linux-kernel - Re: [PATCH 1/4] KVM: MMU: fix forgot reserved bits check in speculative path

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Date:	Wed, 14 Jul 2010 16:06:57 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Xiao Guangrong <xiaoguangrong@...fujitsu.com>
CC:	LKML <linux-kernel@...r.kernel.org>,
	KVM list <kvm@...r.kernel.org>,
	Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: [PATCH 1/4] KVM: MMU: fix forgot reserved bits check in speculative
 path

On 07/13/2010 12:42 PM, Xiao Guangrong wrote:
> In the speculative path, we should check guest pte's reserved bits just as
> the real processor does
>
> Reported-by: Marcelo Tosatti<mtosatti@...hat.com>
> Signed-off-by: Xiao Guangrong<xiaoguangrong@...fujitsu.com>
> ---
>   arch/x86/kvm/mmu.c         |    8 ++++++++
>   arch/x86/kvm/paging_tmpl.h |    5 +++--
>   2 files changed, 11 insertions(+), 2 deletions(-)
>
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index b93b94f..9fc1524 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -2783,6 +2783,9 @@ void kvm_mmu_pte_write(struct kvm_vcpu *vcpu, gpa_t gpa,
>   		break;
>   	}
>
> +	if (is_rsvd_bits_set(vcpu, gentry, PT_PAGE_TABLE_LEVEL))
> +		gentry = 0;
> +
>   	mmu_guess_page_from_pte_write(vcpu, gpa, gentry);
>   	spin_lock(&vcpu->kvm->mmu_lock);
>   	if (atomic_read(&vcpu->kvm->arch.invlpg_counter) != invlpg_counter)
> @@ -2851,6 +2854,11 @@ void kvm_mmu_pte_write(struct kvm_vcpu *vcpu, gpa_t gpa,
>   		while (npte--) {
>   			entry = *spte;
>   			mmu_pte_write_zap_pte(vcpu, sp, spte);
> +
> +			if (!!is_pae(vcpu) != sp->role.cr4_pae ||
> +			      is_nx(vcpu) != sp->role.nxe)
> +				continue;
> +
>    

Do we also need to check cr0.wp?  I think so.

>   			if (gentry)
>   				mmu_pte_write_new_pte(vcpu, sp, spte,&gentry);
>    

Please move the checks to mmu_pte_write_new_pte(), it's a more logical 
place.

It means the reserved bits check happens multiple times, but that's ok.

Also, you can use arch.mmu.base_role to compare:

     static const kvm_mmu_page_role mask = { .level = -1U, .cr4_pae = 1, 
... };

     if ((sp->role.word ^ base_role.word) & mask.word)
          return;

> @@ -640,8 +640,9 @@ static int FNAME(sync_page)(struct kvm_vcpu *vcpu, struct kvm_mmu_page *sp,
>   			return -EINVAL;
>
>   		gfn = gpte_to_gfn(gpte);
> -		if (gfn != sp->gfns[i] ||
> -		      !is_present_gpte(gpte) || !(gpte&  PT_ACCESSED_MASK)) {
> +		if (is_rsvd_bits_set(vcpu, gpte, PT_PAGE_TABLE_LEVEL) ||
> +		      gfn != sp->gfns[i] || !is_present_gpte(gpte) ||
> +		      !(gpte&  PT_ACCESSED_MASK)) {
>   			u64 nonpresent;
>
>   			if (is_present_gpte(gpte) || !clear_unsync)
>    

Eventually we have to reduce the number of paths.  But lets fix things 
first.

-- 
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.

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