linux-kernel - Re: [PATCH V2] KVM/x86: Check input paging mode when cs.l is set

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Date:   Thu, 14 Dec 2017 12:41:06 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Lan Tianyu <tianyu.lan@...el.com>
Cc:     rkrcmar@...hat.com, tglx@...utronix.de, mingo@...hat.com,
        hpa@...or.com, x86@...nel.org, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, dvyukov@...gle.com,
        jmattson@...gle.com
Subject: Re: [PATCH V2] KVM/x86: Check input paging mode when cs.l is set

On 14/12/2017 04:55, Lan Tianyu wrote:
> +	 * When EFER.LME and CR0.PG are set, CR4.PAE and EFER.LMA
> +	 * must be set.
> +	 */
> +	if ((sregs->efer & EFER_LME) && (sregs->cr0 & X86_CR0_PG_BIT)) {
> +		if (!(sregs->cr4 & X86_CR4_PAE_BIT))
> +			return -EINVAL;
> +		if (!(sregs->efer & EFER_LMA))
> +			return -EINVAL;
> +	} else if (sregs->efer & EFER_LMA)

This can just be "(sregs->efer & EFER_LMA) || sregs->cs.l", making the
next "if" redundant.  Even better written as

	if ((sregs->efer & EFER_LME) && (sregs->cr0 & X86_CR0_PG_BIT)) {
		/*
		 * When EFER.LME and CR0.PG are set, the processor is in
		 * 64-bit mode (though maybe in a 32-bit code segment).
		 * CR4.PAE and EFER.LMA must be set.
		 */
		if (... || ...)
			return -EINVAL;
	} else {
		/*
		 * Not in 64-bit mode: EFER.LMA is clear and the code
		 * segment cannot be 64-bit.
		 */
		if (... || ...)
			return -EINVAL;
	}

Paolo

> +		return -EINVAL;
> +
> +	/* When CS.L is set, vcpu should run in 64-bit mode. */
> +	if (sregs->cs.l)
> +		if (!((sregs->cr0 & X86_CR0_PG_BIT) &&
> +		      (sregs->cr4 & X86_CR4_PAE_BIT) &&
> +		      (sregs->efer & EFER_LME)))
> +			return -EINVAL;