linux-kernel - Re: [RFC PATCH 3/3] nSVM: use svm->nested.save to load vmcb12 registers and avoid TOC/TOU races

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Message-ID: <2b1e17416cef1e37f42e9bc8b2283b03d2651cb2.camel@redhat.com>
Date:   Tue, 14 Sep 2021 12:12:11 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Emanuele Giuseppe Esposito <eesposit@...hat.com>,
        kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH 3/3] nSVM: use svm->nested.save to load vmcb12
 registers and avoid TOC/TOU races

On Tue, 2021-09-14 at 12:02 +0300, Maxim Levitsky wrote:
> On Tue, 2021-09-14 at 10:20 +0200, Emanuele Giuseppe Esposito wrote:
> > On 12/09/2021 12:42, Maxim Levitsky wrote:
> > > >   
> > > > -	if (!nested_vmcb_valid_sregs(vcpu, &vmcb12->save) ||
> > > > +	if (!nested_vmcb_valid_sregs(vcpu, &svm->nested.save) ||
> > > >   	    !nested_vmcb_check_controls(vcpu, &svm->nested.ctl)) {
> > > If you use a different struct for the copied fields, then it makes
> > > sense IMHO to drop the 'control' parameter from nested_vmcb_check_controls,
> > > and just use the svm->nested.save there directly.
> > > 
> > 
> > Ok, what you say in patch 2 makes sense to me. I can create a new struct 
> > vmcb_save_area_cached, but I need to keep nested.ctl because 1) it is 
> > used also elsewhere, and different fields from the one checked here are 
> > read/set and 2) using another structure (or the same 
> 
> Yes, keep nested.ctl, since vast majority of the fields are copied I think.

But actually that you mention it, I'll say why not to create vmcb_control_area_cached
as well indeed and change the type of svm->nested.save to it. (in a separate patch)

I see what you mean that we modify it a bit (but we shoudn't to be honest) and such, but
all of this can be fixed.

The advantage of having vmcb_control_area_cached is that it becomes impossible to use
by mistake a non copied field from the guest.

It would also emphasize that this stuff came from the guest and should be treated as
a toxic waste.

Note again that this should be done if we agree as a separate patch.

> 
> Best regards,
> 	Maxim Levitsky
> 
> 
> > vmcb_save_area_cached) in its place would just duplicate the same fields 
> > of nested.ctl, creating even more confusion and possible inconsistency.
> > 
> > Let me know if you disagree.
> > 
> > Thank you,
> > Emanuele
> >