linux-kernel - Re: [PATCH 2/2] kvm, ept: remove the default write bit

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Message-ID: <20100611195052.GC12317@amt.cnet>
Date:	Fri, 11 Jun 2010 16:50:53 -0300
From:	Marcelo Tosatti <mtosatti@...hat.com>
To:	Lai Jiangshan <laijs@...fujitsu.com>
Cc:	Avi Kivity <avi@...hat.com>, kvm@...r.kernel.org,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 2/2] kvm, ept: remove the default write bit

On Fri, Jun 11, 2010 at 07:30:50PM +0800, Lai Jiangshan wrote:
> When ept enabled, current code set shadow_base_present_pte
> including the write bit, thus all pte entries have
> writabe bit, and it means guest os can always
> write to any mapped page (even VMM maps RO pages for
> the guest.)
> 
> We always use get_user_pages(write=1), so this bad code does not
> cause any bad result currently.
> 
> But it is really bad, so fix it, and we will use RO pages future.
> 
> We will set writabe bit when it is really writable (determined by
> the parameters of the set_spte())
> 
> Signed-off-by: Lai Jiangshan <laijs@...fujitsu.com>
> ---
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index fdb18cf..c7565ea 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -4365,8 +4365,7 @@ static int __init vmx_init(void)
>  
>  	if (enable_ept) {
>  		bypass_guest_pf = 0;
> -		kvm_mmu_set_base_ptes(VMX_EPT_READABLE_MASK |
> -			VMX_EPT_WRITABLE_MASK);
> +		kvm_mmu_set_base_ptes(VMX_EPT_READABLE_MASK);
>  		kvm_mmu_set_mask_ptes(0ull, 0ull, 0ull, 0ull,
>  				VMX_EPT_EXECUTABLE_MASK);
>  		kvm_enable_tdp();

You can remove the call to kvm_mmu_set_base_ptes entirely, because
VMX_EPT_READABLE_MASK == PT_PRESENT_MASK.

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