linux-kernel - Re: [PATCH v2] kvm: mmu: don't do memslot overflow check

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Date:	Wed, 15 Apr 2015 17:01:16 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Wanpeng Li <wanpeng.li@...ux.intel.com>
CC:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Xiao Guangrong <guangrong.xiao@...ux.intel.com>,
	Andres Lagar-Cavilla <andreslc@...gle.com>,
	Eric Northup <digitaleric@...gle.com>
Subject: Re: [PATCH v2] kvm: mmu: don't do memslot overflow check



On 15/04/2015 04:24, Wanpeng Li wrote:
> As Andres pointed out:
> 
> | I don't understand the value of this check here. Are we looking for a
> | broken memslot? Shouldn't this be a BUG_ON? Is this the place to care
> | about these things? npages is capped to KVM_MEM_MAX_NR_PAGES, i.e.
> | 2^31. A 64 bit overflow would be caused by a gigantic gfn_start which
> | would be trouble in many other ways.
> 
> This patch drops the memslot overflow check to make the codes more simple.
> 
> Reviewed-by: Andres Lagar-Cavilla <andreslc@...gle.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@...ux.intel.com>
> ---
> v1 -> v2:
>  * Fix Andres's name
>  * Add Andres's Reviewed-by 
> 
>  arch/x86/kvm/mmu.c | 12 ++----------
>  1 file changed, 2 insertions(+), 10 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu.c b/arch/x86/kvm/mmu.c
> index 2a0d77e..9265fda 100644
> --- a/arch/x86/kvm/mmu.c
> +++ b/arch/x86/kvm/mmu.c
> @@ -4505,19 +4505,12 @@ void kvm_mmu_zap_collapsible_sptes(struct kvm *kvm,
>  	bool flush = false;
>  	unsigned long *rmapp;
>  	unsigned long last_index, index;
> -	gfn_t gfn_start, gfn_end;
>  
>  	spin_lock(&kvm->mmu_lock);
>  
> -	gfn_start = memslot->base_gfn;
> -	gfn_end = memslot->base_gfn + memslot->npages - 1;
> -
> -	if (gfn_start >= gfn_end)
> -		goto out;
> -
>  	rmapp = memslot->arch.rmap[0];
> -	last_index = gfn_to_index(gfn_end, memslot->base_gfn,
> -					PT_PAGE_TABLE_LEVEL);
> +	last_index = gfn_to_index(memslot->base_gfn + memslot->npages - 1,
> +				memslot->base_gfn, PT_PAGE_TABLE_LEVEL);
>  
>  	for (index = 0; index <= last_index; ++index, ++rmapp) {
>  		if (*rmapp)
> @@ -4535,7 +4528,6 @@ void kvm_mmu_zap_collapsible_sptes(struct kvm *kvm,
>  	if (flush)
>  		kvm_flush_remote_tlbs(kvm);
>  
> -out:
>  	spin_unlock(&kvm->mmu_lock);
>  }
>  
> 

Thanks, queued for 4.1.

Paolo
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