linux-kernel - Re: [PATCH 1/2] KVM: MMU: fix ept=0/pte.u=0/pte.w=0/CR0.WP=0/CR4.SMEP=1/EFER.NX=0 combo

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Date:	Thu, 10 Mar 2016 13:26:06 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Xiao Guangrong <guangrong.xiao@...ux.intel.com>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org
Cc:	stable@...r.kernel.org, Andy Lutomirski <luto@...capital.net>
Subject: Re: [PATCH 1/2] KVM: MMU: fix
 ept=0/pte.u=0/pte.w=0/CR0.WP=0/CR4.SMEP=1/EFER.NX=0 combo



On 10/03/2016 13:14, Xiao Guangrong wrote:
>> More precisely, ignore_bits is only needed if guest EFER.NX=0 and we're
>> not in this CR0.WP=1/CR4.SMEP=0 situation.  In theory you could have
>> guest EFER.NX=1 and host EFER.NX=0.
> 
> It is not in linux, the kernel always set EFER.NX if CPUID reports it,
> arch/x86/kernel/head_64.S:
> 
> 204         /* Setup EFER (Extended Feature Enable Register) */
> 205         movl    $MSR_EFER, %ecx
> 206         rdmsr
> 207         btsl    $_EFER_SCE, %eax        /* Enable System Call */
> 208         btl     $20,%edi                /* No Execute supported? */
> 209         jnc     1f
> 210         btsl    $_EFER_NX, %eax
> 211         btsq    $_PAGE_BIT_NX,early_pmd_flags(%rip)
> 212 1:      wrmsr                           /* Make changes effective */
> 
> So if guest sees NX in its cpuid then host EFER.NX should be 1.

You're right.  It's just in theory.  But ignoring EFER.NX when it is 1
is technically not correct; since we have to add some special EFER_NX
logic anyway, I preferred to make it pedantically right. :)

Paolo