linux-kernel - Re: [PATCH RFC] KVM: nSVM: Fix L1 state corruption upon return from SMM

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Date:   Wed, 23 Jun 2021 14:39:34 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Cathy Avery <cavery@...hat.com>,
        Emanuele Giuseppe Esposito <eesposit@...hat.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] KVM: nSVM: Fix L1 state corruption upon return from
 SMM

On Wed, 2021-06-23 at 11:39 +0200, Paolo Bonzini wrote:
> On 23/06/21 09:44, Vitaly Kuznetsov wrote:
> > - RFC: I'm not 100% sure my 'smart' idea to use currently-unused HSAVE area
> > is that smart. Also, we don't even seem to check that L1 set it up upon
> > nested VMRUN so hypervisors which don't do that may remain broken. A very
> > much needed selftest is also missing.
> 
> It's certainly a bit weird, but I guess it counts as smart too.  It 
> needs a few more comments, but I think it's a good solution.
> 
> One could delay the backwards memcpy until vmexit time, but that would 
> require a new flag so it's not worth it for what is a pretty rare and 
> already expensive case.
> 
> Paolo
> 

I wonder what would happen if SMM entry is triggered by L1 (say with ICR),
on a VCPU which is in L2. Such exit should go straight to L1 SMM mode.

I will very very soon, maybe even today start testing SMM with my migration
tests and such. I hope I will find more bugs in this area.

Thanks for fixing this issue!

Best regards,
	Maxim Levitsky