linux-kernel - Re: [PATCH v2 1/3] KVM: vmx: fix ept reserved bits for 1-GByte page

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Message-ID: <53F3145D.4080103@redhat.com>
Date:	Tue, 19 Aug 2014 11:09:49 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Wanpeng Li <wanpeng.li@...ux.intel.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	Gleb Natapov <gleb@...nel.org>,
	Zhang Yang <yang.z.zhang@...el.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/3] KVM: vmx: fix ept reserved bits for 1-GByte page

Il 19/08/2014 11:04, Wanpeng Li ha scritto:
> EPT misconfig handler in kvm will check which reason lead to EPT 
> misconfiguration after vmexit. One of the reasons is that an EPT 
> paging-structure entry is configured with settings reserved for 
> future functionality. However, the handler can't identify if 
> paging-structure entry of reserved bits for 1-GByte page are 
> configured, since PDPTE which point to 1-GByte page will reserve 
> bits 29:12 instead of bits 7:3 which are reserved for PDPTE that 
> references an EPT Page Directory. This patch fix it by reserve 
> bits 29:12 for 1-GByte page. 
> 
> Signed-off-by: Wanpeng Li <wanpeng.li@...ux.intel.com>
> ---
> v1 -> v2:
>  * same "if" statement cover both 2MB and 1GB pages
>  * return 0xf8 for level == 4

I think you dropped this check by mistake.

>  * get the level by checking the return value of ept_rsvd_mask 
> 
>  arch/x86/kvm/vmx.c | 19 +++++++------------
>  1 file changed, 7 insertions(+), 12 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index cad37d5..2763f37 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -5521,17 +5521,12 @@ static u64 ept_rsvd_mask(u64 spte, int level)
>  	for (i = 51; i > boot_cpu_data.x86_phys_bits; i--)
>  		mask |= (1ULL << i);
>  
> -	if (level > 2)
> -		/* bits 7:3 reserved */
> -		mask |= 0xf8;
> -	else if (level == 2) {
> -		if (spte & (1ULL << 7))
> -			/* 2MB ref, bits 20:12 reserved */
> -			mask |= 0x1ff000;
> -		else
> -			/* bits 6:3 reserved */
> -			mask |= 0x78;
> -	}
> +	if (spte & (1ULL << 7))

You need to go this way if level == 1 too.  Otherwise, you would report
bits 6:3 reserved if the hypervisor is using the ignored bit 7 (Table
28-6, Format of an EPT Page-Table Entry).

> +		/* 1GB/2MB page, bits 29:12 or 20:12 reserved respectively */
> +		mask |= (PAGE_SIZE << ((level - 1) * 9)) - PAGE_SIZE;
> +	else
> +		/* bits 6:3 reserved */
> +		mask |= 0x78;
>  
>  	return mask;
>  }
> @@ -5561,7 +5556,7 @@ static void ept_misconfig_inspect_spte(struct kvm_vcpu *vcpu, u64 spte,
>  			WARN_ON(1);
>  		}
>  
> -		if (level == 1 || (level == 2 && (spte & (1ULL << 7)))) {
> +		if (level == 1 || (rsvd_bits & 0x38)) {

- rsvd_bits will always be zero here.  You need to check the return
value of ept_rsvd_mask().  Let's call it rsvd_mask in the rest of this
email.

- the test is inverted, you need to check that bits 5:3 are _not_
reserved, hence (rsvd_mask & 0x38) == 0.

- once you do this, the test also covers level 1.

I suggest that you write a testcase for kvm-unit-tests.

Paolo
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