linux-kernel - Re: [PATCH] KVM: X86: Fix SMRAM accessing even if VM is shutdown

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Date:   Thu, 8 Feb 2018 16:57:03 +0800
From:   Xiao Guangrong <guangrong.xiao@...il.com>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Dmitry Vyukov <dvyukov@...gle.com>
Subject: Re: [PATCH] KVM: X86: Fix SMRAM accessing even if VM is shutdown



On 02/07/2018 10:16 PM, Paolo Bonzini wrote:
> On 07/02/2018 07:25, Wanpeng Li wrote:
>> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
>> index 786cd00..445e702 100644
>> --- a/arch/x86/kvm/x86.c
>> +++ b/arch/x86/kvm/x86.c
>> @@ -7458,6 +7458,11 @@ int kvm_arch_vcpu_ioctl_run(struct kvm_vcpu *vcpu, struct kvm_run *kvm_run)
>>   		goto out;
>>   	}
>>   
>> +	if (unlikely(vcpu->run->exit_reason == KVM_EXIT_SHUTDOWN)) {
>> +		r = -EINVAL;
>> +		goto out;
>> +	}
>> +
>>   	if (vcpu->run->kvm_dirty_regs) {
>>   		r = sync_regs(vcpu);
>>   		if (r != 0)
>>
> 
> This most likely breaks triple faults in the usual case where they
> should result in resetting the system; the KVM API doesn't say that you
> should clear vcpu->run->exit_reason before entering.
> 
> What exactly causes the EPT misconfig to reach the WARN?  That is, how
> does kvm_mmu_page_fault end up returning a negative errno value?  If I
> read the code correctly only tdp_page_fault can do so, so my guess would
> be kvm_handle_bad_page:
> 
>          if (pfn == KVM_PFN_ERR_RO_FAULT)
>                  return RET_PF_EMULATE;
> 
>          if (pfn == KVM_PFN_ERR_HWPOISON) {
>                  kvm_send_hwpoison_signal(kvm_vcpu_gfn_to_hva(vcpu, gfn),
> current);
>                  return RET_PF_RETRY;
>          }
> 
> 	/* KVM_PFN_ERR_FAULT */
>          return -EFAULT;
> 
> Maybe it should return RET_PF_EMULATE, which would cause an emulation
> failure and then an exit with KVM_EXIT_INTERNAL_ERROR.

So the root cause is that a running vCPU accessing the memory whose memslot
is being updated (met the condition KVM_MEMSLOT_INVALID is set on the its
memslot).

The normal #PF handler breaks KVM_RUN and returns -EFAULT to userspace, we'd
better to make ept-misconfig's handler follow this style as well.

Actually, the WARN_ON in ept-misconfig's handler is unnecessary as
kvm_mmu_page_fault() will warn us if it is the real ept misconfig, so we can
simply return kvm_mmu_page_fault().