Commit c08800a5 authored Feb 04, 2013 by Dongxiao Xu Committed by Marcelo Tosatti Feb 05, 2013

KVM: VMX: disable SMEP feature when guest is in non-paging mode



SMEP is disabled if CPU is in non-paging mode in hardware.
However KVM always uses paging mode to emulate guest non-paging
mode with TDP. To emulate this behavior, SMEP needs to be manually
disabled when guest switches to non-paging mode.

We met an issue that, SMP Linux guest with recent kernel (enable
SMEP support, for example, 3.5.3) would crash with triple fault if
setting unrestricted_guest=0. This is because KVM uses an identity
mapping page table to emulate the non-paging mode, where the page
table is set with USER flag. If SMEP is still enabled in this case,
guest will meet unhandlable page fault and then crash.

Reviewed-by: Gleb Natapov <gleb@redhat.com>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Dongxiao Xu <dongxiao.xu@intel.com>
Signed-off-by: Xiantao Zhang <xiantao.zhang@intel.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>

parent 4293b5e5

arch/x86/kvm/vmx.c

+8 −0

Original line number	Diff line number	Diff line
		@@ -3227,6 +3227,14 @@ static int vmx_set_cr4(struct kvm_vcpu *vcpu, unsigned long cr4)
		if (!is_paging(vcpu)) {
		hw_cr4 &= ~X86_CR4_PAE;
		hw_cr4 \|= X86_CR4_PSE;
		/*
		* SMEP is disabled if CPU is in non-paging mode in
		* hardware. However KVM always uses paging mode to
		* emulate guest non-paging mode with TDP.
		* To emulate this behavior, SMEP needs to be manually
		* disabled when guest switches to non-paging mode.
		*/
		hw_cr4 &= ~X86_CR4_SMEP;
		} else if (!(cr4 & X86_CR4_PAE)) {
		hw_cr4 &= ~X86_CR4_PAE;
		}

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