been effective (when applied to
the host).

I can verify though that with it disabled in the bios, it is indeed locked
down, without needing the countermeasure.
root at narwhal:~ # kldload cpuctl
root at narwhal:~ # cpucontrol -m 0xc80 /dev/cpuctl0
MSR 0xc80: 0x00000000 0x40000000

I still think the countermeasure is a good idea though!
https://github.com/HardenedBSD/hardenedBSD/commit/da28546280938e19e866d2e11a9ccda3c4ca82fa
is a version of the countermeasure that checks the virtualization flag too so
it will avoid tickling the bhyve limitation while still providing protection.

But at the very least, filter the SDBG feature flag.

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