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: <a1ab92dd-6e76-fb30-d570-582cd3ebecd3@redhat.com>
Date:   Tue, 14 Sep 2021 12:52:54 +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

> 
> I would do it this way:
> 
> struct svm_nested_state {
>          ...
> 	/* cached fields from the vmcb12 */
> 	struct  vmcb_control_area_cached ctl;
> 	struct  vmcb_save_area_cached save;
>          ...
> };
> 
> 

The only thing that requires a little bit of additional work when 
applying this is svm_get_nested_state() (and theoretically 
svm_set_nested_state(), in option 2). In this function, nested.ctl is 
copied in user_vmcb->control. But now nested.ctl is not anymore a 
vmcb_control_area, so the sizes differ.

There are 2 options here:
1) copy nested.ctl into a full vmcb_control_area, and copy it to user 
space without modifying the API. The advantage is that the API is left 
intact, but an additional copy is required.

2) modify KVM_GET_NESTED_STATE and KVM_SET_NESTED_STATE to handle 
vmcb_control_area_cached. Advantage is that there is a lightweight copy 
+ the benefits explained by you in the previous email (no unset field).

I am not sure which one is the preferred way here.

Thank you,
Emanuele