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SubjectRe: [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@linux.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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