On Sun, Jun 14, 2015 at 05:13:05PM +0100, zichao wrote:

I and marc are talking about how to plug the guest debug exploit in an
easier way.

I remembered that you mentioned disabling monitor mode had proven to be
extremely fragile in practice on 32-bit ARM SoCs, what if I save/restore
the debug monitor mode on each switch between the guest and the host,
would it be acceptable?

If you're just referring to DBGDSCRext, then you could give it a go, but
you'll certainly want to predicate any writes to that register on whether
or not hw_breakpoint managed to reset the debug regs on the host.

Like I said, accessing these registers always worries me, so I'd really
avoid it in KVM if you can. If not, you'll need to do extensive testing
on a bunch of platforms with and without the presence of external debug.

Will