linux-kernel - Re: [PATCH v2 4/6] KVM: x86: Introduce KVM_GET_SREGS2 / KVM_SET

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Date:   Mon, 26 Apr 2021 14:32:26 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org
Cc:     "open list:DOCUMENTATION" <linux-doc@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Jonathan Corbet <corbet@....net>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@...r.kernel.org>, "H. Peter Anvin" <hpa@...or.com>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Wanpeng Li <wanpengli@...cent.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Sean Christopherson <seanjc@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH v2 4/6] KVM: x86: Introduce KVM_GET_SREGS2 /
 KVM_SET_SREGS2

On 26/04/21 13:13, Maxim Levitsky wrote:
> +	if (sregs2->flags & KVM_SREGS2_FLAGS_PDPTRS_VALID) {
> +
> +		if (!is_pae_paging(vcpu))
> +			return -EINVAL;
> +
> +		for (i = 0 ; i < 4 ; i++)
> +			kvm_pdptr_write(vcpu, i, sregs2->pdptrs[i]);
> +
> +		kvm_register_mark_dirty(vcpu, VCPU_EXREG_PDPTR);
> +		mmu_reset_needed = 1;
> +	}

I think this should also have

	else {
		if (is_pae_paging(vcpu))
			return -EINVAL;
	}

but perhaps even better, check it at the beginning:

	if ((sregs->cr4 & X86_CR4_PAE) &&
             !!(sregs->efer & EFER_LMA) == !!(sregs2->flags & KVM_SREGS2_FLAGS_PDPTRS_VALID))
		return -EINVAL;

which technically means the flag is redundant, but there is some value in
having the flag and not allowing the user to shoot itself in the foot.

Paolo