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

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Message-ID: <233cfca3-971e-c3c2-f0fe-b50dd69d2546@redhat.com>
Date:   Wed, 7 Feb 2018 15:16:49 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     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 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.

Thanks,

Paolo