Re: [PATCH] KVM: nVMX: Fix L2 guest hang if shadow page tables on EPT

[Paolo Bonzini]

On 17/03/2017 15:41, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
> 
> The L2 guest hang if shadow page tables on EPT, the trace on L1 shows that 
> L2 kvm_exit reason EXCEPTION_NMI and page fault repeatedly:
> 
> qemu-system-x86-2821  [003] d..2    45.848814: kvm_entry: vcpu 0
> qemu-system-x86-2821  [003] ...1    45.848827: kvm_exit: reason EXCEPTION_NMI rip 0xe05b info fe05b 80000b0e
> qemu-system-x86-2821  [003] ...1    45.848827: kvm_page_fault: address fe05b error_code 14
> 
> Commit 7ca29de21362 (KVM: nVMX: fix CR3 load if L2 uses PAE paging and EPT)
> prevents to load L2's PDPTRs according to dereferencing L2's CR3 since it is 
> uninitialized in real mode. Hyper-V L1 will emulate L2 real mode with PAE 
> paging and EPT enabled. However, there is a progress to switch from Legacy 
> mode's such-mode Protected mode to Long mode during system boot, the check 
> in nested_vmx_load_cr3() will prevent to load PDPTRs if it is still in 
> Protected mode w/ PAE paging and nested EPT/shadow page tables on EPT. Actually 
> the original commit should just intended to prevent to dereference L2's CR3 
> if the L1 hypervisor emulates L2's real mode through vm8086.  
> 
> This patch fixes it by allowing load PDPTRs if PAE paing, EPT enabled and 
> !vm86_active.
> 
> Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
> Cc: Ladi Prosek <lprosek@xxxxxxxxxx>
> Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>

Please provide a testcase.  I know this is a regression, but I'm not
going to merge the fix without a corresponding patch to kvm-unit-tests.

Paolo

> ---
>  arch/x86/kvm/vmx.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> index c664365..2b2a05f 100644
> --- a/arch/x86/kvm/vmx.c
> +++ b/arch/x86/kvm/vmx.c
> @@ -9933,7 +9933,7 @@ static bool nested_cr3_valid(struct kvm_vcpu *vcpu, unsigned long val)
>  static int nested_vmx_load_cr3(struct kvm_vcpu *vcpu, unsigned long cr3, bool nested_ept,
>  			       u32 *entry_failure_code)
>  {
> -	if (cr3 != kvm_read_cr3(vcpu) || (!nested_ept && pdptrs_changed(vcpu))) {
> +	if (cr3 != kvm_read_cr3(vcpu) || pdptrs_changed(vcpu)) {
>  		if (!nested_cr3_valid(vcpu, cr3)) {
>  			*entry_failure_code = ENTRY_FAIL_DEFAULT;
>  			return 1;
> @@ -9944,7 +9944,7 @@ static int nested_vmx_load_cr3(struct kvm_vcpu *vcpu, unsigned long cr3, bool ne
>  		 * must not be dereferenced.
>  		 */
>  		if (!is_long_mode(vcpu) && is_pae(vcpu) && is_paging(vcpu) &&
> -		    !nested_ept) {
> +		    !(nested_ept && to_vmx(vcpu)->rmode.vm86_active)) {
>  			if (!load_pdptrs(vcpu, vcpu->arch.walk_mmu, cr3)) {
>  				*entry_failure_code = ENTRY_FAIL_PDPTE;
>  				return 1;
>