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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Date:   Tue, 14 Sep 2021 11:24:35 +0200
From:   Emanuele Giuseppe Esposito <eesposit@...hat.com>
To:     Maxim Levitsky <mlevitsk@...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 14/09/2021 11:12, Maxim Levitsky wrote:
> 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.

So basically you are proposing:

struct svm_nested_state {
	...
	struct vmcb_control_area ctl; // we need this because it is used 
everywhere, I think
	struct vmcb_control_area_cached ctl_cached;
	struct vmcb_save_area_cached save_cached;
	...
}

and then

if (!nested_vmcb_valid_sregs(vcpu, &svm->nested.save_cached) ||
     !nested_vmcb_check_controls(vcpu, &svm->nested.ctl_cached)) {

like that?

Or do you want to delete nested.ctl completely and just keep the fields 
actually used in ctl_cached?

Also, note that as I am trying to use vmcb_save_area_cached, it is worth 
noticing that nested_vmcb_valid_sregs() is also used in 
svm_set_nested_state(), so it requires some additional little changes.

Thank you,
Emanuele

> 
> 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
>>>
> 
>