Re: [PATCH] KVM: nVMX: fix CR3 load if L2 uses PAE paging and EPT

["Radim Krčmář" <rkrcmar@redhat.com>]

2016-11-24 14:29+0100, Ladi Prosek:
> KVM does not correctly handle L1 hypervisors that emulate L2 real mode with
> PAE and EPT, such as Hyper-V. In this mode, the L1 hypervisor populates guest
> PDPTE VMCS fields and leaves guest CR3 uninitialized because it is not used
> (see 26.3.2.4 Loading Page-Directory-Pointer-Table Entries). KVM always
> dereferences CR3 and tries to load PDPTEs if PAE is on. This leads to two
> related issues:
> 
> 1) On the first nested vmentry, the guest PDPTEs, as populated by L1, are
> overwritten in ept_load_pdptrs because the registers are believed to have
> been loaded in load_pdptrs as part of kvm_set_cr3. This is incorrect. L2 is
> running with PAE enabled but PDPTRs have been set up by L1.
> 
> 2) When L2 is about to enable paging and loads its CR3, we, again, attempt
> to load PDPTEs in load_pdptrs called from kvm_set_cr3. There are no guarantees
> that this will succeed (it's just a CR3 load, paging is not enabled yet) and
> if it doesn't, kvm_set_cr3 returns early without persisting the CR3 which is
> then lost and L2 crashes right after it enables paging.
> 
> This patch replaces the kvm_set_cr3 call with a simple register write if PAE
> and EPT are both on. CR3 is not to be interpreted in this case.
> 
> Signed-off-by: Ladi Prosek <lprosek@redhat.com>
> ---
> 
> tl;dr This makes Hyper-V (Windows Server 2016) work on KVM.

Great work!

> @@ -10113,8 +10115,17 @@ static void prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12)
>  	vmx_set_cr4(vcpu, vmcs12->guest_cr4);
>  	vmcs_writel(CR4_READ_SHADOW, nested_read_cr4(vmcs12));
>  
> -	/* shadow page tables on either EPT or shadow page tables */
> -	kvm_set_cr3(vcpu, vmcs12->guest_cr3);
> +	/*
> +	 * Shadow page tables on either EPT or shadow page tables.
> +	 * If PAE and EPT are both on, CR3 is not used by the CPU and must not
> +	 * be dereferenced.
> +	 */
> +	if (is_pae(vcpu) && is_paging(vcpu) && nested_ept_enabled) {

The spec also has a "&& !is_long_mode(vcpu)" condition for PAE paging.
I'm pretty sure that is_pae() and is_paging() is set in long mode ...
the stuff in kvm_set_cr3() is not needed even then?

> +		vcpu->arch.cr3 = vmcs12->guest_cr3;
> +		__set_bit(VCPU_EXREG_CR3, (ulong *)&vcpu->arch.regs_avail);
> +	} else
> +		kvm_set_cr3(vcpu, vmcs12->guest_cr3);
> +
>  	kvm_mmu_reset_context(vcpu);
>  
>  	if (!enable_ept)
> -- 
> 2.7.4
>