linux-kernel - Re: [PATCH v2 1/3] kvm: svm: Add support for additional SVM NPF error codes

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Date:   Thu, 27 Jul 2017 18:27:49 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Brijesh Singh <brijesh.singh@....com>, kvm@...r.kernel.org
Cc:     thomas.lendacky@....com, rkrcmar@...hat.com, joro@...tes.org,
        x86@...nel.org, linux-kernel@...r.kernel.org, mingo@...hat.com,
        hpa@...or.com, tglx@...utronix.de, bp@...e.de
Subject: Re: [PATCH v2 1/3] kvm: svm: Add support for additional SVM NPF error
 codes

On 23/11/2016 18:01, Brijesh Singh wrote:
>  
> +	/*
> +	 * Before emulating the instruction, check if the error code
> +	 * was due to a RO violation while translating the guest page.
> +	 * This can occur when using nested virtualization with nested
> +	 * paging in both guests. If true, we simply unprotect the page
> +	 * and resume the guest.
> +	 *
> +	 * Note: AMD only (since it supports the PFERR_GUEST_PAGE_MASK used
> +	 *       in PFERR_NEXT_GUEST_PAGE)
> +	 */
> +	if (error_code == PFERR_NESTED_GUEST_PAGE) {
> +		kvm_mmu_unprotect_page(vcpu->kvm, gpa_to_gfn(cr2));
> +		return 1;
> +	}


What happens if L1 is mapping some memory that is read only in L0?  That
is, the L1 nested page tables make it read-write, but the L0 shadow
nested page tables make it read-only.

Accessing it would cause an NPF, and then my guess is that the L1 guest
would loop on the failing instruction instead of just dropping the write.

Paolo