linux-kernel - Re: [PATCH v6 01/12] KVM: x86: fix possible infinite loop caused by reexecute

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Date:	Wed, 22 Aug 2012 20:49:26 +0800
From:	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
To:	Avi Kivity <avi@...hat.com>
CC:	Marcelo Tosatti <mtosatti@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>, KVM <kvm@...r.kernel.org>
Subject: Re: [PATCH v6 01/12] KVM: x86: fix possible infinite loop caused
 by reexecute_instruction

On 08/22/2012 08:01 PM, Avi Kivity wrote:
> On 08/21/2012 05:57 AM, Xiao Guangrong wrote:
>> Currently, we reexecute all unhandleable instructions if they do not
>> access on the mmio, however, it can not work if host map the readonly
>> memory to guest. If the instruction try to write this kind of memory,
>> it will fault again when guest retry it, then we will goto a infinite
>> loop: retry instruction -> write #PF -> emulation fail ->
>> retry instruction -> ...
>>
>> Fix it by retrying the instruction only when it faults on the writable
>> memory
>>
>> Signed-off-by: Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>
>> ---
>>  arch/x86/kvm/x86.c |   12 +++++++++++-
>>  1 files changed, 11 insertions(+), 1 deletions(-)
>>
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index fb0d937..704680d 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -4473,6 +4473,7 @@ static int handle_emulation_failure(struct kvm_vcpu *vcpu)
>>  static bool reexecute_instruction(struct kvm_vcpu *vcpu, gva_t gva)
>>  {
>>  	gpa_t gpa;
>> +	pfn_t pfn;
>>
>>  	if (tdp_enabled)
>>  		return false;
>> @@ -4490,8 +4491,17 @@ static bool reexecute_instruction(struct kvm_vcpu *vcpu, gva_t gva)
>>  	if (gpa == UNMAPPED_GVA)
>>  		return true; /* let cpu generate fault */
>>
>> -	if (!kvm_is_error_hva(gfn_to_hva(vcpu->kvm, gpa >> PAGE_SHIFT)))
>> +	/*
>> +	 * Do not retry the unhandleable instruction if it faults on the
>> +	 * readonly host memory, otherwise it will goto a infinite loop:
>> +	 * retry instruction -> write #PF -> emulation fail -> retry
>> +	 * instruction -> ...
>> +	 */
>> +	pfn = gfn_to_pfn(vcpu->kvm, gpa_to_gfn(gpa));
>> +	if (!is_error_pfn(pfn)) {
>> +		kvm_release_pfn_clean(pfn);
>>  		return true;
>> +	}
>>
>>  	return false;
>>  }
>>
> 
> Good catch.  Did this actually happen or did you find it by code inspection?
> 

It did not happen in my test. Actually, it is reported by Marcelo.

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